Immune System Notes PDF

C. Exanthematous rashes: - Enterovirus - Parvovirus B19: - “Slapped-cheek rash” - “Fifth disease” - HHV-6: - Roseola infantum - Measles: - Vaccines preventable - Potentially severe complications (neurological, p...

C. Exanthematous rashes: - Enterovirus - Parvovirus B19: - “Slapped-cheek rash” - “Fifth disease” - HHV-6: - Roseola infantum - Measles: - Vaccines preventable - Potentially severe complications (neurological, pneumonia) - Highly infectious by airborne travel - Rubella: - Vaccine preventable - Congenital rubella syndrome if pregnant women are infected - Joint pain D. Viral haemorrhagic fever: - Ebola virus disease: - Major outbreaks, most cases in Africa - ~ 30,000 cases in recent West African outbreak - HK as a globally connected city is at risk - Other causes: - Severe dengue, Hantavirus hemorrhagic fever renal syndrome, yellow fever virus, Lassa fever, Crimean Congo hemorrhagic fever, Marburg virus E. Common cancers associated with viruses: - Vaccine available against HBV and HPV high-risk genotypes - Drug treatment available for HBV, HCV 8.10.8. OTHER MEDICALLY IMPORTANT VIRUSES A. Monkeypox: - Poxvirus; enzootic in West/ Central Africa in animals (primarily rodents) - Large ongoing international outbreak in 2022 in MSM communities 84  - Clinical presentation: - Skin lesions universal (mostly anogenital area, but can involve trunk/ limbs/ face) - Fever, lethargy, sore throat, lymph node Swelling - Complications: - Proctitis - Bacterial superinfection - Disseminated infection - Treatment/ Prevention: - Smallpox vaccines can be used for pre-exposure and post-exposure prophylaxis B. Arboviruses: - Viruses spread by insects - Many cause rash, some cause encephalitis - HK has two main arboviruses: - Japanese encephalitis - Dengue - Other important arboviruses: - Zika virus, West Nile virus, Yellow Fever virus - Dengue: clinical course and diagnostics: 85 9. IMMUNE DEFENCE 9.1. NON-SPECIFIC DEFENCE 9.1.1. INTRODUCTION TO THE IMMUNE SYSTEM - Immunity: - “Immunis” – “defensive mechanism against diseases” - Immune system can mount an immune response (recognition → defence) - Antigens (infectious microbes, foreign macromolecules) trigger immune responses - Pathogens are antigens that breach the immune system - Cellular location of invading pathogens: - There are 2 types of immunity: - Differ in speed, strength, specificity, and memory 1. Innate (natural) - Macrophages, NK cells - Early, rapid but limited in strength - “Non-specific” 2. Adaptive (acquired) - B, T lymphocytes - Takes time but is powerful - Specific and have memory - Memory in adaptive immunity: 86  9.1.2. SECONDARY IMMUNE RESPONSE - Characteristics of a secondary immune response: - Second or subsequent contact with an antigen - Short lag (1-4 days) and early peak (3-5 days) - Last longer - Stronger (100-1000x more antibodies produced) - Memory B cells - Similar for T cell response - Only thymus (T) dependent antibodies - Occurs mainly in the bone marrow - High antibody affinity: - Affinity maturation due to somatic hyper-mutation of Ig variable region genes - Mainly IgG – due to antibody class switch - Somatic hyper-mutation: - Heavy chain carries V (variable), D (diversity), J (joining) gene segments - Light chain carries V (variable), J (joining) gene segments - There can be mutations in somatic cells that allows for the generation of antibody diversity - If the mutation yields a low affinity antibody, it is deleted - If the mutation yields a high affinity antibody, it is selected and will proliferate to form a clone of antibodies in plasma cells 87 - Isotype switching (antibody class switch): - VDJ in heavy chain locus attached to µ → transcription for IgM → IgM expressing B cell - DNA segment (µ-𝛿-𝛾3) removed → VDJ in heavy chain attached to 𝛾1 instead of µ → transcription for IgG1 → IgG expressing B cell 9.1.3. MORE ON IMMUNE RESPONSE - Immune responses to infections depend on: 1. Type of pathogens: - Extracellular pathogens (bacteria, fungi, parasites) - Intracellular pathogens (bacteria, viruses) 2. Types of immune responses - Humoral responses (B cells) - Cell-mediated responses (T cells) - Immune responses in innate and adaptive immunities: 88  a. Innate immunity: - Protection against infection: - Infected cells secrete Type 1 IFN (𝛼/𝛽) to target cell (infected/ neighbour cells) to induce an antiviral state - Eradication of established infection: - NK cells bind to infected cells to induce apoptosis or cell lysis b. Adaptive: - Protection against infection: - B cells produce antibodies for neutralisation of pathogens and toxins - Eradication of established infection: - CD8+ T cells bind to infected cells to induce apoptosis or cell lysis 9.1.4. HUMORAL RESPONSE - Non-specific: - Complement pathway - Interferons (𝛼/𝛽) - Acute-phase proteins - Specific: - Rely on antibodies (from B cells) - Neutralisation of viruses and toxins - Opsonization: - Bind to pathogens from recognition by other immune cells - Attraction by Fc receptors on phagocytes → easier phagocytosis - C3b from complement system also acts as opsonin - Promote complement-mediated lysis (Classical Pathway) 89  9.1.5. CELL-MEDIATED IMMUNITY - Non-specific, T-independent (early phase): - Phagocytes - Macrophages - Natural killer cells (NK) - Specific, T-dependent: - Cytotoxic T cells (TC): kill infected cells - Helper T cells (TH): help other cells A. TC cells (CD8+): - Cell-mediated cytotoxicity: - Mediated by T cell receptor - Target antigen must be presented by self- antigen (MHC I) - CD8+ helps maintain specificity of TC cells - In a non-specific response, toxic activating ligand binds to NK receptor B. TH cells (CD4+): - Secrete cytokines - Classification: - TH1: IL-2, IFN-𝛾 - Promote type IV (delayed) hypersensitivity - TH2: IL-4, IL-10 - Promote type I (immediate) hypersensitivity - TH17: IL-17 - IL-17 is a pro inflammatory cytokine - Essential in fighting extracellular pathogens and fungi 90 - Function: 1. TH cells release cytokines that activate B cells, enabling antibody production 2. TH aids in NK cell/ macrophage development - HIV infection of CD4+ TH cells: C. Phagocytes: - Macrophages and granulocytes - Process of phagocytosis: - Chemotaxis: directed movement to site of infection - Attachment and uptake allows of digestion - Destruction of pathogen 91 92 - Effector mechanisms for phagocyte-mediated killing of microorganisms: 1. Reactive Oxygen Intermediates (ROIs) - Most common - E.g. Superoxide anion (toxic) 2. Reactive Nitrogen Intermediates (RNIs) - E.g. Nitric oxides 3. Other mediators - E.g. Lysozyme, prostaglandins 4. Cytokines - E.g. IL-1, IL-6, TNF, IFN-𝛾 - T-dependent macrophage activation: D. Interferons and TNFs: - Interferons: - Cytokines play a role in interaction of inflammatory cells - Classification: 93 - TNF 𝛼/𝛽: - Allow bacteria to remain dormant in cell (contain but not deactivate) - Positive feedback loop E. Natural killer cells: - CD16+ CD56dim/neg: cytotoxic NK cells (90%) - CD16- CD56bright: regulatory or cytokine secreting NK cells (10%) 9.1.6. MUCOSAL IMMUNITY - Background: - Most pathogens come through respiratory tract - Mucous acts as the first line of defence - Mechanism: - Antigen is picked up by M cells and passed to dendritic cells - Dendritic cell moves to (mesenteric) lymph node - Antigen presentation leads to differentiation and proliferation of IgA effector B cells - IgA plasma cells formed, secreting IgA dimers - Clinical relevance: - E.g. Breast milk goes to intestinal of baby → increased protection against infections 94  9.1.7. IMMUNODEFICIENCY - Classification: - Primary immunodeficiency: - Intrinsic - Mostly genetic linked - E.g. SCID – lack of T, B cells - Secondary immunodeficiency: - Extrinsic – acquired - Infections, e.g. AIDS – HIV infect TH cells - Drugs, irradiation, malnutrition - Consequences of immunodeficiency: 1. Recurrent (pyogenic) infections: - Defects in immunoglobulins - Defects in complements - Defects in phagocytes - Pathogens – encapsulated bacteria: - Confer advantage in invasion - Must be handled by immunoglobulins, complements and phagocytes at specific binding sites 2. Opportunistic infections: - Opportunistic pathogen: Microorganism that causes infectious disease only in individuals with compromised host defence mechanisms - Defective CMI (defects in T cells) - Pathogens: common viruses, yeast 9.1.8. ANTI-TUMOUR IMMUNITY - Tumour pathogenesis: - Genetic mutations: inherited or acquired - Immunodeficiency (Kaposi’s sarcoma, Burkitt’s lymphoma, etc.) - Virus-induced - Others (chemical carcinogens, radiation, etc.) - Evidence of anti-tumour immunity: - Neonatal or old age > Adults - Immune cells infiltrate in tumours - Spontaneous regression of tumours - Postmortem: Tumours > clinically diagnosed - Graft versus leukaemia (GVL) response → possibility of tumour regression 95 - Treatment by CAR T-cell therapy: - Immune escape mechanisms of tumours: - Tumour surveillance theory - Tumour cells sneak through - Lack of molecules important in immunity - Tumour-derived factors suppress immunity - E.g. Immune checkpoint inhibitors: 96 9.2. NON-SPECIFIC DEFENCE 9.2.1. INNATE IMMUNITY - Major defence mechanisms: 1. Defence barriers 2. Cellular (phagocytes and NK cells) 3. Humoral (soluble factors) 1. Defence barriers: - Anatomical barriers: - Skin, mucosa - Sweat/ Sebaceous glands: secretions inhibit growth of bacteria - Defensin: antimicrobial peptides expressed by leukocytes and epithelial cells - Chemical: - Lysozyme: found in tears and saliva - Pepsin: secreted by gastric glands, involved in breakdown of bacterial cell wall - Microbiological: - Normal flora - Microbiota (new): - For development of immunity and metabolic/CNS diseases 2. Cellular mechanisms: - Phagocytes: - Discovered by Elie Metchnikoff (1883) - Includes macrophages and neutrophils (polymorphonuclear leukocyte) - Chemotaxis → Adherence → Ingestion → Digestion - Phagosome formed will fuse with lysosome and digested - Antigens displayed on cell surface during antigen presentation to activate T cells - Mononuclear phagocytic system: - A family of cells comprising bone marrow progenitors, blood monocytes and tissue macrophages - Changed colours of liver/ spleen (increase in number of macrophages in regions) - In brain (microglia), lungs, liver, kidney, synovial membrane (synovial A cells) - Phagocytosis in erythrocyte clearance: - 1 million RBCs are destroyed every second - Due to oxidative damage and cellular senescence (ageing process) - Clearance of RBCs is enhanced by the loss of RBC membrane constituents such as CD47 and sialic acids - When these constituents are present on RBC at normal levels, they elicit a negative signal to prevent the phagocytosis 97  - Chronic granulomatous disease: - Rare (1 in 106) - Early-onset with severe infections - Defects in neutrophil killing - Mechanism of oxygen metabolites mediated neutrophil killing is defective (no generation of H2O2) - Patients lacking cytochrome b558, unable to generate a respiratory burst - Macrophage-activation syndrome (MAS): - Cytokine storm – severe complication of several chronic rheumatic diseases of childhood (or severe virus infections) - Uncontrolled activation and proliferation of macrophages 3. Humoral (soluble) factors - Produced during the early stage of immune response - Involves: 1) Acute phase proteins - Synthesised normally by liver cells upon microbe stimulation - Increase markedly in serum during infection - Opsonins (e.g. c-reactive proteins, amyloid proteins): bind to surface of invading pathogens to make them more detectable → enhance phagocytosis (opsonization) - Steps: - Bacteria induce macrophages to produce IL-6, which acts on hepatocytes to induce synthesis of acute-phase proteins - C-reactive protein binds phosphorylcholine on bacterial surfaces, acting as an opsonin, and also activating complement - Mannan-binding lectin binds mannose residue on bacterial surfaces, acting as an opsonin, and also activating complement 2) Complements - Consist of at least 30 different proteins - Make up 5% of serum proteins in the bloodstream of vertebrates - Activated by microorganisms or antibodies - Classical pathways, alternative pathways - Complement activation: - By Ab/Ag or microorganisms - Activated C3 produces fragmented C3a and C3b - C3b responsible for cascade of activation (C5, C6, C7, C8, C9) - C5-C9 forms tunnel-like structure (MAC) that inserts into the membrane of bacteria - Leads to leakage of cellular components from bacteria 98  - Functions: 1) Cytolysis/ Lysis – destruction of invading pathogens 2) Chemotaxis – attraction of phagocytes to inflammatory site 3) Enhance phagocytosis by opsonization - Deficiencies of complements: - E.g. patients with C3 deficiency suffer from frequent severe bacterial infection - Associated with an increased risk of infection and immune complex disease such as SLE 3) Interferons - A class of antiviral proteins produced by certain cells - Interferon 𝛼 – produced by virus-infected leukocytes - Interferon 𝛽 – produced by virus-infected fibroblasts - Interferon 𝛾 – produced by T lymphocytes - Enhance adaptive immune response - Killing of infected cells by cytotoxic T cells 99  - Natural killer cells: - Large granular lymphocytes, constitute 5-10% of lymphocytes in human peripheral blood - CD16+ CD56+ CD3- - Cytotoxicity – determined by the integration of signals from inhibitory and activation receptors - Potential use in tumour immunology through immune surveillance - Therapies with interferons: a. IFN-𝛼 (Roferon) has been used for the treatment of hepatitis B and C; for cancer therapy; (SARS) b. IFN-b has emerged as the first drug capable of improving autoimmune neurologic diseases such as multiple sclerosis 9.3. SPECIFIC DEFENCE 9.3.1. BODY’S DEFENCE AGAINST INFECTION 100 - Overview: - Specific immunity = Acquired immunity = Adaptive immunity - The effector cells in specific immunity are the B and T lymphocytes - Lymphatic system and lymphoid organs: 1. Primary lymphoid organs: - For development and maturation of lymphocytes from their precursors - E.g. Thymus, bone marrow 2. Secondary lymphoid organs - Lymphocytes interact with antigens and other accessory cells - Differentiation of resting lymphocytes into effector cells - E.g. Spleen, lymph nodes, tonsils, Peyer’s patches, etc. - Characteristics of specific immunity: 1. Antigen specific: - Have antigen binding receptor for recognition 2. Diverse repertoire - Antigen receptors must be sufficiently diverse to recognise many pathogens 3. Immunological memory - Able to “memorise” the pathogens and mount better response 4. Self and non-self discrimination - To avoid destruction of self 101  9.3.2. HUMORAL RESPONSE - Characteristics: - Effector function is caused by proteins rather than cells - Mainly involves B lymphocytes and antibodies - B lymphopoiesis in bone marrow: - B cells mature from BM and circulate in blood and lymphatics, and in peripheral lymphoid organs recognize antigens → Differentiate to plasma cells that secrete antibodies → Some develop into memory cells for secondary response 9.3.3. ANTIBODIES AND IMMUNOGLOBULINS - Basic structure of an immunoglobulin (Ig)/ antibody (Ab): - Each Ab composes of 2 identical heavy (H) chains and 2 identical light (L) chains linked by disulfide bonds with: - Variable (Fab) region: sequence is highly variable between different Abs, for binding antigens - Constant (Fc) region: no variation within the same sub-class (isotype) of Abs, for biological activity of the Abs - Each Ab has two identical Ag binding sites 102 - Characteristics of antibodies: - Diverse variations in F(ab)’ regions: - Abs recognize the 3-dimension conformational determinants of antigens - Different Abs have different variable domains that recognize different epitopes (antigenic determinants) – i.e. Ag-specific - Estimated human B cells can generate >108 different Abs specificity – diverse repertoire - Functions: - Five major antibody isotypes (immunoglobulins): - Due to variations in the Fc region - IgG, IgM, IgD, IgA, IgE - Soluble forms: IgG, IgE, IgM, IgA - Structural polymers: - E.g. IgA dimer (4 antigen binding sites) - E.g. IgM pentameter (10 antigen binding sites) - E.g. Membrane-bound IgM and IgD 103 - Antibody-dependent cellular cytotoxicity (ADCC): - NK cells and eosinophils express the respective Fc receptors for binding to IgG and IgE - These cells contain cytotoxic granules that mediate cell killing activity in an antibody- dependent manner - Antibody can only mediate cell lysis or killing through complement or other immune cells, binding of Ab per se cannot lyse target cells directly - Cell has a surface antigen bound by IgG - NK cells have receptor on cell surface that will recognise the Fc region of an IgG - NK cells are recruited to target cell to mediate NK lysis - IgE is specialised for worm killing - Eosinophils have high affinity binding receptors that recognise the Fc region of an IgE molecule - Worm is too big to be phagocytosed → activated eosinophils release cytotoxic granules that create an environment that is unfavourable for the worm to survive 9.3.4. CELL-MEDIATED RESPONSE - Characteristics: - Involves T lymphocytes - T lymphopoiesis in thymus: - T cells mature from thymus and circulate in blood and lymphatics, in peripheral lymphoid organs recognise antigens (Ag) → Activated to become effector T cells for cellular functions → Some develop into memory cells for secondary response 104 - T cell receptor: - Defines antigen specificity of a T cell: - T cell receptor (TCR) composes of two chains, the 𝛼 and 𝛽 chains for conventional T cells , account for > 95% T cells in the body - A TCR contains single antigen binding site with highly variable sequence - Only exits in membrane bound form found on T cell surface - TCR antigen recognition: - TCR always complex with CD3 complex for intracellular signalling, and a co-receptor - TCR recognize antigenic peptides presented by Major Histocompatibility Complex (MHC) class I or class II molecules on antigen presenting cells (APCs) - Antigen binding is aided by the co-receptor, either CD4 or CD8 - Examples of APCs: macrophage, B cells, dendritic cells 105 - Mechanisms of target cell killing by TC and LGL: 1. Perforins secreted by killer cell → opens channel that allows water to pass through membrane of infected cell → osmotic lysis 2. Fas ligand binds to Fas on infected cell → caspase and endonuclease activation → apoptosis 3. Granzymes also induce the apoptotic pathway → apoptosis 9.3.5. T CELL LINEAGES 1. CD8+ T cells: - Killer/ Cytotoxic T cells (TC cells) - Mediate MHC-restricted cell cytotoxicity: - Resting CD8+ T cells recognize peptides on target cells presented by MHC class I molecules on APCs for activation - Activated CD8+ T cells develop cytotoxic granules for effector function → killing of target cells through release of cytotoxic granules - Granules contain pore-forming proteins (perforins) and apoptosis-inducing enzymes (granzymes) 106 2. CD4+ T cells: - Helper T cells (TH cells) - Resting CD4+ T cells recognize peptides presented by MHC class II molecules on APCs for activation - Activated CD4+ T cells express additional surface proteins and cytokines for helper function - CD4+ helper T cell modulate functions and development of other immune cells via the release of cytokines and cell-cell contact - E.g. B cells produce different antibody isotypes (IgG, IgA) → activated CD4+ T cells interacts with B cells via CD40L-CD40 interaction and cytokines such as IL-4 and TGF-β - Helper T cell subsets and cytokines: - CD4+ T cells are one of the most potent cytokine producers in the body - Examples of cytokines: IFN-𝛾, TNF-α, transforming growth factor (TGF-β), lymphotoxin, IL-2, IL-4, IL-5, IL-6, etc. - CD4+ T cells development is also modulated by cytokines too, and can be further developed into different subtypes - E.g. TH1, TH2, TH17 and Treg (regulatory) cells, each specialised in producing specific types of cytokines 107  9.3.6. BURNET’S CLONAL SELECTION THEORY - Burnet’s Clonal Selection Theory of Lymphocytes: - Development of humoral response: - Each B cell has many BCRs of the same antigen specificity - Different B cells have different antigen specificity (epitope) - Steps: 1. Antigen recognition by BCR – native structure of Ag 2. Antigen will select the B cell with the right specificity for expansion 3. Clonal expansion and differentiation into plasma cells and memory B cells - Each T cell has many TCRs of the same antigen specificity - Different T cells have different antigen specificity (epitope) - Steps: 1. Antigen recognition by TCR – native structure of Ag 2. Antigen will select the T cell with the right specificity for expansion 3. Clonal expansion and differentiation into effector T cells and memory T cells - Induction of specific immune responses: - Occurs in secondary lymphoid organs - Involves 3 distinct phases: 1. Ag recognition 2. Activation 3. Reaction/ effector phases - Reaction phase occurs at the site of infection/ inflammation - For first encounter of Ag (primary response), this usually takes 10-14 days - After reaction phase, surviving memory lymphocytes circulate in the body, and mount Ag- specific response upon re-exposure to the same antigen 108  9.3.7. IMMUNOLOGICAL MEMORY - Primary vs secondary responses: - Differences: 1. Time of induction - Primary response has longer time of induction (7-10 days) - Secondary response has shorter time of induction (2-3 days) 2. Magnitude of reaction - Smaller magnitude (production of antibodies) in primary response - Larger magnitude (production of antibodies) in secondary response 3. Decline in reaction after reaction phase - Lower memory response (plateau) in primary response - Higher memory response (plateau) in secondary response - Characteristics of immunological memory response: - Faster: - Shortened activation phase - Stronger: - Both qualitatively and quantitatively - More efficient effector response - E.g. more and better antibodies produced, more T cell proliferation - Provide long-lasting protection - Principle of vaccinations: - Makes use of secondary immune response (specificity, memory) - Development of memory T/B cells in vaccination helps to protect subsequent exposure of pathogens - Toxoid (modified toxin) in virus has no/ limited infectivity and pathogenicity, but immunogenicity is retained - E.g. Tetanus vaccine 109  9.3.8. SELF AND NON-SELF DISCRIMINATION - Unique for adaptive immunity - A healthy immune system is effective in generating immune responses to pathogens (‘non-self’), and it does not attack, under normal conditions, body’s own tissues or cells (‘self’) - There are mechanisms to avoid immune reactions against self antigens and control autoimmunity - Early and continuous presence of self-antigens induce self-tolerance: - Central tolerance (“thymic education”): - Deletion of self-reactive T/B cells during lymphopoiesis in primary lymphoid organs (stop maturation and production of self-reactive cells) - Peripheral tolerance: - Deletion, inactivation and suppression of self-reactive T/B cells in peripheral lymphoid tissues 9.3.9. SUMMARY 1. Specificity – immune responses are specific for distinct antigens. Both B and T lymphocytes express membrane receptors (BCR/TCR) that distinguish differences between distinct antigens. 2. Diversity – a diverse lymphocyte repertoire in the mammalian immune system can discriminate at least 108 distinct antigens. This extraordinary diversity is a result of numerous different clones of lymphocytes generated in an individual. 3. Memory – exposure of the immune system to a foreign antigen enhances its ability to respond to that antigen. The secondary immune responses are usually more rapid and stronger. 4. Discrimination of self and non-self – lymphocytes in each individual are able to recognize and respond to foreign antigens but are normally unresponsive to the self antigens, which is called tolerance. 110 9.4. ACUTE INFLAMMATION 9.4.1. INTRODUCTION TO INFLAMMATION - 4 causes of tissue damage: 1. Ischaemia 2. Physical energy 3. Chemicals 4. Infections - Consequences of tissue damage/ cell death: - Organs lose its function - Sometimes molecules are leaked into bloodstream → detectable in blood tests - Scarring after dead cells are cleared up → affect organism function - Dystrophic calcification → hard lumps or shows up on X-ray - Bacterial infection in the necrotic area - Cardinal features of acute inflammation: 1. Redness (rubor) 2. Swelling (tumour) 3. Warmth (calor) 4. Pain (dolor) 5. Loss of function (functio laesa) - Inflammation: - The physiological response of an organism to tissue injury - Not a disease by itself - Two types of inflammation (with overlap): 1. Acute inflammation – initial and often transient series of tissue reactions to injury 2. Chronic inflammation – subsequent and often prolonged tissue reactions following the initial response - Characteristics of acute inflammation: - Transient inciting stimulus - Vascular permeability - Exudation - PMNs (neutrophils, basophils, eosinophils) - Stages: Inflammation → Demolition → Healing/Repair → Resolution 9.4.2. COMPONENTS INVOLVED IN ACUTE INFLAMMATION - Components involved in acute inflammation: 1. Tissue damage 2. Chemical mediators 3. Blood vessels 111 4. Neutrophils 112 1. Tissue damage - Increased blood flow → rubor, calor - Leakage of plasma proteins → tumor - Neutrophil emigration → dolor - Process: - Margination → Diapedesis → Chemotaxis → Phagocytosis 2. Chemical mediators (modulate inflammatory response) - Functions: - Vasodilation - Increased vascular permeability - Attract neutrophils - Initiates plasma protein-derived mediators - 2 types: a. Cell-derived mediators: - Histamine: - Stored in mast cells and basophils - Released from mast cells during: - Physical injury - Binding of antibodies to mast cells - Initiated by the complement system - Effect: Vasodilation, increase vascular permeability 113 - Arachidonic acid: - Omega-6 polyunsaturated fatty acid - Common in cell membrane phospholipids - Converted to prostaglandins and other inflammatory mediators in tissue injury - Originally found in the bark of willow tree → allowed for invention of aspirin - Arachidonic acid derived mediators: 114  b. Plasma protein-derived mediators: - Complement system – complements the inflammatory pathways 3. Blood vessels - Functions: - Vasodilation - Increased vascular permeability - Vasodilation: - Increased blood volume → slower blood flow → vessels packed with slow-moving red cells (stasis) → neutrophils move to the peripheral (plasmatic zone) → helps neutrophils migrate out of the blood vessels - Tissue becomes red and warm - May involve histamine, PGE2, complements (C3a, C5a), bradykinin, etc. 115 - Increased vascular permeability: - Starling’s Law – More fluid pushed out from vessel into the tissue - Initially, there is only the flow of low molecular weight proteins (holes in wall are small) – transudate - As holes become larger they become exudate → higher molecular weight protein and more cells present - Immunoglobulins move into damaged tissue - Help destroy invading microorganisms - Coagulation proteins move into tissue - Help blood clot formation - Mechanism of increased vascular permeability: 116 4. Neutrophils: - Emigration into tissue - Chemotaxis - Phagocytosis - How neutrophils leave the blood vessels: - Neutrophils have binding sites on their surface (e.g. Sialyl-Lewis X glycoprotein and integrins) - Counter-receptors are present on the endothelial cells (selectin) - Neutrophils bind to counter-receptors - Under the influence of chemokines, integrins bind to ligands (e.g. ICAM-1) - Neutrophils bind firmly to endothelial cells allowing for migration - Endothelial cells increase the number of adhesion molecules through cytokines (e.g. TNF) - Adhesion molecules: - Stick leukocytes to endothelial cells - Increased in inflammation - E.g intercellular adhesion molecule-1 (ICAM-1) on endothelial cells and LFA-1 on leukocytes bind like a lock and key - How neutrophils move to site of injury (chemotaxis): - Tissue injury site contains chemoattractants - Bacterial products - Cytokines - Complements (especially C5a) - Neutrophils recognize the concentration gradient of chemoattractants - They move up the concentration gradient to the source of these chemicals (i.e. the site of injury) by amoeboid movement 117 - The vascular/ cellular process: 1. C5a causes mast cells to release histamine and cause vasodilation 2. Neutrophils emigrate 3. Bind to antigen with C3b receptors 9.4.3. PHAGOCYTOSIS AND OXYGEN-DEPENDENT MECHANISMS - Phagocytosis: - Once antigen is opsonized it is phagocytosed - 2 main opsonins are C3b and Fc - Ab binds to Ag - Ab receptors (Fc) on cell binds complex - Pseudopodia engulf complex to form phagosome - Fuses with lysosome - Intracellular digestion and destruction - Oxygen dependent mechanisms: - Oxygen is converted by NADPH oxidase into superoxide - Superoxide is influenced by superoxide dismutase and changes to hydrogen peroxide - H2O2 under the influence of myeloperoxidase enzyme is converted to hypochlorous acid - The superoxide, H2O2 and hypochlorous acid (HClO) damage the bacteria (but also the neutrophil cell) 118  9.4.4. OUTCOMES OF ACUTE INFLAMMATION - Systemic effects: - Fever - Tiredness - Loss of appetite - Leukocytosis - These appear when the cytokines, prostaglandins, etc. spill into the systemic blood circulation - Resolution - Suppuration - Minor – e.g. acne vulgaris - Severe – e.g. brain/lung abscess formation - Repair and organisation, fibrosis - Chronic inflammation - Beneficial effects: - Destroy invading microorganisms - Deliver oxygen and nutrients to the injured tissue - Deliver drugs (e.g. antibiotics) to site of infection - Harmful effects: - Destroys normal tissue (non-specific) - Swelling (oedema) can be harmful in certain anatomic sites - E.g. infection of epiglottis blocks airway - E.g. cerebral oedema leads to brain herniation 119  9.4.5. SYMPTOMS OF ACUTE INFLAMMATION IN APPENDICITIS - Pain: - Central abdominal pain: - Appendix acutely inflamed → oedema → stimulate the pain fibres there - Pain in the midgut is felt in the central abdomen (“referred pain”, “visceral pain”) - Pain moved to right lower quadrant: - Inflammation start to affect the outer layer of peritoneum - Stimulate pain fibres in the abdomen wall - Fever: - Cytokines (PGE2) enter bloodstream → brain (hypothalamus) sets a higher body temperature - Inflamed appendix: - Appendix may be blocked by the faecalith → overgrowth of bacteria trapped→ infection → tissue injury → acute inflammation 9.4.6. CLINICAL SIGNIFICANCE - Clinical significance: 1. The cardinal signs of acute inflammation – red, swelling, pain and heat are typical of acute inflammation 2. Lymphatics may become secondarily inflamed “lymphangitis” - The presence of red streaks near a skin wound is a telltale sign of infection 3. Patients with genetic or acquired deficiencies in leukocytes and adhesions get recurrent infections a. Cancer patients with low neutrophils b. Leukocyte adhesion deficiency (Type 1) – mutations in 𝛽 chain of CD11/CD18 on cells c. Leukocyte adhesion deficiency (Type 2) – mutation in the enzyme needed to make the receptor for neutrophils on vessel wall d. Chronic granulomatous disease – decreased oxidative burst so can’t produce superoxide e. Chediak-Higashi Syndrome – mutations in the protein needed for lysosomal membrane tethering (Vesicles can’t walk on microtubules to get to lysosomes properly) → Delayed fusion of phagosome with lysosome in leukocytes - Side effect: Autophagocytosis of melanosomes in melanocytes → albinism - Granular defects in NK cells & platelets - (Mnemonic – CHINA – Chediak Higashi, Infections, Neuropathy, Albinism) 120  f. Complement deficiencies or excess activity - C2 is most common, resulting in bacterial infections, however some patients are asymptomatic (cope with alternative pathway) - Alternate pathway deficiencies are rare - Patients with C5-9 deficiencies get bacterial infections, especially Neisseria as it has a thin cell wall - C1 inhibitor deficiency – more common than C2 deficiency - Blocks activation of C1 - Inhibits Factor XII (coagulation) and kallikrein system - Unregulated production of kallikrein – hereditary angioedema - Defect in enzyme that tethers complement regulators to cell wall means the complement is not regulated - Excess production can destroy red blood cells - E.g. Paroxysmal nocturnal haemoglobinuria (PNH) g. Many anti-inflammatory agents block action of COX-1 and/or COX-2 h. Proteases released by neutrophils are not destroyed - Excess tissue destruction in patients with α1-antitrypsin deficiency 9.5. CHRONIC INFLAMMATION 9.5.1. INTRODUCTION TO CHRONIC INFLAMMATION - Chronic inflammation: - Inflammation of prolonged duration (weeks or months) - Inflammation, tissue destruction, healing and repair are proceeding at the same time - Causes: 1. Following on acute inflammation: - Persistence of inciting stimulus - Interference of normal healing process - Repeating of acute inflammation - Examples: a. Unresolved acute inflammation (chronic suppurative inflammation): - Chronic osteomyelitis - Chronic abscess b. Repeated episodes of acute inflammation: - Chronic pyelonephritis - Chronic cholecystitis 121 2. Distinct process from the outset: - Brief and minimal acute phase - Low-grade smouldering response - Examples: a. Viral infection - Hepatitis B infection b. Persistent infection: - TB, leprosy - Syphilis - Fungi c. Insoluble particles: - Silica, asbestos - Other foreign bodies - Cholesterol (e.g. atherosclerosis) d. Hypersensitivity: - Non-infective condition, e.g. contact dermatitis - Autoimmune condition, e.g. rheumatoid arthritis - Importance of chronic inflammation: - Pathogenesis of diseases not conventionally thought of as inflammatory disorders: - Alzheimer’s disease - Metabolic syndrome - Type 2 Diabetes Mellitus - Certain Cancers - Comparison of acute and chronic inflammation: 122  9.5.2. FEATURES OF CHRONIC INFLAMMATION - Histologic hallmarks of chronic inflammation: 1. Mononuclear inflammatory cell infiltrate a. Macrophages b. Lymphocytes c. Plasma cells 2. Granulation tissue 3. Tissue destruction and fibrosis 4. Regeneration 1a. Macrophages: - Recruitment from circulating monocytes - Transformation into macrophages - Activation of macrophages - Local proliferation and immobilisation of macrophages - Histology: abundant, clear cytoplasm - Development and differentiation of macrophages: 123  1b. Lymphocytes: - Recruited in: - Antibody mediated immune reaction - Cell-mediated immune reactions - Histology: dark, minimal cytoplasm 1c. Plasma cells: - Synthesis of immunoglobulin - Histology: lots of rER for antibody synthesis, peripheral nucleus, eccentric clock-face nuclei - Other cells in chronic inflammation: - Eosinophils – abundant in immune reaction mediated by IgE and in parasitic infections - Mast cells – response in allergic reactions - Neutrophils – induced by persistent microbes or by mediator produced by activated macrophages and T lymphocytes 2. Granulation tissue: - In-growth of vascular tissue from surrounding connective tissue - Contains thin–walled capillaries, fibroblasts and inflammatory cells - E.g. granulation tissue forms wall of abscesses 3. Tissue destruction and fibrosis: - Tissue destruction is one of the hallmarks of chronic inflammation - Fibrosis and scarring e.g. valvular stenosis, narrowing of hollow organs - Adhesions e.g. serous cavities, joint deformity - E.g. Stomach, chronic ulcer → granulation may lead to bleeding → perforation → peritonitis - E.g. Heart, mitral stenosis/regurgitation, aortic stenosis/regurgitation → fibrosis, calcification - E.g. Lung, repeated bouts of pneumonia → bronchiectasis 4. Regeneration: - Regeneration of destroyed tissue, e.g. epithelial overgrowth or invagination - Endarteritis obliterans, e.g. narrowing of small artery lumen 9.5.3. EFFECTS OF CHRONIC INFLAMMATION - General effects of chronic inflammation: 1. Fever, acute-phase proteins, leukocytosis 2. Hyperplasia of lymphoid system 3. Immune response with antibody production 124 4. Reactive systemic amyloidosis with formation of AA protein 125 A. Granulomatous inflammation: - Distinctive pattern of chronic inflammation with which predominant cell type is activated macrophages showing epithelial-like (epithelioid) arrangement - Induced by T cell and macrophage activation in response to an agent that is resistant to eradication: - T cell response → cytokines released → induced proliferation of macrophages → aggregation of macrophages → formation of epithelioid and giant cell (granuloma tissue) B. Granuloma: - Collection of modified macrophages (histiocytes) - May be surrounded by rim of lymphocytes - Central necrosis may be present - Types of granuloma: 1. Foregin body granuloma: - Incited by inert foreign bodies, e.g. talc, sutures, lipid/cholesterol, collagen 2. Immunologic granuloma: - Incited by insoluble particles capable of inducing cell-mediated immune response - Infective, e.g. TB, leprosy, fungal, parasite ova - Tumours, e.g. Seminoma 126  9.5.4. CHRONIC INFLAMMATION IN TUBERCULOSIS - Characteristic features of tuberculosis: - Tubercle – aggregate of epithelioid cells and Langhans’ multinucleated giant cells - Caseous necrosis - Acid-fast bacilli [Cell-mediated hypersensitivity type IV] - Classification of tuberculosis: 1. Primary TB: - Infection of an individual lacking previous contact with tubercle bacilli 2. Secondary TB/ Post-primary TB: - Arises in previously sensitised individual - Most cases represent reactivation of asymptomatic primary disease 3. Miliary TB: - When mycobacterium gain access to lymphatics and blood to seed distant organs 9.5.5. CHRONIC INFLAMMATION IN TUBERCULOSIS - Multinucleated giant cells: 1. Langhans giant cells - Horseshoe arrangement of nucleus - Caused by tuberculosis 2. Foreign body giant cells - No special arrangement of nucleus - Caused by large foreign body 3. Touton giant cells - Centralised nuclei - Caused by lesions with high fat content 127 9.6. ANTIMICROBIALS 9.6.1. INTRODUCTION TO ANTIMICROBIALS - Definition: - Antimicrobials: A substance that kills or inhibits growth of an infectious microorganism - Antibiotics/ Antibacterial agents: A substance that kills or inhibits growth of bacteria - New antibiotics in the past 10 years: - Classes are not new → derivative of classes of pre-existing antibiotics 9.6.2. CLASSIFICATION OF ANTIBACTERIAL AGENTS - Classification of antibacterial agents: A. By chemical structure B. By target site/ mechanism of action C. By activity (bacteriostatic or bactericidal) A. By chemical structure: 128 1. Beta-lactam structures: - Most important, usually first line treatment of infections - 5 major families: penicillins, cephamycins, carbapenems, cephalosporins, monobactams - Beta lactam ring fused to 5-membered or 6-membered ring 2. Vancomycin – big molecule 3. Quinolone: - Includes fluoroquinolone – fluorine at C6 position, more common form - E.g. Ciprofloxacin - Exception (no fluorine): nalidixic acid 4. Tetracyclines: - 4 cyclical structures - E.g. Doxycycline, minocycline 5. Aminoglycosides: - 3 sugars - E.g. Gentamicin, tobramycin 129 6. Macrolides antibiotics: - Big cyclical ring (14, 15 or 16 members) - E.g. erythromycin, clarithromycin, azithromycin - Importance of drug chemistry: - Common among agents in the same chemical group (or subgroup) - Immunological reactivity – drug allergy from cross-reaction - Pharmacological properties – absorption, tissue distribution, contraindications (children, pregnancy), toxicities - Mechanism of action – the same binding site(s) in bacteria - Bacteria resistance mechanism(s) B. Classification by mechanism of action: - Gram-positive vs Gram-negative bacteria: - Cross-linkages: [Red: Transglycosylation; Blue: Transpeptidase] 130 - E.g. Staphylococcus aureus vs E. coli - Reaction involving beta-lactam: - Active site – serine amino acid residue in TPase domain - B-lactam ring opens up an occupies active site → interfere with activity of transpeptidase → affect cross-linkage involving the short peptides [TPase – transpeptidase domain; TGase – transglycosylase] [One protein may have both TPase and TGase domains] - The binding site of penicillin (all beta-lactams) is called penicillin binding protein (PBP) - Bacteria have multiple PBPs that may be classified into two groups: - Class A PBPs are enzymes with both TGase and TPase activity - Class B PBPs only have TPase activity - Vancomycin binds to the D-alanine-D-alanine portion of cell wall precursors → Osmotic lysis 131 C. By activity - Definitions: - Bactericidal: - Indicate that the action of the agent will kill the targeted microbe - Working definition: >99.9% decrease in the initial inoculum - Bacteriostatic: - Indicate that the action of the agent will inhibit growth of the targeted microbe but will not kill it - Working definition: prevent visible growth - Evaluation of activity: 1. Assessment by growth curve: [Y: log(number of organisms), X: number of hours after incubation] 2. Assessment by MIC and MBC: - The minimal inhibitory concentration (MIC) is the lowest concentration of antibacterial agent that inhibits visible growth after overnight (16-24 hours) incubation - The minimal bactericidal concentration (MBC) is identified by determining the lowest concentration of antibacterial agent that kills the initial bacterial inoculum by ≥99.9% 132 - E.g. MIC=1, MBC=2 (need to use back-titration and calculation) 133 - Common bacteriostatic antibacterial agents: 1. Macrolides 4. Chloramphenicol 2. Clindamycin 5. Sulphonamides 3. Tetracyclines 6. Trimethoprim [Act on ribosomes or specific metabolic pathways] - Common bactericidal antibacterial agents: 1. Beta-lactams 3. Aminoglycosides 2. Vancomycin 4. Fluoroquinolones [Act on cell wall, DNA or ribosomes] - Comparison: - Designation of bactericidal and bacteriostatic agents is arbitrary - An agent can be bacteriostatic against one organism but bactericidal against another organism - An agent may be bacteriostatic at low concentration but bactericidal at higher concentration - Also affected by bacterial inoculum - Distinction is of doubtful clinical significance: - Unlikely to be relevant for pneumonia, skin infections, soft tissue infections, intra- abdominal infections - No data on meningitis, endocarditis, neutropenic infections (usually use bactericidal agents where possible) 9.6.3. IMMUNISATION - Definition: - The process of artificially inducing immunity or providing protection from disease - Vaccine – an antigenic preparation used to induce immunity - Classification of vaccines: 134 A. Active immunisation: - Induction of “acquired” immunity - By “priming” with antigen - Provide pre-existing immunity → prevent infection - Secondary response upon exposure → mild or subclinical infection; little or no damage - Specific antibody and/or T lymphocytes - Primary responses result from the activation of previously unstimulated naive B cells - Secondary responses are due to the stimulation of expanded clones of memory B cells - Secondary response develops more rapidly than the primary response does, and larger amounts of antibodies are produced in the secondary response 1. Live, attenuated vaccine: - Benefits: - Mimics natural infection: - Stimulate T-lymphocytes “naturally” - Induce mucosal immunity - Also protect the unvaccinated - Vaccine strain is infectious - Herd immunity - Limitations: - Vaccine strains could cause disease - Healthy recipients – rare but risk exists (reversion to virulence) - Immunocompromised recipients – immunised before diagnosis of condition, person to person transmission of vaccine strain puts them at risk 2. Inactivated vaccine: - Impossible to “attenuate” - Too risky - Unknown stability (reversion risk) - Methods: - Heat - Vibrio cholera - Chemical - Formaldehyde – Hepatitis A, Diphtheria toxin, tetanus toxin - B-propiolactone – Influenza virus - Limitations: - Relatively less immunogenic - Poorer stimulation of memory cell - Shorter duration of protection 135 - Improving the immunogenicity of inactivated vaccines: a. Adjuvant: - Substances added and used to boost immune response - Non-specific immunostimulant - Aluminium salts – most widely used - Effective at inducing antibody responses but less active in inducing cell-mediated immunity b. Protein conjugation: - Difficult to produce effective vaccine against encapsulated bacteria - Capsule is a major virulence factor - Many polysaccharide antigen fail to stimulate T cells - Ineffective in children 1000 donors) 2. Specific Ig: specific preparations obtained from donor pools pre-selected for a high antibody content against a specific disease, e.g. Hepatitis B Ig, Varicella-zoster Ig, Tetanus Ig); given mostly as secondary prophylaxis 137  - FDA approved products for passive immunisation and immunotherapy: - Benefits: - Immediate protection provided - Useful for those unable to response immunologically - Limitations: - Protection is relatively short-lasting (IgG has a half life of ~21 days) - Use of anti-serum raised in animals can cause serum sickness - Residual risk of blood-born infections (e.g. HIV, HBV, HCV) 9.7. INFECTION CONTROL 9.7.1. INTRODUCTION TO INFECTION CONTROL - Definition: - Evidence-based practices and procedures that, when applied consistently in health care settings, can prevent or reduce the risk of transmission of microorganisms to health care providers, clients, patients, residents and visitors - Hospital-acquired (nosocomial) infections: - An infection originating in a medical facility, e.g. occurring in a patient in a hospital or other health care facility in whom the infection was not present or incubating at the time of admission - Includes infections acquired in the hospital but appearing after discharge and such infections among staff 138