Histology of the Blood, Immune & Lymphoid System (PDF)

 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) It is a specialized type of connective tissue that composed of: 1. The plasma: represents the matrix. 2. The formed blood elements: represent the cells. Blood has no fibers. The formed blood elements: 1- Red b...

 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) It is a specialized type of connective tissue that composed of: 1. The plasma: represents the matrix. 2. The formed blood elements: represent the cells. Blood has no fibers. The formed blood elements: 1- Red blood corpuscles (RBCs). 2- White blood cells (WBCs) or leukocytes including:  Granular leukocytes: neutrophils, eosinophils and basophils.  Agranular leukocytes: lymphocytes and monocytes. 3- Blood platelets. Preparation of a blood smear: 1-Giemsa's stain (methylene blue+ eosin) is used which differentiate blood cells by their nuclei and cytoplasmic granules: basophilic (violet) eosinophilic (pink) azurophilic (red purple) 2-Blood count: It is the average number of a formed blood element per cubic millimeter blood.  RBCs count.  Total leukocytic count.  Platelets count.  Differential leukocytic count: it is the percentage of each type of leukocytes relative to the total number of WBCs. Red Blood Corpuscles (RBCs) Average RBCs count:  In males: 5-5.5 millions/mm3.  In females:4.5-5 millions/mm3. Life span of RBCs:  It is 100-120 days.  Old & deformed RBCs are phagocytosed by the macrophages in the spleen & liver. Function of Red blood corpuscles: Contain hemoglobin for gas exchange. Blood & Immune system Module Page 1  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) LM: Shape: eosinophilic biconcave discs. Diameter: 7.2-7.8µm (=the size of the smallest blood capillaries). Nucleus: absent. Cytoplasm: hemoglobin occupies about 33% of the corpuscular volume & is more concentrated at the periphery. EM: Shape: membranous electron dense biconcave discs. Cell organelles: no nucleus& no organelles. RBCs Membrane: 1- From outside: It is covered by a well-developed glycocalyx which is responsible for the blood grouping. 2- From inside: It is supported by a well-developed cytoskeleton, which is responsible for the flexibility of RBCs.  It is formed of microfilaments (actin &spectrin) that are attached to a peripheral membrane protein (ankyrin). How do the RBCs adapt to their function? 1- The biconcave discoid shape increases the surface area of the RBCs by 20-30% more than if they had a spherical shape. 2- The lack of a nucleus and organelles allows more space in the cytoplasm for carrying the maximal amount of hemoglobin. 3- The concentration of hemoglobin at the periphery of the cytoplasm facilitates its binding to oxygen and carbon dioxide. 4- The structure of the cytoskeleton helps the RBCs to be squeezed through the narrowest blood capillaries without being damaged. 5- The selective permeability of the cell membrane to oxygen and carbon dioxide to enhance their diffusion rates during gas exchange. Myeloid tissue Sites: 1- The central bone marrow cavity in young long bones. 2- The marrow cavities of cancellous bones. Types of bone marrow: 1- Red bone marrow: active in hemopoiesis. 2- Yellow bone marrow: Blood & Immune system Module Page 2  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil)  inactive bone marrow in which the blood formation has stopped.  Its yellow color is due to the presence of fat cells.  It can revert to the red type in hemorrhage & anemia. Structure of myeloid tissue: I-The stroma: A. Reticular fibers: support the myeloid cells & blood sinusoids. B. Stromal cells: the fixed cell population of the bone marrow. They include: 1- Reticular cells: have 3 functions:  Produce the reticular fibers.  Secrete growth factors that stimulate the hemopoietic stem cells.  In yellow bone marrow, these cells are changed to fat cells. 2- Macrophages:  Phagocytosis of aged RBCs & the malformed blood elements.  Storage of iron for reuse. 3- Fat cells: One of the largest cells in bone marrow. II-The blood sinusoids:  Wide & irregular blood vessel.  Lined by fenestrated endothelial cells. III-The myeloid cells: The free cell population of the bone marrow. They include: Multipotent SC CFU-LY CFU-E CFU-B 1- Pluripotent hemopoietic stem cells (PHSCs): CFU-GEMM Shape: small cells with pale basophilic cytoplasm and pale nuclei. Function: they divide, give rise to daughter cells (half remains as a reserve of the HPSCs while the other half develops into multipotent hemopoietic stem cells). 2- Multipotent hemopoietic stem cells (MHSCs): They give rise to cell colonies for each of the formed blood elements. 3- Colony forming units (CFUs): 2 types: A. Colony forming unit- lymphocyte (CFU- Ly): give rise to lymphocytes. B. Colony forming unit- granulocyte, erythrocyte, monocyte, megakaryocyte (CFU- GEMM): give rise to unipotent colony: CFU-Erythrocyte (CFU-E). CFU-Megakaryocyte (CFU-Meg). CFU-monocyte (CFU-M) CFU-Granulocyte: give rise to CFU-Neutrophil. Blood & Immune system Module Page 3  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) CFU-Eosinophil (CFU-Eo). CFU-Basophil (CFU-Ba). These cells are larger with more basophilic cytoplasm (abundant ribosomes for synthesis of the specific products of each blood element). Erythropoiesis Definition: It is the process of formation of RBCs. Duration: 7 days. 1- PHSCs. 2- MHSCs. 3- Colony forming unit- erythrocytes (CFU-E). 4- Proerythroblasts:  They are the first recognizable erythrocyte precursor.  They are large cells with basophilic cytoplasm. 5- Basophilic erythroblasts:  Hemoglobin synthesis is most active.  The cytoplasm is strongly basophilic due to abundant polysomes. 6- Polychromatophilic erythroblasts:  Hemoglobin is accumulating in large amount.  Ribosomes are still abundant.  The cytoplasm shows eosinophilic areas alternating with basophilic spots.  They are the last stage in which the cells undergo cell divisions. The next phases involve morphological changes of the erythroblasts. 7- Orthochromatophilic erythroblasts (normoblasts):  The synthesis of hemoglobin is completed.  Ribosomes are much reduced in number.  The nucleus becomes small and condensed, pushed towards the periphery, extruded from the cell and phagocytosed by macrophages. 8- Reticulocytes:  Non-nucleated.  They differ from mature RBCs in that: Slightly larger. Their cytoplasm contains remnants of ribosomes and other organelles which on staining with cresyl blue stain gives a reticulate pattern.  They represent about 1% of all RBCs in a normal blood film (an increase in their percentage indicates an increase in the rate of erythropoiesis as in case of anemia or hemorrhage). 9- Mature RBCs. Blood & Immune system Module Page 4 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) platelets Normal count: 150,000-400,000/ mm3. Structure: LM:  Shape: biconvex discs.  Diameter:2-3µm.  Nucleus: absent.  The granulomere: granular central core contains azurophilic granules.  The hyalomere: pale basophilic area at the periphery. Function of Blood Platelets: Blood platelets are concerned with the process of thrombus formation (blood clotting) in response to any vascular endothelial injury to prevent excessive blood loss. How do the blood platelets adapt to their function? EM:  Shape: irregular outline (when activated) due to presence of pseudopodia  motility.  The platelet membrane: is covered by a well-developed glycocalyx helps in platelets adhesion & aggregation.  The hyalomere contains: 1- Actin and myosin filaments  contraction of the platelet for motility and clot retraction. 2- A circumferential bundle of microtubules  maintains the discoid shape of platelets. 3- Two systems of membranes  regulation of the intraplatelet calcium concentration and thus the control of platelet degranulation. These are: The open canalicular system: small canaliculi that open on the platelet surface. The dense tubular system: remnants of the endoplasmic reticulum of the platelets mother cell (the megakaryocyte).  The granulomere contains: 1- Few ribosomes, mitochondria, and glycogen particles. 2- Three types of membrane bounded granules which are released by exocytosis during the process of thrombus formation. These are: Alpha granules: abundant, large granules which contain fibrinogen and other coagulation factors. Delta granules: less abundant, medium- sized granules which contain histamine, serotonin, ATP, ADP. Blood & Immune system Module Page 5 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Lambda granules (lysosomes): few small dense granules, contain lysosomal hydrolytic enzymes. Thrombopoiesis Definition: It is the formation of blood platelets. The release of mature platelets from the bone marrow into the bloodstream takes about 10 days 1- PHSCs. 2- MHSCs. 3- Colony forming unit-GEMM. 4- Colony forming unit- megakaryocyte (CFU- Meg). 5- Megakaryoblasts. 6- Megakaryocytes. Megakaryocyte LM: Shape: Giant cells (50-150µm). Nucleus: large, lobulated containing the polyploid number of the ordinary DNA sets. Cytoplasm: basophilic contains many organelles together with the alpha, delta, and lambda granules. EM: the cytoplasm shows membranous demarcation lines surrounding groups of granules (lines of cleavage of the megakaryocytes into about 1200 platelets per megakaryocyte). Megakaryocytes are located near the bone marrow sinusoids; they extend their processes to release the platelets in bloodstream. Megakaryoblast Blood & Immune system Module Page 6  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) White Blood Cells (Leukocytes) 3 Normal count: 4000-10,000 / mm blood. Types:  Granular leukocytes (granulocytes) which have specific granules in their cytoplasm: 1- Neutrophils. 2- Eosinophils. 3- Basophils.  Agranular leukocytes (agranulocytes) which lack specific granules in their cytoplasm: 1- Monocytes. 2- Lymphocytes. Function of white blood cells: They do not function within the bloodstream but, they circulate in the blood stream and when they receive a stimulus, they leave the bloodstream to enter the connective tissue and perform an immune function. Granular Leukocytes I-Neutrophils (polymorphonuclear leukocytes): Why are they named neutrophils? because their granules have a neutral staining affinity to the stains in blood film. Why are they named PNLs? Because the cells may contain from 2-5 lobes of nuclei according to its age. Differential count: 60-70% of the total leukocytic count. LM:  Cytoplasm: lightly stained.  Nucleus: multilobed formed of 2-5 segments that are connected with a thin chromatin thread.  In females with a normal karyotyping (44+XX chromosomes), one of the X- chromosomes is condensed and attached to the nucleus forming a drumstick appearance (Barr body). EM: (How do the neutrophils adapt to its function?)  Shape: have an irregular outline due to extension of pseudopodia  motility& phagocytosis  Cytoplasm: contains two types of granules  phagocytosis: 1- Small specific granules: contain protease enzymes e.g., collagenase. 2- Larger azurophilic granules (non-specific): containing lysosomal hydrolytic enzymes and the antibacterial defensins. Blood & Immune system Module Page 7  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Function of the neutrophils: (The first line of defense against any invading micro-organism) Killing strategy 1- Go to battlefield: they leave the circulation and migrate to the site of action in connective tissue. 2- Identify the enemy. 3- Phagocytosis (so they are called microphage). During the inflammatory reaction, neutrophils themselves are damaged by the bacterial toxins causing their death and they are called pus cells. II-Eosinophils: Why are they called eosinophils? because their granules have a strong affinity to the eosin component of the stains. Differential count:1-4% of the total leukocytic count. LM:  Nucleus: dense, bilobed, C-shaped.  Cytoplasm: contains large, shiny reddish granules. EM: (How do the eosinophils adapt to its function?)  The cytoplasm contains two types of granules: 1- Large specific granules: with electron dense crystalloid core), containing major basic protein and histaminase enzyme. 2- Small nonspecific azurophilic granules: containing lysosomal hydrolytic enzymes. Functions of the eosinophils: 1- They are responsible for regulation of allergic reactions (secrete histaminase). 2- In parasitic infections: killing of parasites. Killing Strategy 1- Go to battlefield: They circulate in the blood and then migrate to the connective tissue (especially mucosa of digestive, respiratory, genital, urinary & skin). 2- Identify the enemy. 3- The proteins in their specific granules act on the surface of the parasitic worm  forming pores  facilitating entry of lethal substances in the granules to the parasite  death of parasite. III-Basophils = mast cell of the blood: Why are they called basophils? Because their granules have strong affinity to the basic component of the stains. Why are they called mast cell of the blood? Because they secrete heparin and histamine like the connective tissue mast cells. Blood & Immune system Module Page 8 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Differential count: 0.5- 1 % of the total leukocytic count. LM:  Nucleus: bilobed, S-shaped, obscured by the abundant basophilic granules in the cytoplasm.  Cytoplasm: contains abundant basophilic granules  Basophils show a metachromasia when stained with toluidine blue (the granules appear red instead of blue because of their content of heparin). EM: (How do the basophils adapt to its function?)  The cytoplasm contains two types of granules: 1- Large specific granules: containing heparin and histamine. 2- Small nonspecific azurophilic granules: containing lysosomal enzymes. Function of basophils: Basophils are responsible for the release of histamine and heparin in systemic hypersensetivity allergic reaction. Cell type Neutrophil Eosinophil Basophil Differential 60-70% 1-4% 0.5-1% leukocytic count LM Nucleus Multilobed, Barr body in females. Bilobed C shaped. Bilobed S shaped. Cytoplasm Lightly stained. Shiny reddish Basophilic granules granules. obscuring the nucleus. Metachromatic staining. EM Small=collagenase Large=major basic Large= histamine, Specific granules Large protein, histaminase. heparin. Azurophilic Small Small granules Functions First line of defense against any 1-Activated in parasitic Involved in invading microorganism; microphage. infection. hypersensitivity reaction. 2- down regulate allergic reaction. Blood & Immune system Module Page 9  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Granulopoiesis Promyelocytes:  Cytoplasm is more basophilic: as active protein synthesis occurs and the production of azurophilic granules begins. Myelocytes:  The specific granules for each type of the granulocytes start to appear in addition to the azurophilic granules.  The cytoplasmic affinity changes: it shows decreased basophilia and increased eosinophilia in neutrophils and eosinophils, whereas basophilia dominates in basophils.  This stage represents the last stage in which the cell can undergoes cell division, the next stages represent only morphological changes. Metamyelocytes:  The specific granules are more abundant.  The cytoplasm shows the staining affinity characteristic for each of the granular leukocytes.  Specifically, the neutrophils present by the end of these stage with an additional phase known as the band cells: 1- immature neutrophils 2- with a band-shaped nucleus. 3- They are 0-3% in the blood. This percentage increases if there is an acute infection. Mature granulocytes:  The nucleus becomes segmented in the pattern characteristic for each type of the granulocytes. Blood & Immune system Module Page 10  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Agranular Leukocytes I-Monocyte: Why are they called monocyte? Because they have a single, non-segmented nucleus. Differential count: 3-8 % of the total leukocytic count. LM:  Size: the largest leukocytes.  Nucleus: large, eccentric and kidney shaped or horseshoe.  Cytoplasm: abundant, pale basophilic and finely granular. EM: (How does the monocyte adapt to its function?)  Shape: has an irregular outline due to extension of pseudopodia  motility& phagocytosis  Cytoplasm: contains small dense azurophilic granules containing lysosomal hydrolytic enzymes. Function of monocytes: Monocytes circulate in the blood and then migrate to the different body tissues where they differentiate to specific tissue phagocytic cells that constitute the mononuclear phagocytic system:  CT: Macrophages.  Bone: Osteoclasts.  CNS: Microglia.  Liver: Von Kupffer cells.  In chronic infections: macrophages fuse with one another forming multinucleated giant cells.  Phagocytosis of foreign antigens. Killing Strategy: 1- Go to battlefield. 2- Differentiate into specific phagocytic cell. 3- Identify the enemy. 4- Phagocytosis.  Antigen presenting cells: they process antigens and present them to T lymphocytes. Monopoiesis 1- PHSCs. 2- MHSCs. 3- CFU-GEMM. 4- CFU-M. 5- Monoblast. Blood & Immune system Module Page 11  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) 6- Promonocyte: it has large indented nucleus & basophilic cytoplasm. It is the last cell capable of division. 7- Monocyte. II-Lymphocytes: Why are they called lymphocyte? Because they are the main cell population of the lymphoid organs. Differential count: 20-30 %of the total leukocytic count. Life span: for several months. The memory cells can live for several years. LM: Shape& size: rounded cells with variable diameter. Nucleus: large, dense and slightly indented. Cytoplasm: narrow rim of basophilic cytoplasm. EM:  Cytoplasm: small dense azurophilic granules containing lysosomal enzymes.  The cell coat: surface molecules acting as: A- Receptors for the antigen. B- The cluster of differentiation (CD markers)  classifying the lymphocytes into subtypes according to their functions. Types of lymphocytes: According to their size:  Large lymphocytes.  Medium sized lymphocytes.  Small lymphocytes: represents most of the circulating lymphocytes in blood. They represent the mature form of lymphocytes. According to the function: 1- T lymphocytes: small lymphocytes,60-80% of circulating lymphocytes. 2- B lymphocytes: small lymphocytes, 20-30% of circulating lymphocytes. Blood & Immune system Module Page 12  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) 3- Natural Killer cells: 5-10% of circulating lymphocytes. Lymphopoiesis 8- PHSCs. 9- MHSCs. CFU-LY 10- Colony forming unit- lymphocyte (CFU- Ly):  Some daughters of these cells: migrate during fetal and CFU-B CFU-T early postnatal life to the cortex of the thymus where they differentiate into colony forming unit -T lymphocyte.  The rest: remains in the bone marrow and differentiate to colony forming unit- B lymphocyte. 11- Colony forming unit- T lymphocyte: They divide in the thymus giving rise to T- lymphoblasts. T lymphoblasts divide to give immunocompetent T- lymphocytes (expressing specific surface receptors). Colony forming unit- B lymphocyte: They divide in the bone marrow giving rise to B- lymphoblasts. They divide to give immunocompetent B- lymphocytes (expressing specific surface receptors). The Immune System There are three main lines of defense mechanisms: I-Surface protective mechanisms: 1- Skin: constitutes a barrier to the entry of microorganisms. 2- The mucous surfaces: as the conjunctiva and oral cavity are protected by antibacterial lysozyme secreted in tears and saliva. 3- The acidic environment in the stomach and vagina: inhibits the growth of pathogens. 4- The ciliated respiratory epithelium: is protected by a layer of mucus which is continuously removed with the trapped particles. The first line of defense. When these defenses fail, the two other types of defense mechanisms are activated. II-Non-specific immune response (innate immune system): 1- The complement system: mediates a wide range of inflammatory processes. 2- The phagocytic cells. 3- Natural killer cells. Blood & Immune system Module Page 13  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) III-Specific immune response: This depends on the recognition of specific antigens by specific surface receptors on the immune cells. There are two types of specific immune responses:  Humoral immune response.  Cell mediated immune response. Cells of the immune system: All lymphocytes are similar in either light or electron microscopic pictures. They can be distinguished by immunohistochemical methods into 3 different types according to their surface receptors or surface markers:  T-Lymphocyte: Origin: HPSCs in bone marrow and then their daughter cells migrate to thymus. Development: in the thymus where they proliferate, mature, and acquire T cell receptors (TCR) on their surface specific for certain antigen. Site of action: T cells migrate to the peripheral lymphoid tissues (lymph nodes, spleen, and Peyer’s patches in the ileum) where they come into contact with the antigens When T cells are activated by the specific antigen, they proliferate and differentiate into four functional subtypes:  Cytotoxic T cells (killer cells): They recognize foreign cells and destroy them by release of perforins (create holes in the membrane of the target cells) and granzymes (induce apoptosis of the target cells).  T helper cells: they produce lymphokines that activate B and cytotoxic T lymphocytes.  T regulatory (suppressor cells): they decrease the activities of other T and B lymphocytes in order to suppress immune response.  T memory cells: long lived cells responding more rapidly and strongly upon subsequent exposure to the same antigen.  B-lymphocytes: Origin & development: the bone marrow where they proliferate, mature and acquire surface immunoglobulins receptors specific for certain antigen recognition. Site of action: B cells migrate to the peripheral lymphoid tissues. Blood & Immune system Module Page 14  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) where they come into contact with antigens. When they are activated by the specific antigen, they proliferate and differentiate into: Plasma cells: secrete antibodies (humoral immune response). B memory cells: long lived cells responding more rapidly and strongly upon subsequent exposure to the same antigen.  Natural killer cells: Origin: in bone marrow, from the same precursors of T and B cells. They lack the surface markers characteristic of B and T cells. They act nonspecifically to kill virally infected cells and malignant cells. Origin: the bone marrow. They recognize, phagocytose and present the foreign antigens to T lymphocytes. They include:  Macrophages.  Langerhans cells of the skin.  B lymphocytes.  Epithelial reticular cells of the thymus. Blood & Immune system Module Page 15  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Lymphoid Organs 1- Primary (central) lymphoid organs: These are the sites for development & maturation of lymphocytes. They include:  Thymus.  Bone marrow. 2- Secondary (peripheral) lymphoid tissues: These are the sites where lymphoid cells react with foreign antigens to elicit an immunological response. They include:  Lymph nodes.  Spleen.  Lymphoid nodules in the tonsils, Peyer’s patches in the ileum and appendix. Site: in the superior mediastinum behind the sternum. It is active in childhood, but it progressively atrophies at puberty where most of the lymphoid tissue is replaced by adipose tissue. Histological Structure of Thymus: Stroma:  Connective tissue capsule.  Connective tissue septa (trabeculae) extend into the parenchyma  divide it into incomplete lobules. Parenchyma: each lobule is divided into:  A peripheral dark cortex.  An inner pale medulla. I-The thymic cortex: The cortex is darkly stained because of densely packed cells. It is composed of: 1. An extensive population of thymocytes (developing T lymphocytes). 2. Epithelial reticular cells: provide a supporting framework (cytoreticulum) for the developing T lymphocytes. 3. Macrophages. 5Nos: lymphoid nodules, B lymphocytes, plasma cells, reticular fibers and reticular cells. II- Thymic medulla: The medulla is pale because of its loosely- packed cells. The medulla of adjacent lobules is connected together due to the incomplete lobulation of the gland. The medulla contains: 1. A small number of mature small lymphocytes. 2. Few macrophages. 3. Epithelial reticular cells: large in number, form the cytoreticulum of the medulla. 4. Hassall’s corpuscles. Blood & Immune system Module Page 16  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Hassall’s corpuscles: Site: in the medulla of the thymus. Structure: Concentric whorls of eosinophilic flattened epithelial reticular cells. The epithelial reticular cells in the center of the corpuscle are filled with keratohyaline granules and cytokeratin filaments and they may calcify. Function: is still not fully understood. Vascular supply of the thymus: The cortex is supplied by looped capillaries that extend from the arterioles at the cortico- medullary junction. The lymphocytes that reach maturity  enter the circulation through the post capillary venules  migrate to the thymic-dependent areas of secondary lymphoid organs  homing and activated. The Blood-Thymus Barrier: Definition: It is a barrier between the developing T lymphocytes and the blood in the cortical capillaries. Function: this barrier prevents the foreign antigens from reaching the thymic cortex where T lymphocytes are still in state of maturation. Components: Continuous (non-fenestrated) endothelial cells of the blood capillaries joined together by occluding junctions. Basal lamina of the endothelial cells. The pericytes surrounding the capillary wall. Macrophages in the perivascular connective tissue. Basal lamina of the epithelial reticular cells. Epithelial reticular cells joined together by occluding junctions. Functions of epithelial reticular cells: 1- Form the cytoreticulum of the cortex& medulla. 2- Form Hassall’s corpuscles. 3- Form the blood thymic barrier. 4- Secrete several growth factors that stimulate T cell proliferation and differentiation e.g., thymopoietin. Site: They are located along the course of the lymphatic vessels. Function: filtration of the lymph. Shape: small kidney shaped bodies (indented at the hilum where arteries enter while veins and efferent lymphatic vessels leave the lymph node). Blood & Immune system Module Page 17  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Histological Structure of lymph node: I- Stroma:  Capsule: Fibroelastic.  Trabeculae:  Perpendicular to capsule,  Divide the cortex into incomplete compartments.  In the medulla: they run in different directions.  Reticular CT: Support the parenchymal cells& lymph sinuses. II-Parenchyma: It is divided into an outer dark cortex and inner pale medulla. A- Cortex: it is divided into two regions: 1- Outer cortex:  It is situated under the capsule.  It consists of the cortical lymphoid nodules* separated by the cortical lymph sinuses. 2- Inner cortex (thymus- dependent zone/the paracortical area): it consists mainly of T lymphocytes which are not arranged as nodules. B-Medulla:  It is formed of dense lymphoid tissue of cells called the medullary cords consisting of densely packed B lymphocytes, plasma cells and macrophages.  These cords branch and anastomose and are separated by medullary lymph sinuses. C-The loose lymphoid tissue: it is formed of loosely scattered lymphocytes, plasma cells and macrophages with lymph sinuses surrounding the dense cortical lymphoid nodules and medullary cords. Lymph circulation in the lymph node: Afferent lymphatic vessels cross the capsule and drain lymph into the subcapsular sinuses  the trabecular sinuses  the medullary sinuses  the efferent lymphatic vessels. Blood & Immune system Module Page 18 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) (*) The lymphoid nodules: Definition: rounded or oval dense lymphoid tissues formed by B lymphocytes & macrophages. Types: Primary lymphoid nodules: which are not activated yet by lymph borne antigen. Secondary lymphoid nodules: When the lymph node is activated by lymph- borne antigens. the B lymphocytes at the center of the nodule develop into lymphoblasts (larger, loosely packed) forming pale central areas called germinal center. The result of germinal center formation is the production of an expanded population of B memory cells & plasma cells. Types of lymphoid nodules Primary lymphoid Secondary lymphoid nodules nodules 1- Structure B lymphocytes & Contain B lymphocytes and macrophages without macrophages + germinal center germinal center. (consists of less closely packed lymphoblasts and plasma cells). 2- Function Not activated by foreign Activated by foreign antigen with production of plasma cells antigen. and B memory cells. Lymphadenopathy Blood & Immune system Module Page 19  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) The spleen is the largest lymphoid organ in the body. Function: filtration of the blood. Structure of the spleen: 1- Stroma: 1- A thin capsule:  Musculo-elastic,  Covered by peritoneum (mesothelium). 2- Trabeculae:  Musculo-elastic,  Arise from the capsule at the hilum, run different directions carrying the blood vessels. 3- Reticular connective tissue:  Support the parenchymal cells and the blood sinusoids. 2- Parenchyma: 1- White pulp (The Malpighian corpuscles):  It consists of lymphoid nodules, scattered in the substance of the spleen.  These nodules are formed of B lymphocytes and macrophages. They may primary or secondary nodules contain germinal centers which give rise to plasma cells.  The nodules are traversed by the central artery (follicular artery) which is eccentric in position.  The areas around the central arteries are called periarterial lymphoid sheaths which are thymic dependent areas consisting of T-lymphocytes. 2- The red pulp: It is formed of blood sinuses (sinusoids), separated from each by cords of cells called splenic cords (Billroth’s cords):  The splenic cords: consist of RBCs, macrophages, lymphocytes, plasma cells and granulocytes.  The blood sinuses: have slits between the endothelial cells, so aged or malformed RBCs are damaged when they pass through these slits and phagocytosed by the macrophages. The marginal zone: Definition: it is the area around the margins of the lymphoid nodules between the red and white pulps. Importance: it is the site where: 1- The circulating T & B lymphocytes enter into their specific location in the splenic parenchyma. 2- Blood borne antigens have their first access to the parenchyma of the spleen. Blood & Immune system Module Page 20 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Splenic Circulation: There are 2 blood circulations within the spleen: 1- Open circulation: occurs in human. Splenic artery  Trabecular arteries  Central artery  send branches to:  White pulp.  Marginal sinuses. Central artery continues into red pulp  Penicillar arterioles  continue as arterial capillaries. Some of the capillaries are surrounded by aggregations of macrophages (sheathed capillaries)  empty into reticular network of the red pulp (exposed to macrophages)  squeezed through the wall of the splenic sinuses  larger veins  trabecular veins  Splenic vein. Advantage: exposes the blood more efficiently to macrophages in red pulp. 2- Closed circulation: in some species. Some of the blood from the sheathed capillaries  splenic sinuses directly. Functions of Spleen: 1- White pulp:  Lymphoid nodules.  Periarterial lymphoid sheath.  Marginal zone (*). 2- Red pulp:  Blood sinusoids.  Splenic cords. I-Immune functions: 1- Antigen presenting cells: for initiation of the immune response. 2- Activation and proliferation of B& T lymphocytes. 3- Production of antibodies against antigen in the blood. The role of the red pulp is primarily blood filtration through: 1- Removal of foreign antigens from the blood. 2- Removal of aged, abnormal, or damaged RBCs and platelets. This function is accomplished by the macrophages in the red pulp. Activation & proliferation of T and B lymphocytes and antibodies production occur in the white pulp. II-Hemopoietic functions: 1- Formation of RBCs in early fetal life. 2- Destruction of aged and damaged RBCs& platelets. 3- Retrieval of iron from RBCs hemoglobin. 4- Storage of blood especially RBCs in some species. Blood & Immune system Module Page 21 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) (*) Marginal Zone: Definition: it is the area around the margins of the lymphoid nodules between the red and white pulps. Importance: it is the site where: 1- The circulating T & B lymphocytes enter into their specific location in the splenic parenchyma. 2- Blood borne antigens have their first access to the parenchyma of the spleen. Contents: 1- Marginal sinuses. 2- B & T lymphocytes. 3- Plasma cells. 4- Interdigitating dendritic cells (Antigen presenting cells). 5- Macrophages. Blood & Immune system Module Page 22 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Organ Thymus Lymph node Spleen Structure Fibroelastic + I-Stroma smooth muscles, covered  Capsule Fibrous Fibroelastic. by peritoneum.  Trabeculae Divide it into incomplete lobules. Arise from capsule, Arise from hilum, run in perpendicular, divide different directions. cortex into compartments, run in different directions in the medulla.  Reticular Absent Present. Present. fibers. II-Parenchyma  Cortex.  Cortex  White pulp 1-T- lymphcytes A- Outer cortex: scattered lymphatic 2-macrophages 1- Lymphoid follicles. nodules contain central 3-epithelial reticular cells. 2- Lymph sinuses artery+ Periarterial  Medulla. B- Inner cortex (thymic lymphoid sheath. 1- Small lymphocytes dependant area) Marginal zone 2-macrophage  Medula 3-epithelial reticular cells 1- Medullary cords  Red pulp 4- Hassall, s corpuscles 2- Medullary sinuses Splenic cords (Billroth  Loose cords) lymphatic +Blood sinuses tissues. Site of development & maturation Filtration of lymph. Filtration of the blood. Function of T lymphocytes. Blood & Immune system Module Page 23 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Definition: organs composed of aggregations of partially encapsulated lymphoid tissue. Site: beneath the epithelium of the initial portion of the digestive and respiratory tracts. Types: the palatine tonsils, pharyngeal tonsil , lingual & tubal tonsils. I-Palatine tonsils: Anatomy: They are located in the lateral walls of the oropharynx. Structure: 1- The anterior and medial surfaces:  Covered by non-keratinized stratified squamous epithelium which dips inside the parenchyma to form 1ry & 2ry crypts.  The lumen of the crypts may contain desquamated epithelial cells and bacteria. 2- Lymphoid nodules: either primary or secondary are present beneath the epithelium and around the crypts. 3- Loose lymphoid tissue: present in between the lymphoid nodules. It consists of lymphocytes, plasma cells, macrophages and neutrophils. 4- The lateral and posterior surfaces:  Covered by a connective tissue capsule separating the tonsil from adjacent organs (acts as a barrier against spreading of infections).  Mucous acini (minor salivary glands): present within the capsule. #NO afferent lymph vessels or blood sinuses are present. II- Lingual tonsils: Site: on the dorsal surface of the posterior third of the tongue. Number: multiple. Structure:  Lymphoid nodules.  Covered by stratified squamous epithelium.  Each tonsil has a single crypt, where ducts of the lingual mucous glands open. Blood & Immune system Module Page 24 Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) III-Pharyngeal tonsil: Site: in the posterosuperior portion of the pharynx(nasopharynx). Number: single. Structure:  Lymphoid nodules.  Covered with pseudostratified columnar ciliated epithelium.  The surface is folded with no crypts. Chronic inflammation of pharyngeal tonsil leads to its hypertrophy (adenoids). IV- Tubal tonsils: Site: around the opening of the Eustachian tube in the lateral wall of the nasopharynx. Number: Two. Structure:  Lymphoid nodules.  Covered by pseudostratified columnar ciliated epithelium.  The epithelium dips inside the parenchyma to form few crypts. Waldeyer ring Types of tonsils Palatine Lingual tonsils Pharyngeal tonsil Tubal tonsils tonsils 1- Number Two. Multiple. Single. Two. 2- Site In the lateral On the dorsal In the posterosuperior Around the opening walls of the surface of the portion of the of the Eustachian oropharynx. posterior third pharynx(nasopharynx). tube in the lateral of the tongue. wall of the nasopharynx 3- Structure  Epithelium Stratified Stratified Pseudostratified Pseudostratified squamous squamous columnar ciliated columnar ciliated epithelium. epithelium. epithelium. epithelium.  Crypts Primary& Each tonsil has a Absent. Few. secondary. single crypt. Blood & Immune system Module Page 25  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Abnormalities Of Blood Elements: Normal blood elements count: Average RBC count: 3  In males: 5-5.5 millions/mm. 3  In females:4.5-5 millions/mm. 3 Normal platelets count: 150,000-400,000/ mm. 3 Normal WBCs count: 4000-10,000 / mm blood. Differential leukocytic count Abnormalities of the RBCs: A- Abnormal Count: 1- Polycythemia: increase in RBCs number. 2- Anaemia: decrease in RBCs number. B- Abnormal Size: 1- Microcytosis: when the RBC diameter is less than 6 microns. 2- Macrocytosis: when the RBC diameter is more than 9 microns. 3- Anisocytosis: when the RBCs have variable sizes. C- Abnormal Shape: 1- Rouleaux formation: when there is a slowing of the blood circulation or stagnation, the RBCs aggregate in rows. 2- Crenation: when RBCs are placed in hypertonic solution, they lose their water content & shrink. The shrinkage is not uniform, so their surfaces show spiny protrusions & the RBCs are called echinocytes. 3- Erythrocytes ghosts: when the RBCs are placed in a hypotonic solution, they imbibe water & swell (spherocytosis). They may burst leaving remnants of RBCs (RBCs ghosts) and they undergo hemolysis. 4- Poikilocytois: when RBCs have variable shapes. Blood & Immune system Module Page 26  Histology of the Blood, Immune & Lymphoid system(By Prof. Dr. Iman Nabil) Abnormal Platelets count: 1- Thrombocythemia: increase in platelets count. 2- Thrombocytopenia: decrease in platelets count. Abnormalities of WBCs: 3 1- Leukocytosis: increase in total WBCs count more than 10.000/mm :  Infection, inflammation & allergy.  Leukemia. 3 2- Leukopenia: decrease in total WBCs count less than 4.000/mm :  Viral infection.  Bone marrow suppression (irradiation& drugs). Abnormal Neutrophil count: 1- Neutrophilia: increase in differential count of the neutrophils:  Acute infection & inflammation (acute appendicitis, acute tonsillitis, abscess…) 2- Neutropenia: decrease in differential count of the neutrophils:  Influenza, measels, tuberculosis... Abnormal Eosinophil count: 1- Eosinophilia: increase in differential count of the eosinophils:  Allergic reaction: urticaria, bronchial asthma, atopic dermatitis...  Parasitic infection: bilharziasis. 2- Eosinopenia: decrease in differential count of the neutrophils:  Prolonged corticosteroid therapy. Abnormal Basophil count Basophilia: increase in differential count of the basophils  infection, allergy… Abnormal Monocyte count: Monocytosis: increase in differential count of the monocytes leukemia, chronic inflammation… Abnormal Lymphocyte count: 1- Lymphocytosis: increase in differential count of the lymphocytes:  Lymphocytic leukemia.  Viral infection (measels, mumps…).  Chronic inflammation: chronic infection especially intracellular (tuberculosis & syphilis), autoimmune diseases and malignancy. 2- Lymphopenia: decrease in differential count of the lymphocytesBone marrow suppression. Blood & Immune system Module Page 27