The Function and Structure of the Skin PDF

The Function and Structure 2 of the Skin The skin is the interface between humans and their envi- tion of the cell changes in a process known as terminal ronment. It weighs an average of 4 kg and covers an area differ...

The Function and Structure 2 of the Skin The skin is the interface between humans and their envi- tion of the cell changes in a process known as terminal ronment. It weighs an average of 4 kg and covers an area differentiation. A vertical section through the epidermis of 2 m2. It acts as a barrier, protecting the body from harsh summarizes the life history of a single epidermal cell (Fig- external conditions and preventing the loss of important ure 2.2). body constituents, especially water. A death from destruc- The basal layer, the deepest layer, rests on a basement tion of skin, as in a burn, or in toxic epidermal necrolysis membrane, which attaches it to the dermis. It is a single (p. 121), and the misery of unpleasant acne, remind us of layer of columnar cells, whose basal surfaces sprout many its many important functions, which range from the vital fine processes and hemidesmosomes, anchoring them to to the cosmetic (Table 2.1). the lamina densa of the basement membrane. The skin has three layers. The outer one is the epider- In normal skin some 30% of basal cells are preparing for mis, which is firmly attached to, and supported by con- division (growth fraction). Following mitosis, a cell enters nective tissue in the underlying dermis. Beneath the der- the G1 phase, synthesizes RNA and protein, and grows in mis is loose connective tissue, the subcutis or hypo-dermis, size (Figure 2.3). Later, when the cell is triggered to divide, which usually contains abundant fat (Figure 2.1). DNA is synthesized (S phase) and chromosomal DNA is replicated. A short post-synthetic (G2 ) phase of further growth occurs before mitosis (M). DNA synthesis contin- Epidermis ues through the S and G2 phases, but not during mitosis. The G1 phase is then repeated, and one of the daughter The epidermis consists of many layers of closely packed cells moves into the suprabasal layer. It then differenti- cells, the most superficial of which are flattened and filled ates (Figure 2.2), having lost the capacity to divide, and with keratins; it is therefore a stratified squamous epithe- synthesizes keratins. Some basal cells remain inactive in a lium. It adheres to the dermis at the basement membrane so-called G0 phase but may re-enter the cycle and resume where downward projections (epidermal ridges or pegs) proliferation. Stem cells reside amongst the interfollicu- interlock with upward projections of the dermis (dermal lar basal cells, the base of sebaceous glands, and amongst papillae) (Figure 2.1). the cells of the external root sheath at the bulge in the The epidermis contains no blood vessels. It varies in hair follicle at the level of attachment of the arrector pili thickness from less than 0.1 mm on the eyelids to nearly muscle. These cells divide infrequently, but can generate 1 mm on the palms and soles. As dead surface squames are new proliferative cells in the epidermis and hair follicle in shed (accounting for some of the dust in our houses), the response to damage. thickness is kept constant by cells dividing in the deep- est (basal) layer. A generated cell moves, to the surface, Keratinocytes passing through the prickle and granular cell layers before The spinous or prickle cell layer (Figure 2.4) is composed dying in the horny layer. The journey from the basal layer of keratinocytes. These differentiating cells, which syn- to the surface (epidermal turnover or transit time) takes thesize keratins, are larger than basal cells. Keratinocytes about 30 days. During this time the appearance and func- are firmly attached to each other by small interlocking Clinical Dermatology, Fifth Edition. Richard B. Weller, Hamish J.A. Hunter and Margaret W. Mann. © 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd. 7 8 Chapter 2 gap junctions, specialized areas on opposing cell walls. Table 2.1 Functions of the skin. Tonofilaments are small fibres running from the cyto- Function Structure/cell involved plasm to the desmosomes. They are more numerous in cells of the spinous layer than of the basal layer, and Protection against: are packed into bundles called tonofibrils. Many lamellar chemicals, particles Horny layer granules (otherwise known as membrane-coating gran- ultraviolet radiation Melanocytes ules, Odland bodies or keratinosomes), derived from the antigens, haptens Langerhans cells Golgi apparatus, appear in the superficial keratinocytes microbes Langerhans cells of this layer. They contain polysaccharides, hydrolytic Preservation of a balanced Horny layer enzymes, and stacks of lipid lamellae composed of phos- internal environment pholipids, cholesterol and glucosylceramides. Their con- Prevents loss of water, Horny layer tents are discharged into the intercellular space of the electrolytes and granular cell layer to become precursors of the lipids in the macromolecules intercellular space of the horny layer (see Barrier function Shock absorber Dermis and subcutaneous fat below). Strong, yet elastic and Cellular differentiation continues in the granular layer, compliant which normally consists of two or three layers of cells Temperature regulation Blood vessels that are flatter than those in the spinous layer, and have Eccrine sweat glands more tonofibrils. As the name of the layer implies, these Insulation Subcutaneous fat cells contain large irregular basophilic granules of kerato- Sensation Specialized nerve endings hyalin, which merge with tonofibrils. These keratohyalin granules contain proteins, including involucrin, loricrin Lubrication Sebaceous glands and profilaggrin, which is cleaved into filaggrin by spe- Protection and prising Nails cific phosphatases as the granular cells move into the Calorie reserve Subcutaneous fat horny layer. Vitamin D synthesis Keratinocytes As keratinocytes migrate out through the outermost Body odour/pheromones Apocrine sweat glands layers, their keratohyalin granules break up and their contents are dispersed throughout the cytoplasm. Filag- Psychosocial, display Skin, lips, hair and nails grin peptides aggregate the keratin cytoskeleton, collaps- ing it, and thus converting the granular cells to flat- tened squames. These make up the thick and tough cytoplasmic processes, by abundant desmosomes, and peripheral protein coating of the horny envelope. Its by other cadherins separated by an intercellular layer of structural proteins include loricrin and involucrin, the glycoproteins and lipoproteins. Under the light micro- latter binding to ceramides in the surrounding inter- scope, the desmosomes look like ‘prickles’. They are spe- cellular space under the influence of transglutaminase. cialized attachment plaques that physically bind adjacent Filaggrin, involucrin and loricrin can all be detected keratinocytes to one another and connect keratin inter- histochemically and are useful as markers of epidermal mediate filaments within keratinocytes to the cell mem- differentiation. brane. Desmosomes are composed of transmembranous The horny layer (stratum corneum) is made of piled-up desmoglein–desmocollin pairs, which bind to the tonofil- layers of flattened dead cells (corneocytes) – the bricks – aments via desmoplakins, plakoglobin and plakophilin-1. separated by lipids – the mortar – in the intercellular There are four types of desmoglein found in the epider- space. Together these provide an effective barrier to water mis. Desmoglein (Dsg) 1 is expressed in the upper epi- loss and to invasion by infectious agents and toxic chemi- dermis while Dsg 3 is mostly expressed in the basal epi- cals. The corneocyte cytoplasm is packed with keratin fil- dermis. Desmoglein 1 is expressed at lower levels in the aments, embedded in a matrix and enclosed by an enve- mucosal epithelium than Dsg 3. Autoantibodies to the lope derived from the keratohyalin granules. This enve- desmogleins are found in pemphigus (p. 113), when they lope, along with the aggregated keratins that it encloses, are responsible for the detachment of keratinocytes from gives the corneocyte its toughness, allowing the skin to one another and so for intraepidermal blister formation. withstand all sorts of chemical and mechanical insults. Cytoplasmic continuity between keratinocytes occurs at Horny cells normally have no nuclei or intracytoplasmic  The Function and Structure of the Skin 9 Thick (hairless) skin Thin (hairy) skin Hair shaft Opening of sweat duct Epidermis Dermal papillae Superficial arteriovenous plexus Papillary dermis Reticular dermis Arrector pili Dermis Meissner’s corpuscle muscle Sweat duct Sebaceous Subcutis/hypodermis gland Deep arteriovenous plexus Subcutaneous fat Hair follicle Dermal nerve fibres Eccrine sweat duct Eccrine sweat gland Eccrine sweat gland Pacinian corpuscle Figure 2.1 Three-dimensional diagram of the skin, including a hair follicle. organelles, these having been destroyed by hydrolytic and The keratins are a family of more than 30 proteins, each degrading enzymes found in lamellar granules and the produced by different genes. These separate into two gene lysosomes of granular cells. families: one responsible for basic and the other for acidic keratins. The keratin polypeptide has a central helical por- Keratinization tion with a non-helical N-terminal head and C-terminal All cells have an internal skeleton made up of micro- tail. Individual keratins exist in pairs so that their double filaments (7 nm diameter; actin), microtubules (20– filament always consists of one acidic and one basic ker- 35 nm diameter; tubulin), and intermediate filaments atin polypeptide. The intertwining of adjacent filaments (10 nm diameter). Keratins (from the Greek keras mean- forms larger fibrils. ing ‘horn’) are the main intermediate filaments in epithe- Different keratins are found at different levels of lial cells and are comparable to vimentin in mesenchy- the epidermis depending on the stage of differentia- mal cells, neurofilaments in neurons, and desmin in tion and disease; normal basal cells make keratins 5 muscle cells. Keratins are not just a biochemical curiosity, and 14, but terminally differentiated suprabasal cells as mutations in their genes cause a number of skin dis- make keratins 1 and 10 (Figure 2.2). Keratins 6 and 16 eases including simple epidermolysis bullosa (p. 123) and become prominent in hyperproliferative states such as bullous ichthyosiform erythroderma (p. 47). psoriasis. 10 Chapter 2 Major Layer keratin Organelle pairs Keratins Horny K1 + K10 Desmosomal remnants Horny envelope Lipid layer Lamellar granule Granular Keratohyalin granule K1 + K10 Degenerating nucleus K5 + K14 Desmosome Golgi apparatus Prickle Ribosomes Tonofibrils Rough endoplasmic reticulum Mitochondrion Nucleus Basal K5 + K14 Scattered tonofilaments Hemidesmosome Lamina densa Figure 2.2 Changes during keratinization. During differentiation, the keratin fibrils in the cells Cell cohesion and desquamation of the horny layer align and aggregate, under the influ- Firm cohesion in the spinous layer is ensured by ‘stick ence of filaggrin. Cysteine, found in keratins of the horny and grip’ mechanisms. A glycoprotein intercellular sub- layer, allows cross-linking of fibrils to give the epidermis stance acts as a cement, sticking the cells together, and strength to withstand injury. the intertwining of the small cytoplasmic processes of the prickle cells, together with their desmosomal attach- ments, accounts for the grip. The cytoskeleton of tonofib- rils also maintains the cell shape rigidly. Resting, The typical ‘basket weave’ appearance of the horny layer Resting,GG00 Differentiation Differentiation in routine histological sections is artefactual and decep- tive. In fact, cells deep in the horny layer stick tightly together and only those at the surface flake off; this is G1 in part caused by the activity of cholesterol sulfatase. This enzyme is deficient in X-linked recessive ichthyosis (p. 46), in which poor shedding leads to the piling up of S corneocytes in the horny layer. Desquamation is normally responsible for the removal of harmful exogenous sub- stances from the skin surface. The cells lost are replaced by newly formed corneocytes; regeneration and turnover G2 of the horny layer is therefore continuous. M The epidermal barrier The horny layer prevents the loss of interstitial fluid Figure 2.3 The cell cycle. from within, and acts as a barrier to the penetration of  The Function and Structure of the Skin 11 (a) (b) Figure 2.4 Layers of the epidermis. (a) Light microscopy and (b) electron micrograph. potentially harmful substances from outside. Solvent permeable to water, but relatively impermeable to ions extraction of the epidermis leads to an increased perme- such as sodium and potassium. Some other substances ability to water, and it has been known for years that essen- (e.g. glucose and urea) also penetrate poorly, whereas tial fatty acid deficiency causes poor cutaneous barrier some aliphatic alcohols pass through easily. The penetra- function. These facts implicate ceramides, cholesterol, tion of a solute dissolved in an organic liquid depends free fatty acids (from lamellar granules; p. 10) and smaller mainly on the qualities of the solvent. quantities of other lipids, in cutaneous barrier formation. Natural moisturising factor (NMF), predominantly made Epidermopoiesis and its regulation up of amino acids and their metabolites, also helps main- Both the thickness of the normal epidermis, and the num- tain the properties of the stratum corneum. Barrier func- ber of cells in it, remain constant, as cell loss at the surface tion is impaired when the horny layer is removed – exper- is balanced by cell production in the basal layer. Locally imentally, by successive strippings with adhesive tape, or produced cytokines, transcription factors and integrins clinically, by injury or skin disease. It is also decreased by stimulate or inhibit epidermal proliferation and differen- excessive hydration or dehydration of the horny layer and tiation, interacting in complex ways to ensure homeosta- by detergents. sis (Table 2.2). The relative impermeability of the stratum corneum and ‘moving staircase’ effect of continually shedding the outer corneocytes provides a passive barrier to infec- Table 2.2 Regulators of epidermopoiesis. tive organisms. In addition to this, protection is given by the various antimicrobial peptides (AMPs) found on Designation Function epithelial surfaces. The two major families of AMPs are P63 Probable stem cell marker. Encourages the defensins and the cathelicidins which have a broad stem cell proliferation range of antimicrobial activity and form the first line of β1 integrin Drives stem cell proliferation immune defence of the body. TGF-α The speed at which a substance penetrates through the C-myc epidermis is directly proportional to its concentration dif- TGF-β Inhibits stem cell proliferation ference across the barrier layer, and inversely proportional Notch signalling Controls epidermal differentiation to the thickness of the horny layer. A rise in skin temper- PPARα ature aids penetration. A normal horny layer is slightly 12 Chapter 2 Vitamin D synthesis outside and thus prone to low sun exposure and vitamin The steroid 7-dehydrocholesterol, found in keratinocytes, D levels. is converted by sunlight to cholecalciferol. The vitamin becomes active after 25-hydroxylation in the kidney. Kid- Other cells in the epidermis ney disease and lack of sun, particularly in dark-skinned Keratinocytes make up about 85% of cells in the epider- peoples, can both cause vitamin D deficiency and rickets. mis, but three other types of cell are also found there: Low vitamin D levels have been linked to a wide range of melanocytes, Langerhans cells and Merkel cells (Fig- diseases including multiple sclerosis, cardiovascular dis- ure 2.5). ease, hypertension and solid organ tumours. However, results of vitamin D supplementation trials have been Melanocytes variable. Vitamin D may merely be a marker for sun Melanocytes are the only cells that can synthesize exposure, with other sun-related factors such as cuta- melanin. They migrate from the neural crest into the basal neous nitric oxide leading to health improvements. Alter- layer of the ectoderm where, in human embryos, they are natively, the direction of association may be the other way, seen as early as 8 weeks’ gestation. They are also found in with unhealthy patients unable to spend as much time hair bulbs, the retina and pia arachnoid. Each dendritic Langerhans cell Dendritic Suprabasal No desmosomes Contains characteristic cytoplasmic organelles Epidermis Keratinocytes Lamina densa Dermis Melancocyte Merkel cell Dendritic No dendrites Mostly basal Basal No desmosomes Desmosomes Contains melanosomes Contains neuro- secretory granules Figure 2.5 Melanocyte, Langerhans cell and Merkel cell.  The Function and Structure of the Skin 13 Figure 2.6 Melanocyte (electron micrograph), with melanosomes (inset). melanocyte associates with a number of keratinocytes, forming an epidermal melanin unit (Figure 2.5). The den- dritic processes of melanocytes wind between the epider- mal cells and end as discs in contact with them. Their cytoplasm contains discrete organelles, the melanosomes, containing varying amounts of the pigment melanin (Fig- ure 2.6). This is ‘injected’ into surrounding keratinocytes to provide them with pigmentation to help protect the skin against damaging ultraviolet radiation. Melanogenesis is described at the beginning of Chap- ter 19 on disorders of pigmentation. Figure 2.7 Adenosine triphosphase-positive Langerhans cells Langerhans cells in an epidermal sheet. The network provides a The Langerhans cell is a dendritic cell (Figures 2.5 and reticulo-epithelial trap for contact allergens. 2.7) like the melanocyte. It also lacks desmosomes and tonofibrils, but has a lobulated nucleus. The specific gran- ules within the cell look like a tennis racket when seen in (MHC) Class II antigens (HLA-DR, -DP and -DQ). They two dimensions in an electron micrograph (Figure 2.8), or are best thought of as highly specialized macrophages. like a sycamore seed when reconstructed in three dimen- Langerhans cells have a key role in many immune reac- sions. They are plate-like, with a rounded bleb protruding tions. They take up exogenous antigen, process it and from the surface. present it to T lymphocytes either in the skin or in the Langerhans cells come from a mobile pool of precur- local lymph nodes (p. 26). They probably play a part in sors originating in the bone marrow. There are approxi- immunosurveillance for viral and tumour antigens. Top- mately 800 Langerhans cells per mm2 in human skin and ical or systemic glucocorticoids reduce the density of epi- their dendritic processes fan out to form a striking net- dermal Langerhans cells as does ultraviolet radiation. work seen best in epidermal sheets (Figure 2.7). Langer- hans cells are alone among epidermal cells in possess- Merkel cells ing surface receptors for C3b and the Fc portions of IgG Merkel cells are found in normal epidermis (Fig- and IgE, and in bearing major histocompatibility complex ure 2.5) and act as transducers for fine touch. They are 14 Chapter 2 Figure 2.8 Langerhans cell (electron micrograph), with characteristic granule (inset). non-dendritic cells, lying in or near the basal layer, and are bodies to BP antigens 1 and 2 are found in pemphigoid of the same size as keratinocytes. They are concentrated (p. 117), a subcutaneous blistering condition. The intra- in localized thickenings of the epidermis near hair folli- cellular component of the hemidesmosome binds to the cles (hair discs), and contain membrane-bound spherical intermediate filament network within keratinocytes. granules, 80–100 nm in diameter, which have a core of Fine anchoring filaments, largely composed of laminin- varying density, separated from the membrane by a clear 332, cross the lamina lucida and connect the lamina densa halo. Sparse desmosomes connect these cells to neigh- to the plasma membrane of the basal cells by interacting bouring keratinocytes. Fine unmyelinated nerve endings with the α6 β4 integrin component of the hemidesmo- are often associated with Merkel cells, which express some. Anchoring fibrils (of type VII collagen), dermal immunoreactivity for various neuropeptides. microfibril bundles and single small collagen fibres (types I and III), extend from the papillary dermis to the deep Epidermal appendages part of the lamina densa. The skin appendages are derived from epithelial germs Laminins, large non-collagen glycoproteins produced during embryogenesis and, except for the nails, lie in the by keratinocytes, aided by nidogen, promote adhesion dermis. They include hair, nails, and sweat and sebaceous between the basal cells above the lamina lucida and type glands. They are described, along with the diseases that IV collagen, the main constituent of the lamina densa, affect them, in Chapters 12 and 13, respectively. below it. Laminin 332 is the most important member of this family, which also includes laminins 311 and 511. Laminins bind to cells by interactions with cellular Dermo-epidermal junction integrins. The structures within the dermo-epidermal junction The basement membrane lies at the interface between provide mechanical support, encouraging the adhesion, the epidermis and dermis. With light microscopy it can growth, differentiation and migration of the overlying be highlighted using a periodic acid–Schiff (PAS) stain, basal cells, and also act as a semipermeable filter that reg- because of its abundance of neutral mucopolysaccharides. ulates the transfer of nutrients and cells from dermis to Electron microscopy (Figure 2.9) shows that the lamina epidermis. densa (rich in type IV collagen) is separated from the basal cells by an electron-lucent area, the lamina lucida. Hemidesmosomes span the basal plasma membrane of Dermis basal keratinocytes and structurally bind the epidermis to the dermis. They are complex structures made up of The dermis lies between the epidermis and the subcuta- bullous pemphigoid antigens 1 (a 230-kDa plakin) and 2 neous fat. It supports the epidermis structurally and nutri- (180 kDa collagen XVII), plectin and α6 β4 integrin. Anti- tionally. Its thickness varies, being greatest in the palms  The Function and Structure of the Skin 15 Epidermis Desmosome Basal keratinocyte (desmoglein-1 and -3, (keratins 5 + 14) desmoplakin) Tonofilaments (keratin) Basal cell membrane Hemidesmosome (BPAg, collagen Lamina lucida XVII, α6 β4 integrin) (laminin-1) Anchoring filament Lamina densa (laminin-5) (type IV collagen) Anchoring fibril Sub-lamina densa (collagen VII) Dermis Figure 2.9 Structure and molecular composition of the dermo-epidermal junction. and soles and least in the eyelids and penis. In old age, the ual collagen molecules. These molecules consist of three dermis thins and loses its elasticity. polypeptide chains (molecular weight 150 kDa) forming The dermis interdigitates with the epidermis (Fig- a triple helix with a non-helical segment at both ends. ure 2.1) so that upward projections of the dermis, the der- The alignment of the chains is stabilized by covalent mal papillae, interlock with downward ridges of the epi- cross-links involving lysine and hydroxylysine. Collagen dermis, the rete pegs. This interdigitation is responsible is an unusual protein as it contains a high proportion of for the ridges seen most readily on the fingertips (as fin- proline and hydroxyproline and many glycine residues; gerprints). It is important in the adhesion between epider- the spacing of glycine as every third amino acid is a mis and dermis as it increases the area of contact between prerequisite for the formation of a triple helix. Defects them. in the enzymes needed for collagen synthesis are Like all connective tissues the dermis has three compo- nents: cells, fibres and amorphous ground substance. Table 2.3 Functions of some resident dermal cells. Cells of the dermis Fibroblast Synthesis of collagen, reticulin, The main cells of the dermis are fibroblasts, but there are elastin, fibronectin, also small numbers of resident and transitory mononu- glycosaminoglycans, collagenase clear phagocytes, lymphocytes, dermal dendritic cells and Mononuclear Mobile: phagocytose and destroy mast cells. Other blood cells (e.g. polymorphs) are seen phagocyte bacteria during inflammation. The main functions of the resi- Secrete cytokines dent dermal cells are listed in Table 2.3 and their role in Lymphocyte Immunosurveillance immunological reactions is discussed later in this chapter. Langerhans cell In transit between local lymph node and dermal and epidermis Fibres of the dermis dendritic cell Antigen presentation The dermis is largely made up of interwoven fibres, prin- Mast cell Stimulated by antigens, complement cipally of collagen, packed in bundles. Those in the pap- components, and other substances illary dermis are finer than those in the deeper retic- to release many inflammatory ular dermis. When the skin is stretched, collagen, with mediators including histamine, its high tensile strength, prevents tearing, and the elastic heparin, prostaglandins, fibres, intermingled with the collagen, later return it to the leukotrienes, tryptase and unstretched state. chemotactic factors for eosinophils Collagen makes up 70–80% of the dry weight of the and neutrophils dermis. Its fibres are composed of thinner fibrils, which Merkel cell Act as transducers for fine touch are in turn made up of microfibrils built from individ- 16 Chapter 2 r it provides bulk, allowing the dermis to act as a shock Table 2.4 Distribution of some types of collagen. absorber. Collagen Muscles type Tissue distribution Both smooth and striated muscle are found in the skin. I Most connective tissues including tendon and The smooth arrector pili muscles (see Figure 13.1) are bone used by animals to raise their fur and so protect them Accounts for approximately 85% of skin collagen from the cold. They are vestigial in humans, but may help II Cartilage to express sebum. Smooth muscle is also responsible for III Accounts for about 15% of skin collagen ‘goose pimples’ (bumps) from cold, nipple erection and Blood vessels the raising of the scrotum by the dartos muscle. Striated IV Skin (lamina densa) and basement membranes fibres (e.g. the platysma) and some of the muscles of facial of other tissues expression, are also found in the dermis. V Ubiquitous, including placenta VII Skin (anchoring fibrils) Blood vessels Fetal membranes Although the skin consumes little oxygen, its abundant blood supply regulates body temperature. The blood ves- sels lie in two main horizontal layers (Figure 2.10). The responsible for some skin diseases, including Ehlers– deep plexus is just above the subcutaneous fat, and its Danlos syndrome (see Chapter 21), and conditions arterioles supply the sweat glands and hair papillae. The involving other systems, including lathyrism (fragility superficial plexus is in the papillary dermis and arterioles of skin and other connective tissues) and osteogenesis from it become capillary loops in the dermal papillae. An imperfecta (fragility of bones). arteriole arising in the deep dermis supplies an inverted There are many, genetically distinct, collagen proteins, cone of tissue, with its base at the epidermis. all with triple helical molecules, and all rich in hydrox- The blood vessels in the skin are important in ther- yproline and hydroxylysine. The distribution of some of moregulation. Under sympathetic nervous control, arte- them is summarized in Table 2.4. riovenous anastamoses at the level of the deep plexus can Reticulin fibres are fine collagen fibres, seen in fetal skin shunt blood to the venous plexus at the expense of the cap- and around the blood vessels and appendages of adult illary loops, thereby reducing surface heat loss by convec- skin. tion. Elastic fibres account for about 2% of the dry weight Cutaneous lymphatics of adult dermis. They have two distinct protein com- Afferent lymphatics begin as blind-ended capillaries in ponents: an amorphous elastin core and a surrounding the dermal papilla and pass to a superficial lymphatic elastic tissue microfibrillar component. Elastin (molecu- plexus in the papillary dermis. There are also two deeper lar weight 72 kDa) is made up of polypeptides (rich in horizontal plexuses, and collecting lymphatics from the glycine, desmosine and valine) linked to the microfibrillar deeper one run with the veins in the superficial fascia. component through their desmosine residues. Abnormal- ities in the elastic tissue cause cutis laxa (sagging inelastic skin) and pseudoxanthoma elasticum (see Chapter 21). Ground substance of the dermis The amorphous ground substance of the dermis consists largely of two glycosaminoglycans (hyaluronic acid and dermatan sulfate) with smaller amounts of heparan sul- fate and chondroitin sulfate. The glycosaminoglycans are complexed to core protein and exist as proteoglycans. The ground substance has several important functions: r it binds water, allowing nutrients, hormones and waste products to pass through the dermis; r it acts as a lubricant between the collagen and elastic fibre networks during skin movement; and Figure 2.10 Blood vessels of the skin (carmine stain).  The Function and Structure of the Skin 17 Nerves mechanical barrier from which contaminating organisms The skin is liberally supplied with an estimated 1 mil- and chemicals are continually being removed by wash- lion nerve fibres. Most are found in the face and extrem- ing and desquamation. Only when these breach the horny ities. Their cell bodies lie in the dorsal root ganglia. layer do the cellular components, described below, come Both myelinated and non-myelinated fibres exist, with the into play. latter making up an increasing proportion peripherally. The skin is involved in so many immunological reac- Most free sensory nerves end in the dermis; however, a tions, seen regularly in the clinic (e.g. urticaria, aller- few non-myelinated nerve endings penetrate into the epi- gic contact dermatitis, psoriasis, vasculitis), that a spe- dermis. Some of these are associated with Merkel cells cial mention has to be made of the peripheral arm of (p. 13). Free nerve endings detect the potentially damag- the immune system based in the skin – the skin immune ing stimuli of heat and pain (nocioceptors), while special- system (SIS). The coordinated actions of the innate and ized end organs in the dermis, Pacinian and Meissner cor- acquired immune systems are mediated by cells such as puscles, register deformation of the skin caused by pres- macrophages, dendritic cells and T cells, and by effector sure (mechanoreceptors) as well as vibration and touch. proteins such as cytokines and antibodies. Although it is Autonomic nerves supply the blood vessels, sweat glands beyond the scope of this book to cover general immunol- and arrector pili muscles. ogy, this section outlines some of the intricate ways in Itching is an important feature of many skin diseases. which the skin defends itself and the body, and how anti- It follows the stimulation of fine free nerve endings lying gens are recognized by specialized skin cells, such as the close to the dermo-epidermal junction. Areas with a high Langerhans cells. It also reviews the ways in which anti- density of such endings (itch spots) are especially sensi- bodies, lymphocytes, macrophages and polymorphs elicit tive to itch-provoking stimuli. Impulses from these free inflammation in skin. endings pass centrally in two ways: quickly along myeli- nated A fibres, and more slowly along non-myelinated C fibres. As a result, itch has two components: a quick local- Cellular components of the skin ized pricking sensation followed by a slow burning diffuse immune system itching. Many stimuli can induce itching (electrical, chemi- Keratinocytes (p. 7) cal and mechanical). In itchy skin diseases, pruritogenic The prime role of keratinocytes is to make the protective chemicals such as histamine and proteolytic enzymes horny layer (p. 7) and to support the outermost epithelium are liberated close to the dermo-epidermal junction. of the body, but they also have important immunologi- The detailed pharmacology of individual diseases is still cal functions in their own right and act as a link between poorly understood but prostaglandins potentiate chemi- the innate and acquired immune systems. Keratinocytes cally induced itching in inflammatory skin diseases. synthesize and release the cationic antimicrobial peptides cathelicidin and β-defensin. They can recognize pattern- associated molecular patterns (PAMPs) on bacteria by the Learning points toll-like receptors (TLRs) that they carry on their surface 1 More diseases are now being classified by abnormalities of and in the cytosol. Activation of these leads to interferon function and structure rather than by their appearance. release and a T-helper 1 (Th1) immune response. Pro- 2 Today’s patients are inquisitive and knowledgeable. If you inflammatory pathways are activated after TLR ligation understand the structure and function of the skin, your via an organized cluster of proteins known as the inflam- explanations to them will be easier and more convincing. mosome in the keratinocyte cytosol. Keratinocytes pro- duce chemokines that attract cells of the immune system to the skin, and large numbers of cytokines that can acti- The skin immune system vate and guide an immune response. Their release of IL-1 after injury initiates various immune and inflammatory The skin acts as a barrier to prevent injury of underly- cascades (Figure 2.11). γ-interferon induces the expres- ing tissues and to prevent infections from entering the sion of MHC Class II molecule on keratinocytes enabling body. Simply put, it keeps the inside in and the outside them to act as non-professional antigen-presenting cells. out. The horny layer is a physical barrier that minimizes Keratinocytes play a central part in healing after epider- the loss of fluid and electrolytes, and also stops the pene- mal injury by self-regulating epidermal proliferation and tration of harmful substances and traumas (p. 7). It is a dry differentiation (Figure 2.11). 18 Chapter 2 Injury 1 Epidermis Keratinocyte Proliferation 5 3 Activated Migration keratinocyte Other cytokines More e.g. GM-CSF Figure 2.11 The keratinocyte and wound cytokines healing. The injured keratinocyte turns on TNF-α wound healing responses. When a TGF-α keratinocyte is injured (1), it releases Amphiregulin 2 interleukin-1 (IL-1) (2). IL-1 activates endothelial cells causing them to express selectins that slow down lymphocytes passing over them. Once lymphocytes stop Dermis on the endothelial cells lining the vessels, IL-1 acts as a chemotactic factor to draw IL-1 lymphocytes into the epidermis (4). At the same time, IL-1 activates keratinocytes by 4 binding to their IL-1 receptors. Activated Migration keratinocytes produce other cytokines (3). Among these is tumour necrosis factor α (TNF-α) which additionally activates Fibroblast keratinocytes and keeps them in an activated state (5). Activation of keratinocytes causes them to proliferate, Secrete migrate and secrete additional cytokines. extracellular Blood vessel GM-CSF, granulocyte–macrophage Proliferation matrix colony-stimulating factor; TGF, transforming growth factor. Langerhans cells (p. 13) carry, and their roles are gradually being elucidated. Dif- These dendritic cells come from the bone marrow and ferent subsets are seen in inflamed and uninflamed skin, move into the epidermis and mucous membranes. Their and activated dermal dendritic cells can secrete cytokines. dendrites intercalate between keratinocytes and their Dermal dendritic cells can migrate to lymph nodes and peripheral location in the body makes them an early sen- stimulate T-cell proliferation, but they are probably more tinel in defence against infection. They can be identified specifically involved in regulating the humoral antibody- in tissue sections by demonstrating their characteristic mediated immune response. surface marker langerin, and also express MHC Class II molecules, CD1a antigen, and carry Birbeck granules. The T lymphocytes function of Langerhans cells is an area of active research, Like T cells elsewhere, these develop and acquire their but it is clear that they are key players in the induction of antigen receptors (T-cell receptors, TCR) in the thymus. T-cell responses. Following antigen uptake and activation, They differentiate into subpopulations, recognizable by they migrate to the draining lymph nodes and can induce their different surface molecules (CD meaning cluster either immunity or tolerance to an antigen, depending on of differentiation markers), which are functionally dis- the circumstances in which it is presented. tinct. T lymphocytes that express CD4 on their surfaces work to induce immune reactions and elicit inflamma- Dermal dendritic cells tion. T cells that express CD8 are cytotoxic and can lyse Several subsets of dendritic cells are found within the infected, grafted and cancerous cells. A subset of these, dermis, particularly just below the basement membrane. the CD8 skin resident memory T cells, remain perma- They are identified by the different surface markers they nently in the skin and can rapidly amplify a reaction to  The Function and Structure of the Skin 19 an infectious challenge without circulating to the regional that attract and activate other inflammatory cells (see lymph nodes. CD4 (helper) cells are subdivided into Th1 Figure 2.18). cells that produce interleukin-2 (IL-2; T-cell growth fac- Some skin diseases display a predominantly Th1 tor) and γ-interferon. Th2 cells produce other interleukins response (e.g. tuberculoid leprosy), others a mainly Th2 such as IL-4, IL-5, IL-9 and IL-13. A third subset of CD4 response (e.g. atopic dermatitis) and others a Th17 cells has been described, called (somewhat confusingly) response (psoriasis). Th17 cells. IL-23 directs these to release IL-17 and IL- 22. The amounts of IL-12 and IL-4 secreted by antigen- T-cytotoxic (Tc) cells processing cells seem important in determining exactly These lymphocytes are capable of destroying allogeneic which path of differentiation is followed. and virally infected cells, which they recognize by the Th1 cells induce cell-mediated immune reactions in MHC Class I molecules on their surface. They express the skin – for example, allergic contact dermatitis and CD8. delayed hypersensitivity reactions – and are involved in elicitation reactions as well. Th2 cells help B cells pro- T-regulatory (Treg) cells duce antibody. Th17 cells are involved in the clearance of This subset of T cells are strongly immunosuppressive and infectious agents, and also mediate autoimmune inflam- are characterized by the expression of the transcription mation and psoriasis. Th cells recognize antigen in asso- factor Foxp3. Tregs are found in the skin and the circu- ciation with MHC Class II molecules (Figures 2.12 and lation and help balance inflammatory responses. Ultra- 2.13) and, when triggered by antigen, release cytokines violet radiation induces Tregs in the skin, and both the T cell Augmentation Signal 1 Signal 2 T cell β α β α TCR TCR CD4 CD4 CD2 LFA-1 CD28 CD2 LFA-1 CD28 Jβ Jα Jβ Jα Dβ Dβ Vβ Vα Vβ Vα LFA-3 ICAM-1 B7 LFA-3 ICAM-1 B7 CD80/86 CD80/86 MHC-II MHC-II α β α β Langerhans cell Langerhans cell (a) Antigen ( ) presentation (b) Superantigen ( ) presentation Figure 2.12 T-lymphocyte activation by (a) antigen and (b) superantigen. When antigen has been processed it is presented on the surface of the Langerhans cell in association with major histocompatibility complex (MHC) Class II. The complex formation that takes place between the antigen, MHC Class II and T-cell receptor (TCR) provides signal 1, which is enhanced by the coupling of CD4 with the MHC molecule. A second signal for T-cell activation is provided by the interaction between the co-stimulatory molecules CD28 (T cell) and B7 (Langerhans cell). CD2/LFA-3 and LFA-1/ICAM-1 adhesion augment the response to signals 1 and 2. Superantigen interacts with the TCR Vβ and MHC Class II without processing, binding outside the normal antigen binding site. Activated T cells secrete many cytokines, including IL-1, IL-8 and γ-interferon, which promote inflammation (Figure 2.13). 20 Chapter 2 TGF-β IL-23 Th0 TNF-α IL-4 IL-12 Th17 Th1 Th2 IL-22 IL-17 IL-2 IL-4 TNF-α IL-5 IFN-γ IL-10 Cell-mediated Antibody-mediated Tissue inflammation immunity immunity Figure 2.13 Characteristics of Th1, Th2 and Th17 responses. sensitization and elicitation phases of the contact hyper- marrow, but have no antigen-specific receptors, reacting sensitivity response are moderated by these cells. instead with self antigens. They especially kill tumour and virally infected cells. These cells can sometimes recognize T-cell receptor and T-cell gene receptor glycolipid antigens using CD1 surface molecules that do rearrangements not require presentation by antigen-presenting cells. Most T-cell receptors are composed of an α and a β chain, each with a variable (antigen binding) and a con- Killer cells stant domain, which are associated with the CD3 cell sur- These are cytotoxic T cells, NK cells, or monocytic leuco- face molecules (Figure 2.12). The amino acid sequence cytes that can kill target cells sensitized with antibody. In of the variable portion determines its antigen-binding antibody-mediated cellular cytotoxicity, antibody binds specificity – different sequences bind different antigens. to antigen on the surface of the target cell: the K cell binds To provide diversity and the ability to bind almost any to the antibody at its other (Fc) end by its Fc receptor and antigen, the genes coding for the amino acid sequence the target cell is then lysed. undergo rearrangent. Antigenic stimulation results in expansion of appropriate clones carrying TCR capable of Mast cells binding to the antigen. Most responses are polyclonal. On These are present in most connective tissues, predom- the other hand, malignant transformation is associated inantly around blood vessels. Their numerous gran- with proliferation of a unique clone. In fact, an analysis ules contain inflammatory mediators (see Figure 8.1). of the degree of clonality of rearrangements of the gene In rodents – and probably in humans – there are two for the receptor can be used to determine whether a T-cell distinct populations of mast cells, connective tissue and infiltrate in skin is likely to be malignant or reactive. mucosal, which differ in their staining properties, con- tent of inflammatory mediators and proteolytic enzymes. Other (non-T, non-B) lymphocytes Skin mast cells play a central part in the pathogenesis of Some lymphocytes express neither CD4 nor CD8. These urticaria (p. 99) and also influence the development of leucocytes have some properties of T lymphocytes and contact allergic responses to haptens. some properties of myelomonocytic cells. Most have receptors for FcIgG. This subpopulation contains natural killer (NK) and killer (K) cells. Molecular components of the skin immune system Natural killer cells Antigens These are large granular leucocytes that can kill virally Antigens are molecules that are recognized by the infected cells, or tumour cells that have not previously immune system thereby provoking an immune reaction, been sensitized with antibody. They develop in the bone usually in the form of a humoral or cell-mediated immune  The Function and Structure of the Skin 21 response. The immune system can usually identify its own factors. Interleukins are produced predominantly by leu- molecules so that it does not direct a reaction against cocytes, have a known amino acid sequence and are active them. If it does, autoimmune reactions occur. Otherwise, in inflammation or immunity. the skin immune system readily responds to non-self anti- There are many cytokines, and each may act on more gens, such as chemicals, proteins, allografted cells and than one type of cell, causing many different effects. infectious agents. The process of recognizing antigens and Cytokines frequently have overlapping actions. In any developing immunity is called induction or sensitization. inflammatory reaction some cytokines are acting syner- gistically while others will antagonize these effects. This Superantigens network of potent chemicals, each acting alone and in Some bacterial toxins (e.g. those released by Staphylococ- concert, moves the inflammatory response along in a con- cus aureus) are prototypic superantigens. They align with trolled way. Cytokines bind to high affinity (but not usu- MHC Class II molecules of antigen-presenting cells out- ally specific) cell surface receptors, and elicit a biologi- side their antigen-presentation groove and, without any cal response by regulating the transcription of genes in cellular processing, may directly induce massive T-cell the target cell via signal transduction pathways involv- proliferation and cytokine production leading to disor- ing, for example, the Janus protein tyrosine kinase or cal- ders such as the toxic shock syndrome (p. 217). Strepto- cium influx systems. The biological response is a balance coccal toxins act as superantigens to activate T cells in the between the production of the cytokine, the expression pathogenesis of guttate psoriasis. of its receptors on the target cells and the presence of inhibitors. Antibodies (immunoglobulins) Antibodies are immunoglobulins that react with antigens. Adhesion molecules r Immunoglobulin G (IgG) is responsible for long-lasting Cellular adhesion molecules (CAMs) are surface glyco- humoral immunity. It can cross the placenta, and binds proteins that are expressed on many different types of cell; complement to activate the classic complement path- they are involved in cell–cell and cell–matrix adhesion way. IgG can coat neutrophils and macrophages (by and interactions. CAMs are fundamental in the interac- their FcIgG receptors), and acts as an opsonin by cross- tion of lymphocytes with antigen-presenting cells (Fig- bridging antigen. IgG can also sensitize target cells for ure 2.12), keratinocytes and endothelial cells, and are destruction by K cells. important in lymphocyte trafficking in the skin during r IgM is the largest immunoglobulin molecule. It is the inflammation (Figure 2.11). CAMs have been classified first antibody to appear after immunization or infection. into four families: cadherins, immunoglobulin superfam- Like IgG, it can fix complement but unlike IgG it cannot ily, integrins and selectins. E-cadherins are found on cross the placenta. the surface of keratinocytes between the desmosomes. r IgA is the most common immunoglobulin in secre- γ-Interferon causes upregulation of Fas on epidermal lym- tions. It acts as a protective paint in the gastrointestinal phocytes. Interaction of these with Fas ligand on ker- and respiratory tracts. It does not bind complement but atinocytes causes e-cadherins to ‘disappear’, leading to can activate it via the alternative pathway. intercellular edema (spongiosis) between desmosomes. r IgE binds to Fc receptors on mast cells and basophils, CAMs of special relevance in the skin are listed in where it sensitizes them to release inflammatory media- Table 2.5. tors in type I immediate hypersensitivity reactions (Fig- ure 2.14). Histocompatibility antigens Like other cells, those in the skin express surface anti- Cytokines gens directed by genes. The human leucocyte antigen Cytokines are small proteins secreted by cells such as lym- (HLA) region lies within the major histocompatability phocytes and macrophages, and also by keratinocytes. locus (MHC) on chromosome 6. In particular, HLA-A, They regulate the amplitude and duration of inflamma- -B and -C antigens (the Class I antigens) are expressed tion by acting locally on nearby cells (paracrine action), on all nucleated cells including keratinocytes, Langerhans on those cells that secreted them (autocrine) and occa- cells and cells of the dermis. HLA-DR, -DP, -DQ and -DZ sionally on distant target cells (endocrine) via the circu- antigens (the Class II antigens) are expressed only on lation. The term cytokine covers interleukins, interfer- some cells (e.g. Langerhans cells and B cells). They are ons, colony-stimulating factors, cytotoxins and growth usually not found on keratinocytes except during certain 22 Chapter 2 I II III IV V Plasma cell makes IgE attaches to Antigen attaches Mast cell degranulates Mediators of inflammation circulating IgE mast cell to IgE on mast cell after influx of calcium released into tissues Histamine Leukotrienes Antigen (e.g. drug) Platelet-activating factor Plasma cell Fc receptor Eosinophil and neutrophil chemotactic factors Proteases Cytokines (IL-6, IL-8) VI Ca2+ Development of urticarial reaction Mast cell (vasodilatation, Mast cell degranulates oedema, inflammation) Figure 2.14 Urticaria: an immediate (type I) hypersensitivity reaction. reactions (e.g. allergic contact dermatitis) or diseases (e.g. tions. On the other hand, Class I antigens mark target cells lichen planus). Helper T cells recognize antigens only in for cell-mediated cytotoxic reactions, such as the rejection the presence of cells bearing Class II antigens. Class II of skin allografts and the destruction of cells infected by antigens are also important for certain cell–cell interac- viruses. Table 2.5 Cellular adhesion molecules important in the skin. Family Nature Example Site Ligand Cadherins Glycoproteins Desmoglein Desmosomes in Other cadherins Adherence dependent epidermis on calcium Immunoglobulin Numerous molecules ICAM-1 Endothelial cells LFA-1 that are structurally Keratinocytes similar to Langerhans cells immunoglobulins CD2 T lymphocytes LFA-3 Some NK cells VCAM-1 Endothelial cells VLA-4 Integrins Surface proteins (β1-VLA) T lymphocyte VCAM comprising two LFA-1 T lymphocyte ICAM-1 non-covalently Macrophages C3b component of bound α and β chains Mac-1 Monocytes complement Granulocytes Selectins Adhesion molecules E selectin Endothelial cells CD15 with lectin-like domain which binds carbohydrate CD, Cluster of differentiation antigen; ICAM, Intercellular adhesion molecule; LFA, leucocyte function antigen; Mac, macrophage activation; VCAM, vascular cell adhesion molecule; VLA, very late activation proteins.  The Function and Structure of the Skin 23 Types of immune reactions in the skin and powerfully when they encounter their antigen again – even years later. Specific immune responses allow a targeted and ampli- Innate immune system fied inflammatory response. To induce such a response, The epidermal barrier is the major defence against infec- an antigen must be processed by an antigen-presenting tion in human skin. When it is breached, cells in the der- cell such as a Langerhans or dermal dendritic cell, and mis and epidermis can telegraph the danger and engage it must be presented to a T cell, with unique recep- the innate immunity and inflammatory systems. Innate tor molecules on its surface, that can bind the antigen immunity allows reaction to infectious agents and nox- presented to it. To elicit an inflammatory response, this ious chemicals, without the need to activate specific lym- antigen processing, presenting and binding process is phocytes or use antibodies. This is fortunate. If an infected repeated but, this time, with the purpose of bringing in person had to wait for immunity to develop, the onset of inflammatory, phagocytic and cytotoxic cells to control the reaction might take a week or two, and by then the the inflammation within the arena. infection might be widespread or lethal. It is still helpful, if rather artificial, to separate these For example, defensins in the epidermis inhibit bac- elicited specific immune responses into four main types terial replication there. Complement can be activated by using the original classification of Coombs and Gell. All many infectious agents via the alternative pathway with- of these types cause reactions in the skin. out the need for antigen–antibody interaction. Comple- ment activation generates C5a, which attracts neutrophils, Type I: Immediate hypersensitivity reactions and C3b and C5b, which opsonize the agents so they can These are characterized by vasodilatation and an out- be more readily engulfed and killed by the phagocytes pouring of fluid from blood vessels. Such reactions can when these arrive. Chemicals such as detergents can acti- be mimicked by drugs or toxins, which act directly, but vate keratinocytes to produce cytokines leading to epi- immunological reactions are mediated by antibodies, and dermal proliferation and eventual shedding of the toxic are manifestations of allergy. IgE and IgG4 antibodies, agent. After infection or stimulation, certain cells can produced by plasma cells in organs other than the skin, non-specifically secrete chemokines that bring inflam- attach themselves to mast cells in the dermis. These con- matory cells to the area. The main effector cells of the tain inflammatory mediators, both in granules and in innate immune system are neutrophils, monocytes and their cytoplasm. The IgE antibody is attached to the mast macrophages. cell by its Fc end, so that the antigen combining site The antigen-presenting cells have a role in both innate dangles from the mast cell like a hand on an arm (Fig- and acquired immunity. They can recognize certain pat- ure 2.14). When specific antigen combines with the hand terns of molecules or chemicals common to many infec- parts of the immunoglobulin (the antigen-binding site or tious agents. The lipo-polysaccharide of Gram-negative Fab end), the mast cell liberates its mediators into the bacteria is an example of such a pathogen-associated surrounding tissue. Of these mediators, histamine (from molecular pattern. The receptors for these reside on cell the granules) and leukotrienes (from the cell membrane) membranes and are genetically derived. induce vasodilatation, and endothelial cells retract allow- Toll-like receptors provide the innate immune sys- ing transudation into the extravascular space. The vasodi- tem with a certain specificity. These are transmembrane latation causes a pink colour, and the transudation causes proteins, which also recognize patterns, and different swelling. Urticaria and angioedema (p. 99) are examples toll receptors recognize different patterns and chemicals. of immediate hypersensitivity reactions occurring in the For example, toll-like receptor 2 recognizes lipoproteins, skin. while toll-like receptor 3 recognizes double-stranded Antigen may be delivered to the skin from the outside RNA. Toll-like receptors also upregulate the expression of (e.g. in a bee sting). This will induce a swelling in every- co-stimulatory molecules that allow appropriate recogni- one by a direct pharmacological action. However, some tion and response of the adaptive immune system. people, with IgE antibodies against antigens in the venom, swell even more at the site of the sting as the result of Adaptive immune system a specific immunological reaction. If they are extremely Adaptive immunity is not only more specific, but is also sensitive, they may develop wheezing, wheals and anaphy- long-lasting. It generates cells that can persist in a rel- lactic shock (see Figure 25.5) because of a massive release atively dormant state. These are ready to react quickly of histamine into the circulation. 24 Chapter 2 Antigens can also reach mast cells from inside the when these arrive (Figure 2.15). Under certain circum- body. Those who are allergic to shellfish, for example, stances, activation of complement can kill cells or organ- may develop urticaria within seconds, minutes or hours isms directly by the membrane attack complex (C5b6789) of eating one. Antigenic material, absorbed from the gut, in the terminal complement pathway. Complement can passes to tissue mast cells via the circulation, and elicits also be activated by bacteria directly through the alterna- an urticarial reaction after binding to specific IgE on mast tive pathway; antibody is not required. The bacterial cell cells in the skin. wall causes more C3b to be produced by the alternative pathway factors B, D and P (properdin). Aggregated IgA Type II: Humoral cytotoxic reactions can also activate the alternative pathway. In the main, these involve IgG and IgM antibodies, which, Activation of either pathway produces C3b, the piv- like IgE, are produced by plasma cells and are present in otal component of the complement system. Through the the interstitial fluid of the skin. When they meet an anti- amplification loop, a single reaction can flood the area gen, they fix and activate complement through a series of with C3b, C5a and other amplification loop and termi- enzymatic reactions that generate mediator and cytotoxic nal pathway components. Complement is the mediator of proteins. If bacteria enter the skin, IgG and IgM antibod- humoral reactions. ies bind to antigens on them. Complement is activated Humoral cytotoxic reactions are typical of defence through the classic pathway, and a number of mediators against infectious agents such as bacteria. However, they are generated. Amongst these are the chemotactic factor, are also involved in certain autoimmune diseases such as C5a, which attracts polymorphs to the area of bacterial pemphigoid (see Chapter 9). invasion, and the opsonin, C3b, which coats the bacte- Occasionally, antibodies bind to the surface of a cell ria so that they can be ingested and killed by polymorphs and activate it without causing its death or activating I Epidermis IgG antibody reacts to Basement membrane zone basement membrane zone antigen (bullous IgG Dermis pemphigoid antigen, ). C Complement CIq Complement Complement fixation activation II C Complement fixation Complement activation Neutrophil influx C3b C5a Membrane chemotactic attack complex Opsonin factor Neutrophil III Damage to basement Cell or membrane zone, leading Cell or membrane to subepidermal blister C membrane damage or ( ). Figure 2.15 Bullous pemphigoid: a humoral damage phagocytosis cytotoxic (type II) reaction against a basement membrane zone antigen.  The Function and Structure of the Skin 25 complement. Instead, the cell is stimulated to produce a morphs liberates lysosomal enzymes that damage the ves- hormone-like substance that may mediate disease. Pem- sel walls. phigus (see Chapter 9) is a blistering disease of skin in Antigen–antibody complexes can also be formed in the which this type of reaction may be important. circulation, move to the small vessels in the skin and lodge there (Figure 2.16). Complement will then be activated Type III: Immune complex-mediated reactions and inflammatory cells will injure the vessels as in the Antigen may combine with antibodies near vital tissues so Arthus reaction. This causes oedema and the extravasa- that the ensuing inflammatory response damages them. tion of red blood cells (e.g. the palpable purpura that char- When an antigen arrives in the dermis, for example after acterizes vasculitis; see Chapter 8). a bite or an injection, it may combine with appropriate antibodies on the walls of blood vessels. Complement is Type IV: Cell-mediated immune reactions activated, and polymorphonuclear leucocytes are brought As the name implies, these are mediated by lympho- to the area (an Arthus reaction). Degranulation of poly- cytes rather than by antibodies. Cell-mediated immune IgG Antigen I Formation of circulating immune complexes. Circulating immune complex Venule Neutrophil wall Complement C II Immune complex lodges on vessel wall. Complement fixes to C complex and is activated, releasing C5a and C3b. Complement activation C3b C5a Chemotactic Opsonin factor Vessel damage III Neutrophils are attracted to site, and lysosomal enzymes, liberated by degranulating neutrophils, damage vessel walls. Vessel necrosis with Figure 2.16 Immune complex-mediated extravasation of RBC vasculitis (type III reaction). 26 Chapter 2 reactions are important in granulomas, delayed hyper- to self-proteins – activate the innate immune system sensitivity reactions and allergic contact dermatitis. They via Toll-like receptors or the inflammosome in ker- probably also play a part in some photosensitive disorders, atinocytes. Cytokines such as IL-1β, IL-18 or TNF are in protecting against cancer and in mediating delayed released by activated keratinocytes, and this helps steer reactions to insect bites. the response of the skin resident dendritic cells. Among During the elicitation phase, most protein and chem- the keratinocytes are Langerhans cells, highly specialized ical antigens entering the skin are processed by antigen- intraepidermal macrophages with tentacles that inter- presenting cells such as macrophages and Langerhans twine amongst the keratinocytes, providing a net (Fig- cells (Figure 2.17) and then interact with sensitized lym- ure 2.7) to ‘catch’ antigens falling down on them from the phocytes. The lymphocytes are stimulated to enlarge, surface, such as chemicals or the antigens of microbes or divide and to secrete cytokines that can injure tissues tumours. Dermal dendritic cells perform a similar role directly and kill cells or microbes. below the basement membrane. During the initial induc- tion phase, the antigen is trapped by these dendritic cells then leave the skin and migrates to the regional lymph Allergic contact dermatitis node. The Langerhans cells must retract their dendrites Induction (sensitization) phase (Figure 2.17) and ‘swim upstream’ from the prickle cell layer of the When the epidermal barrier is breached, the immune epidermis towards the basement membrane, against the system provides the second line of defence. Antigens – ‘flow’ of keratinocytes generated by the epidermal basal either in isolation (e.g. nickel), or bound as haptens cells. Entering the lymphatic system, the dendritic cells Langerhans Epidermis I cell First exposure to antigen. Antigen trapped on membranes of Langerhans cells and dermal dendritic cells. Dermis II Dermal Afferent dendritic Langerhans and dermal lymphatic cell dendritic cells migrate to afferent lymphatic, and CD4 +ve naive process antigen intracellularly. T-lymphocyte III Dendritic cell re-expresses processed antigen on surface. Interaction with naive T cell. Production of clone of primed (memory) T cells (CD4, Lymph CD45 +ve). node IV Efferent Memory T cells pass into lymphatic general circulation via efferent lymphatic and thoracic duct. Circulation Figure 2.17 Induction phase of allergic contact dermatitis (type IV) reaction.  The Function and Structure of the Skin 27 process the antigens they are carrying and by the time phocyte divides many times. This continuing division they reache the regional lymph re-express them on their depends upon the persistence of antigen (and the antigen- surface in conjunction with MHC Class II molecules. In presenting cells that contain it) and the T-cell growth fac- the node, the dendritic cells mingles with crowds of lym- tor IL-2. Eventually, a whole cadre of memory T cells is phocytes, and are quite likely to find a T cell with just the available to return to the skin to attack the antigen that right T-cell receptor to bind its now processed antigen. stimulated their proliferation. Helper (CD4+ ) T lymphocytes recognize antigen only in CD4+ , CD45+ memory T lymphocytes then leave the the presence of cells bearing MHC Class II antigens, such node and circulate via lymphatic vessels, the thoracic as Langerhans cells and dermal dendritic cells. The inter- duct and blood. They return to the skin aided by hom- actions between surface molecules on a CD4+ T cell and ing molecules (cutaneous lymphocyte antigen, CLA) on a Langerhans cell are shown in Figure 2.12. To activate, their surfaces that guide their trip so that they preferen- the T lymphocyte must also bind itself tightly to certain tially enter the dermis. In the absence of antigen, they accessory molecules, also called co-stimulatory molecules. merely pass through it, and again enter the lymphatic ves- If these are not engaged, then the immune response does sels to return and recirculate. These cells are sentinel cells not occur. (Figure 2.18), alert for their own special antigens. They When a T cell interacts with an antigen-presenting cell accumulate in the skin if the host again encounters the carrying an antigen to which it can react, the T lym- antigen that initially stimulated their production. This Antigen Epidermis I On further exposure to same antigen , antigen is trapped by epidermal Langerhans cells and dermal dendritic cells, processed intracellularly and re-expressed on their surface. Dermis II Antigen is recognized by Sentinel sentinel (memory) lympho- lymphocyte cytes (CD4, CD45 +ve). Interaction between antigen, dendritic cells and T lympho- cytes leads to cytokine release. III Cytokines γ-Interferon activates endothelium to capture IFN-γ IL-8 IL-2 lymphocytes in blood vessel. Chemokines attracts lymphocytes into dermis (chemotaxis). IL-2