Innate Immune Cells Cheat Sheet

Immunology: Innate Immune cells rapidly recognise pathogens Ags via invariant (nonspecific) receptors that trigger killing, phagocytosis or antigen presentation. Adaptive immune cells: recognise pathogens following presentation by APCs to activate immune cell expansion followed by pathogen killing. Innate Adaptive Non-specific receptors Specific receptors Respond quickly Slow 1st response No memory Memory 1st line of defence: surface barriers such as: Hair, Keratin (key structure material making the outer layer of skin), mucous membrane and antimicrobial secretion (destroys pathogens before the enter the body) 2nd line of defence: Innate immunity 3rd line of defence: Adaptive Immunity Lymphatic system: -Lymph system similar to cardiac system, has one way flow (valves), begins in the tissues and ends at the heart -3 main parts: vessels carry fluid, lymph (fluid in vessels), lymph nodes (filters lymph of pathogens) -Plasma from blood vessels seeps into tissues -Lymphatic vessels depend of muscle contraction for lymph flow back to heart -Lymph and lymphocytes empty into the left subclavian vein of the heart Lymphoid primary organs: Bone Marrow: Site of haematopoiesis, formation of all blood cells form CD34+ HSC, B cell receptor assembly. BM MSC assist B cell maturation by providing: 1. a source of self Ag -> B cell apaptosis 2. Cytokine (IL-7) for B cell development. Haematopoiesis: HSC (CD34+ Haematopoietic Stem Cell) -resides in the bone marrow, pluripotent, 1/50000 BM cells, extremely proliferative if need arises. HSC differentiates into common lymphoid or myeloid progenitor cells (lineage commitment) Growth factors, cytokines determine cell lineage Mesenchymal stromal cells (MSC) support HSC development in BM and in vitro Haematopoietic Growth Factors – and cytokine determine cell lineage commitment: Myeloid Growth Factors: (Multi-CSF (IL-3), M-CSF (Macrophage CSF), G-CSF (granulocyte CSF), GM-CSF (Granulocyte monocyte CSF)) and Erythropoietin (Epo)- induces production of RBCs Thymus: Bilobed organ on top of heart: T Cells move the BM to thymus for T cell receptor assembly and selection for release in periphery. Involves 2 steps: 1. Thymic stomate cells express high levels of MCH I & II. Presents various Ag and T cells 2. T cells recognise self-Ag -> apoptosis, T cells that fail to recognise foreign Ag -> apoptosis. 99% of T Cells die in thymus. Cell Death: Apoptosis – orderly: Reduction In cell volume, Chromatin condensation, DNA degradation, Mφ Ingest membrane bound bodies, no inflammation. Necrosis – chaos: Bursting of cell due to injury, Contents released to environment, Inflammation. Lymphoid Secondary Organs (Maturation): Lymph Node: Ags trapped by dendritic cells, DC process and present Ag with MCH class II, Activate Th Cells, secrete cytokines, allows for activation of B cells, B cells interact with soluble Ag, activated lymphocytes proliferate. Spleen: Fictionally different from lymph nodes. (Lymph node traps Ag in circulatory system, allows spleen to activate immune system in response to systematic infection), Ag in blood captured by dendritic cells, presented to Th cells with MCH II, Activation of Th allows subsequent activation of B cells, Blood leaving spleen enriched with activated lymphocytes and antibodies Lymphoid Tertiary Tissues (Maturation) Mucosal Associated Lymphoid Tissue (MALT): Mucous membrane SA = 400m2, MM most common pathogen entry site, MM protected by MALT, Organisation varies, M cell allows AG entry, unique architecture allow B cell activation of IgA secretion that prevents pathogen attachment. Cells of the Immune System: Neutrophils: Only enter tissues when instructed, Phagocytosis, Bactericidal killing Eg, Respiratory burst. Basophils: Smallest WBC in circulation, Function Unknown, maybe similar to mast cells Mast cells: Mainly in tissue, release of granules containing histamine and other compounds. Important in inflammation and allergy Mononuclear cells: Innate immune cells, Monocytes in blood, Mφ in tissue (monocytes 5-10 times smaller), Mφ increase phagocytic ability, Secrete cytocines and produce hyrodlyic enzymes, Named based on tissue they reside – alveolar (lungs), Kupffer (liver), Microglial (brain), Oleoclasts (bone), Activated by phagocytosis of cytokines, Antigen presenting capacity via MHC II. Lymphoid Cells: adaptive immune cells, B cells, T cells, Null cells (NK cells), 20-40% of body’s leukocytes, 99% of lymph node, Nycleous occupies almost entire cell, 6μm diameter. Phagocyte Mobiilisation: Steps for phagocyte mobilisation: 1.Leukocytosis – releaase of neutrophils from bone marrow in response to leukocytosis inducing factors from injured cells. 2.Margination – neutrophils cling to walls of capillaries in inflamed areas in response to CAMs 3. Diapedesis – of neutrophils 4. Chemotaxis – Inflammatory chemicals promote positive chemotacis of neutrophils NK Cells: Innate immune cells, don’t express lymphocyte markers. Eliminate tumour and virally infected cells, use CD16 to carry out ADCC, Reduction of MHC I can activate them (loss of self) Dendritic cells: Antigen recognition/uptake, Antigen presentation – produces cytokines to direct T cell response, Several types = Langerhans (skin), Circulating DCs. Interstitial HCs, populate organs eg, Heart, lungs, liver and intestines. Interdating DCs = T cell areas of lymph nodes and thymic medulla. Scarce cell type, grown in vitro early 90s, intense research area (tumour treatment) Glands which contain antimicrobial substances. Mucosal Membrane: Line all body cavities that open to the exterior (digestive, reproductive, respiratory, urinary), Secretion of HCl and enzymes into stomach which destroy pathogens, Enzymes in saliva (eg. Lysozymes) break down microbes, production of mucus traps mucus (contains numerous anti microbial surfaces), Modifications eg. Cilia trap and sweep away microbes from lower respiratory system. Antimicrobial peptides (AMPs) Two families of AMPs: cathelicidins and defensins. Cationic short polypeptides which found on epithelial surfaces, Preferential interaction with bacterial membrane: (+) charged AMPP bind to (-) phospholipid. Weak interaction with eukaryotic/host cell membrane: (-) phospholipids not found on extracellular face. AMP bind to bacterial membrane Oligomerisation of AMPs create a pore in bacterial of membrane Lysis of bacteria Other types of Antimicrobial found in Epithelium. Immunoglobulins – especially IgA, part of adaptive immune system, bind to bacteria and enhance phagocytosis Surfactant – bind to carbohydrates expressed by bacteria and enhance phagocytosis, ‘Normal’ flora found in epithelium, competition with pathogenic microbes Hormonal Immunity Soluble factors (non-soluble) mediate immunity, generally proteins 3 types – antibodies, complement system and C-reactive proteins Antibodies part of adaptive immunity (can have innate properties) Complement and CRP innate immune system Complement System Group of >30 soluble proteins (enzymes) found in serum in inactive form, Frequently referred to as a cascade – sequential activation by cleavage (Deficiency in one complement protein can affect the whole system). 3 Pathways = Classical, Lectin and alternative. Results in microbial killing, opsonisation and inflammation Complement Activating proteins Classical Pathway – induced by C1q binding directly to microbial surface components, indirectly binding antibodies already attached to microbial surfaces Alternate Pathway – Spontaneous hydrolysis and activation of C3, binds directly to microbial surfaces Lectin Pathway – carbohydrate binding proteins (mannose binding lectin) bind to carbohydrate structure on microbial surfaces Effector functions of the complement system 1.Direct Lysis of pathogens via Membrane attack complex (MAC) Complement components form complex, exposure of hydrophobic phase allows insertion into microbial membrane. C9 binds to complex and polymerizes forming a pore 2.Enhancement of Phagocytosis (opsonisation) Receptors on surface of cell recognise complement components attached to microbial surfaces or to antibodies 3.Recruitment and activation to immune cells Interactions of complement fragments with receptors on endothelial cells and mast cells produce local inflammatory responses, Phagocytic cells recruited. Cells of Innate Immunity Phagocytes: Any cell capable of ingesting foreign particles, Phagocytosis: active process, bacteria and foreign bodies are engulfed and internalised into membrane enclosed vesicles (phagosomes). Phagosomes become acidified, fused with lysosomes and kills microbes. Internalised microorganisms and lysed products recognised as PAMPs. Phhagocytes include: Neutrophils, Macrophages and Dendritic cells Innate Immunity Neutrophils – Function: Phagocytosis and activation of bacterial mechanisms. Differentiate from multi-potential stem cells in BM, released mature into blood, 30-70% of WBCs in peripheral blood, Contains lysosomal granules containing lysozyme, acid hydrolyses, proteases, peroxidase. Does not recirculate Macrophages – Function: Phagocytosis and activation of bacterial mechanisms, antigen presentation. In blood and matured from macrophages, in tissue specialised – Kupffer cells in liver. Differentiate from BM Stem Cells, released as monocytes (Phagocytic, Bactericidal). 27% of WBCs in peripheral blood. Emigrates to tissues, differentiate to macrophages. Contains hydrolytic enzymes. Dendritic Cells – Function: antigen uptake in peripheral sites, antigen presentation. Most cell types in immune system, different types in different body parts. Main function is to uptake and process antigenic material and present it to immune system (bridge between innate and adaptive immunity). Heterogeneous, derived from monocytes. Characteristics of innate immunity: Ancient – In all eukaryotes, Immediate/rapid response to pathogen (first line of defence), How is the unique binding site on an antibody generated? Antigen Natural Killer Cells: Innate lymphoid cells, not considered professional phagocytes. Responds to virally infected cells, tumour cells. Contains cytotoxin granules, perforin (pore forming proteins) and granzymes (proteases enter through perforin pore), Expression of ligands that activated “death” receptors on target cells. How do NK cells recognise tumour/virus infected cells? One mechanism evades the adaptive immune system – down-regulate MHC class I expression on virus infected or malignant cells. Normal cells express MHC I on surface, NK cells ‘sense’ a lack of MHC I expression on abnormal cells and kill them inhibitory and activation receptors combined. What are Granulocytes? Type of leukocytes characterised by granules in cytoplasm. Granules intracellular vesicles contain a variety of antimicrobial substances, released upon activation (defensins, proteolytic enzymes, cytokines). Neutrophils, Basophils and Eozinophils. Basophil functions: Unknown – Contains large basophilic granules – contains histamine, heparin, proteolytic enzymes. Least common granulocyte, constitutes 0.1-0.3% of WBCs in peripheral blood. Important role in certain inflammatory reactions. Eosinophil functions: Killing of antibody coated parasites, Contains eosinophilic granules containing histamine, peroxidase and lipases. 1-5% of peripheral blood and is an important roll in parasite infections, allergies and asthma. How does innate immunity tell self from non self? Pathogen-Associated Molecular Patterns (PAMPs), Molecules expressed by a group of pathogens that are not present in host Bacterial Cell Membranes: Lipoteichoic acid, lipopolysaccharide (LPS) and peptidoglycan are unique to bacterial. Membranes serve as potent PAMPs to innate immune system PAMPs: Double stranded RNA synthesised by viruses (eg. Rotavirus), Unmethylated CpG motids produced by viruses , Flaggellin on bacteria (eg. Helicobacter).Zymosan – carbohydrate found on some fungal species Pattern Recognition Receptors (PRRs): PRRs recognise PAMPs, Expressed on surface or intracellularly by innate immune cells (toll-like, C-lectin, NODlike, RIG-I-like receptors), PRR ligation by PAMP “activates” innate cells – phagocytosis, release of cytotoxic mediators, secretion of proinflammatory cytokines Toll-Like Receptors (TLRs): Major PRR gene family, “leucine-rich repear” zipper structure, Recognise conserved pattern motifs found on variety of pathogens , Signalling: Ligation of TLRs by pathogen activates cell. Activation results in expression of proinflammatory cytokines. Cytokines – activate nearby immune cells, help induce antiviral state. What happens when a macrophage encounters an extracellular microbe? 1.Microbe coated in humoral mediator eg complement proteins 2.Macrophage captures microbe by microbe by PRR and complement receptors, begins to express lysosomes. 3. Microbe is phagocytosed 4. Lysosomes fuse with phagosome – contain enzymes, oxidases pump free radicals into phagolysosome 5. Microbe digested, microbe derived peptides transported via MHC proteins to cell surface, presented to T cells. 6.Indigestible materials expelled. Dendritic cells act as bridge between innate and adaptive immunity. Endocytose/phagocytose pathogen (antigen), Digest antigen into small peptides. Peptides bound to MHC, transported to cell surface for T-cells. Antigen Presenting Challenges for the mammalian immune system: Antigenic drift & shift Difference between innate and adaptive immunity: Timing/lag phase, Specificity, Diversity, Memory, Humoral vs Cellular responses Antigen Presenting Cells (APCs): Types: DCs, Macrophages, B cells T cells and B cells: Clonal selection theory, Importance of Self-MHC recognition by T cell receptor (Transplant rejection can occur when T cells recognise non-self MHC molecule (in the presence of peptide, whether it is self or not)) MHC molecules: MHC Class I: interacts with CD8+ T cells, Present on all nucleated cells, Present 8-10 AA (amino acid) peptides,Endogenous pathway for antigen generation. MHC Class II: interacts with CD4+ T cells, Present mainly on APCs (and thymic epithelial cells), Present 3-20 AA peptides. Exogenous pathway for antigen generation Cross presentation (exception to MHC-I only processing endogenous proteins) e.g. phagocytosis of apoptotic cells. Recognition of molecular patterns common across pathogens, Low specificity and lack of memory. Can direct to the adaptive immune system. 1st Line of defence – Surface barriers Skin: Heavily keratinised – physical barrier, Weakly acidic (pH 3-5) which inhibits growth of bacteria and fungi, Sebum secreted by sebaceous Endogenous Pathway: Antigens arise from the cytoplasm (viral antigens are common) Transporters associated with antigen processing (TAP) transport small cytosolic peptides into the lumen of the endoplasmic reticulum (ER) – 1st diagram Exogenous pathway: Antigens are extracellular in origin (phagocytosis) – 2nd diagram. Interactions between antigenic peptides and MHC: MHC molecules can bind to lots of peptides due to high genetic variation (HLA genes), due to Polymorphism, Polygeny and Co-dominance, Leads to greater chance of survival of a population. Conserved anchor residue (allows peptide stabilisation to MHC, even when there are lots of different types of antigenic peptides): MHCI – at P(position) 2 and P9 AA residue, MHCII – at P1, P4, P6 and P9. Non-classical MHC molecule: CD1: Similarities to both MHCI (structure) and MHCII (Ag processing), binds glycolipids from mycobacteria, Presents to γδ T cells. Adaptive Immunity Self/Non-selfdiscrimination Lag Phase Adaption Present, reaction is against foreign Present, response takes atleast a few days High, response directed only to the agents that initiated it Diversity Extensive, resulting in wide range of antigen receptors Memory Present, subsequent exposures to same agent induced amplified Dynamics of the adaptive immune response Specificity Innate and Adaptive Immunity Work together Innate Adaptive Number of 102 - 103 109 pathogen receptors Time to develop Immediate Days/Weeks response Memory No Yes Similar Greater Strength of response after 2nd challenge Adaptive (specific) Immune System Two cell types comprise system – T lymphocytes, B lymphocytes From lymphoid precursor : ‘B’ bursa of fabricus, ‘T’ thymus T Cells – primary development organ is thymus B cells – Primary development organ is bone marrow and spleen, diverse in antigen recognition What are antigens? Antigens = antibody generators, proteins that elicit immune response. (Proteins, carbohydrates, nuclei acids, lipids, haptens + carriers Eg. Nickel How does the Adaptive immune system recognise so many antigens? Each T or B cell clone has one specific or antigen receptor (TCR or BCR (antibody)), so one cell recognises one antigen Unique antigen receptors generated by random rearrangement of gene segments which encode specificity of antigen receptors B cells with receptors for self are deleted in bone marrow Result = huge pool of B cells and T cells with different Ag specificity Clonal Selection Theory – adaptive immune cells which encounter antigen in periphery induced to survive, proliferate and differentiate, generating antigen-specific clones (Clonally selected lymphocytes differentiate into effector cells eg. Memory cells, theory explains specificity and memory) B cells – Type of lymphocyte, round cell with small cytoplasm, round nucleus (mononuclear), 5% of all WBCs, primary role is to secrete antibodies as plasma cells which have large cytoplasm, rough ER, protein factory (2000 Abs/s) Plasma cells – Large cytoplasm with numerous sheets rough ER, Protein factory – can synthesise, low expression of BCR, Small number found in blood – reside primarily as bone marrow and spleen, Short lived 1-2 weeks. Antibodies – Membrane bound (BCR) or secreted (by plasma cells) soluble molecules, Fab region – responsible for antigen binding (via domain), Fc region – interacts with Fc receptors on immune cells, recruits complement. One antibody – One antigen binding occurs in ‘variable domain’ called VH or VL, During B cell development, gene encoding heavy or light chain variable regions generated at random from selection of gene segments, VDJ recombination of heavy and light chain Variable Regions: Gene encoding heavy chain – variable domain ‘stitched’ together from three different gene segments (V: variable, D: diversity and J: joining) Process relies on Recombinant Activating gene (RAG) 1 and 2. Heavy chain contains 1V 1D 1J, Light chain contains 1V 1J. Negative Selection – Controlling Autoreactive B Cells: Stromal cells present self-antigen to B cells in bone barrow, 10% B cells reach periphery. Receptors editing may save some B cells. When B Cells leave the BM – Clonal selection: Each naïve B cell clone expresses unique BCR (antibody) on their surface. When B cell binds to antigen it is specific for, it becomes inactivated and clonally expanded B Cell Receptors (BCR) – Two components – Antibody, Iga and Igb, signalling occurs through Iga and Igb complex only 2nd and 3rd Signals – T cell activates B cells – T cells provide – CD40 ligation and cytokines The Germinal Centre (GC) – B Cell Activation and Differentiation Mass of leukocytes that form within lymph nodes during infection, primarily composed of proliferating (and differentiating) B cells. Fine Tuning the Immune Response Somatic Hypermutation – Introduction of point mutations into rearranged VL, VH genes; caused by enzyme: Activation-Induced (Cytidine) Deaminase or AID; C region unaffected Affinity Maturation – B cells with BCRs that have increase antigen affinity selected for survival. The GC Reaction – Activated B cells travelled to dark zone of GC and proliferate (clonal expansion) -Fine tuning the BCR: Dark zone B cells (Centroblasts) undergo somatic hypermutation and begin to express BCRs with higher or lower affinity for antigen. -B cells migrate to light zone (centrocytes), encounter follicular DCs that present antigen to B Cells (natural selection) B cells migrate to light zone, cells with strongest specificity selected (others die by apoptosis) Centrocytes receive signals from T cells to undergo isotype switching, leave GC and become memory B Cells or antibody secreting plasma cells. Class Switching in GC: B cells the enter GC receive signals from cytokines to undergo antibody isotype switching. Isotype switching allows transmembrane expression and secretion of other isotypes. Cytokines secreted by CD4+ helper T cells present in light zone. Isotype Switching allows a given VH exon to associated with CH genes. Antigen dependant. Irreversible (deletional) DNA recombination simulated by external signals (DNA looped out). Heavy chain VH remain the same, light chain unaffected, antibody retains same specificity for antigen. Cytokines secreted by T follicular helper cells determine the body isotype. Cytokine from Tfh determines isotype: IGNg induces IgG switch IL-4 or IL-3 induces IgE and TGB-b induces IgA switch Without appropriate cytokines, B cells don’t switch, keep producing IgM. CD40 ligation +: IFNg induces IgG switch, IL-4 or IL-13 induces IgE and TGB-b induces IgA switch. X-linked Hyper IgM T cells do not express CD40L; cannot isotype switch and are susceptible to upper respirator tract infection. T Cells Thymus: where T cell development occurs (immature T cells migrate form bone marrow). Region: Cortex, Medulla. Types of cells present: thymic epithelial cells; DCs/macrophages. T cell development pathway: Positive selection (self-MHC restriction) – single positive; Negative Selection (self-tolerance) – Elimination of selfpeptide recognising T cells. Structure and function of T Cell Receptors TCR: αβ TcR: Expressed on classical T cells (95%; express either CD4 or CD8); Extensive gene rearrangement = hypervariable (due to many variable and joining in germline DNA) γδ TcR: Expressed on γδ T cells (5%; don’t express CD4 or CD8); No MHC requirements; limited gene rearrangement = ‘innate like’’ Binds Antibody Isotypes glycolipids and increased in number when infected with mycobacterium. Antibodies (Immunoglobulins): Made up of heavy and light chains, each Gene rearrangement of TcR: Sequence of events: 1. Β Chain with constant and variable regions linked by disulphide bonds) rearrangement occurs first (also γ and δ -> γδ T cells), successful β chain Can be found in serum (secreted by plasma cells) or attached to surface will pair with pre-Tα and form pre TcR, cell proliferates, then α chain of B cells as part of BCR complex. (Variable region – encode antigen rearrangement second. The presence of surface markers at each stage specificity of antibody, Constant region of antigen chain – biological (i.e double negative -> double positive -> single positive) effector function of antibody); 5 different isotypes with varied function Only β chain includes Diversity regions (both β and αincludes Variable, defined by constant region: IgA, IgD, IgE, IgG, IgM. Joining and Constant regions). β chain is limited in rearrangement events When a B cell becomes activated for the first time during primary but αchain can undergo multiple rearrangement events (and therefore immune response, it secretes IgM. When B cell activated – receives signal can rescue non-functional αchain); Requires Recombinase Activation to switch isotype. Different Isotype of antibodies have different biological Gene-1 and -2 enzymes (RAG-1 and -2) to recognise regions of germline effects DNA to ‘cut’ and to allow gene rearrangement to occur. Antibody Effector Function: Bind to surface of microbes, Induces Molecules that Participate in the TcR Immune Synapse (in both CD4 and phagocytosis, ADCC (Antibody Dependent Cellular Cytotoxicity), Lysis of CD8 T cells): TcR complex (TcR & CD3): CD3 (negatively charged AAs for microbes and induces inflammation. Immunoglobulin G (IgG) – G Heavy chain, 80% of Ab Serum. Long half life interaction with positively charged TcR transmembrane region) -> induces signalling pathways. of 7-25 days. Monomer – 2 identical antigen binding sites with 3 constant TcR co-receptors (CD4 & CD8): Binds to non-polymorphic regions, allows regions: IgG1, IgG2, IgG3. FC portion: Recruit complement (Complement TcR complex to be close to MHC (CD8 (35% of T cells) -> MHCI; CD4 (65% Dependant Cytotoxicity), Engage Fc receptor on macrophage, Engage DCs of T cells) -> MHCII) and neutrophils to enhance phagocytosis, Engage Fcg receptors on NK Cell adhesion molecules/integrins (on T cell –on APC or target cell): cells to induce target cell cytotoxicity (Antibody Dependant Cellular Mediate initial association; LFA-1 –ICAM1, CD2 –LFA-3 Cytotoxicity) Co-stimulatory signalling molecules (on T cell –on APC or target cell): Enhancement of Phagocytosis Allows further activation of T cells, CD28 –B7.1 or B7.2 (aka = CD80 and CD86 respectively) Importance of immunological synapse: Confinement of cytotoxic molecules (and cytokines); Donut structure created by adhesion molecules/integrins. Phagocyte expresses multiple Fc receptors. Crosslinking = induces phagocytes to phagocytose microbes. Antibody Dependent Cellular Cytotoxicity: Antibodies bind to surface of infected cell, NK cell (expressed Fc gamma receptors) crosslinked by antibodies. Antibody coated cell is killed. Complement Dependant Cytotoxicity (CDC): Antibody binds to surface, Fc portion recruits c1q; Recruitment of the complement cascade results in lysis of the target cell. IgA: A heavy chain, highest daily production, 10-15% of serum Ab. Predominant in external serum (breast milk, saliva, tears, mucus); prevents pathogens binding to mucosa, neutralisation of bacteria, viruses, toxins. Exists as a monomer in secretion (high avidity) IgE: E Heavy chain, low concentration in serum (0.002 of Ig); mediates allergy. 4 heavy chain C domains. Potent biological actions (low conc.) Provides immunity to helminths but responsible for allergy (hay fever, asthma, hives, anaphylaxis, peanut allergy, exzema) IgE sensitizes mast cells to antigen by binding FcRe. T cell mediated immunity: what happens during T cell: activation/priming/ differentiation; Transcription genes for IL-2 (important cytokine for T cell activation) and CD25 (α chain of the IL-2 receptor) are highly increased; IL-2 binds to IL-2 receptor on T cells to drive proliferation and differentiation; CD8+ T cells require CD4+ T cells for full activation Regulation of the T Cell Receptors: B cell expressing antibody specific for antigen, binds to it, switches, begins secreting IgE. Allergen re-enters body, crosslinks on surface of mast cell. Degranulation of mast cell, release of inflammatory mediators. IgD: D heavy chain, low serum concentration (0.2%), As a secreted component- has no biological effect. Primarily found on B cells as a part of BCR complex. Naïve B cell express both IgM and IgD. Germinal Centre Reaction Review: Activated B cells received 1st signal, B cell receptor recognition of antigen, travel to dark zone and proliferate. Dark zone B cells undergo somatic hypermutation, express BCRs with higher or lower affinity for antigen Cytotoxin (CD8+) T cells: kills cells infected with virus or intracellular pathogens. Perforins (molecules that make pores) and granzymes (cytolytic). Apoptosis pathways: Fas-FasL (CD95-CD178; Target cell – T cell) pathway. (DISC complex induced, Perforin and granzyme induced, Connection to the mitochondrial mediated apoptosis pathway) Helper (CD4+) T cells: Induces activation of other cells by providing signals (often in cytokine form); T helper 1/ T helper 17 cells (proinflammatory); T helper 2/T regulatory cells (anti inflammatory/regulatory) and T follicular helper cells (helps B cells) T regulatory cells: Mechanisms of action: Production of inhibitory cytokines, Cytolysis, Metabolic disruption, Modulation of DC maturation/function. Uses in therapy (depending on context of disease) Regulation of T cell response: CTLA-4 (inhibitor receptor competing with CD28 to bind to B7);Host defence peptide (often anti microbial) Cytokine: IL-2: Expansion of activated T cells, In adoptive cell therapy IL-10: Secreted by T regulatory cells: inhibitory cytokine.