Unit 1 - Immunology HYFS PDF

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Blue Highlight = Lecture Title Blue Text = Clinical Correlate Yellow Highlight = Subtopic Purple Text = Important Info Unit 1 - Immunology HYFS Blood and Hematopoiesis Blood Composition and Key Parameters Erythrocytes (Red Blood Cells): 45% of blood volume Structure: Anucleate, with virtually no organelles. Lifespan of ~120 days. Function: Primary oxygen/CO₂ transporter due to high hemoglobin content. o Contains a unique lattice of peripheral membrane proteins (e.g., spectrin, ankyrin) providing flexibility. o Hereditary spherocytosis/elliptocytosis: associated with mutations in α-spectrin, β- spectrin, ankyrin, and band 4.2 Leukocytes (White Blood Cells): 1-2% of blood volume, divided into granulocytes and agranulocytes. Granulocytes: Neutrophils, Eosinophils, Basophils. Agranulocytes: Lymphocytes, Monocytes. CBC provides a snapshot of cell counts: o MCV: Mean Corpuscular Volume, or average RBC size. o MCH: Mean Corpuscular Hemoglobin, average amount of hemoglobin in RBCs. o MCHC: Hemoglobin concentration relative to cell volume. o RDW: Red Cell Distribution Width, or variation in size and volume of RBCs. Plasma: 53-54% of blood volume Hematocrit Measures RBC percentage in blood volume. o Males: 39-50% o Females: 35-45% Leukocytes (WBCs) Neutrophils (PMNs) Structure: Multi-lobed nucleus (polymorphonuclear), with granules: 1. Azurophilic (Primary) granules: Myeloperoxidase-rich, important in killing pathogens. 2. Specific (Secondary) granules: Contain enzymes like lysozyme and collagenase. 3. Tertiary and Ficolin-1-rich granules Function: First responders in infection. o Diapedesis: Exit from circulation once the neutrophil is slowed by S-Lex (Sialyl Lewisx carbohydrate) interaction with selectins on the endothelium. Cells roll and adhere to endothelium, responding to IL-8 and using integrins to recognize cell adhesion molecules (e.g. ICAM-1) on endothelial cells o Phagocytosis: Targets recognized either directly or after opsonization, engulfed, and destroyed in phagolysosomes. o Synthesis of ROS (reactive oxygen species): ▪ Phagocyte oxidase system (Phox) – Multisubunit NADPH oxidase complex: uses NADPH from HMP shunt to form superoxide (O 2-) and hydroxyl (OH-) anions ▪ Myeloperoxidase system (MPO): uses heme as a cofactor to produce HOCl from H2O2 and Cl- ions Eosinophils Structure: Bilobed nuclei, granules include: 1  1. Azurophilic (Primary) granules: Lysosomal enzymes. 2. Specific (Secondary) granules: characterized by a crystalloid body and contain Major Basic Protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), enzymes like histaminase. Function: Defense against parasites (e.g., helminths), modulation of allergic reactions via cytokine release. Basophils Structure: Contain lots of granules staining with basic dyes, sometimes obscuring bilobed nuclei. o Contain heparin, histamine, and cytokines (exhibit IL-4, IL-13). Function: Involved in hypersensitivity and anaphylaxis, reacting to IgE-bound antigens. Lymphocytes Subtypes: o T Cells: Cellular immunity. CD3 positive. ▪ CD8+ Cytotoxic: TCR recognizes MHC-I bound antigens, release perforin and granzymes. ▪ CD4+ Helper: TCR recognizes MHC II-bound antigens, secrete IL-2, differentiate into Th1 or Th2. ▪ Regulatory (Treg): CD4+ cells suppress immune responses, maintaining tolerance. FoxP3 positive. ▪ γ/δ T Cells: Epithelial defense, do not recirculate. o B Cells: Humoral immunity, differentiate into plasma cells to produce antibodies. ▪ Exhibit CD9 (B1 and MZ B-cells), CD19, CD20, CD24 on cell surface (along with IgM, IgD, and MHC II molecules) o NK Cells: Recognize and kill virally infected and tumor cells via perforin and granzymes, secrete IFN-γ. ▪ Exhibit CD16, CD56 and CD94 on cell surface Monocytes Structure: Largest leukocytes, agranular, indented nucleus. Function: Circulate for ~3 days before tissue differentiation into: o Macrophages: Phagocytose and present antigens on MHC II. o Specialized Phagocytes: Osteoclasts, Kuppfer cells (liver), etc. Platelets and Megakaryocytes Megakaryocytes Polyploid cells undergoing endomitosis (DNA replication without cell division) under thrombopoietin influence. o Platelet demarcation channels form to shed platelets into circulation. Platelets Derived from megakaryocytes in bone marrow, life span of 7-10 days. o Normal count: 150,000-450,000/mm³. o Zones: 1. Peripheral: Glycocalyx for clotting. 2. Structural: Actin, myosin provide shape. 3. Organelle: Includes alpha, beta, gamma granules for coagulation and repair. 4. Membrane: Calcium storage for activation. Function: Clot formation, wound repair. 2 Hematopoiesis Erythropoiesis Stages: 1. Proerythroblast 2. Basophilic Erythroblast: Basophilic due to lots of free ribosomes that synthesize hemoglobin. 3. Polychromatophilic Erythroblast 4. Orthochromatophilic Erythroblast (Normoblast): eosinophilic due to high levels of hemoglobin in cytoplasm, cell division has ended. 5. Polychromatophilic Erythrocyte (Reticulocyte): Expelled nucleus, retains some organelles; matures in blood. 6. Erythrocyte Degradation: Heme and globin dissociate, globin is degraded and iron from heme is scavenged as hemosiderin or ferritin in spleen. Heme is degraded to bilirubin and excreted by gall bladder Granulopoiesis Stages: Prior to start, CMP (common myelocytic progenitor) gives rise to GMP (granulocyte/monocyte progenitor) under influence of GM-CSF, G-CSF and IL-3 1. Myeloblast: derived from GMP, azurophilic (primary) granules appear at this stage 2. Promyelocyte: Specific (secondary) granules appear at this stage. 3. Myelocyte: cell is now committed to a 1 of the 3 lineages (e.g. neutrophilic myelocyte, eosinophilic myelocyte, or basophilic myelocyte) 4. Metamyelocyte: Lineage specific granules can be distinguished 5. Neutrophil/Eosinophil/Basophil Monopoiesis Development: From GMP (granulocyte/monocyte precursor) → MoP (monocyte precursor) under IL-3 influence, with transcription factors PU.1, Egr-1. Function: Mature monocytes circulate, differentiate to tissue macrophages. Thrombopoiesis Stages: 1. CMP (common myelocytic progenitor) → MEP (megakaryocyte/erythrocyte progenitor) under influence of GM-CSF and IL-3 2. MEP → MKP (megakaryocyte committed progenitor) 3. MKP→ Megakaryoblast 4. Megakaryoblast undergoes endomitosis under influence of thrombopoietin to form a megakaryocyte, characterized by platelet demarcation channels Stem Cells Potency Totipotent: Can form ALL cell types, including both embryonic and extraembryonic (e.g., zygote, morula). Pluripotent: Can form any cell type within the embryo (all three germ layers) (e.g. inner cell mass of blastocyst, embryonic stem (ES) cells) Multipotent: Limited to cells within a particular lineage (e.g., hematopoietic, gut, skin stem cells). Unipotent: Restricted to producing a single cell type (e.g., testis stem cells, erythroblasts). 3 Stem Cell Types and Niches Epidermal Stem Cells: Found in the basal layer of the skin, with a turnover of about two months. Intestinal Stem Cells: Located in the crypt base, governed by Wnt signaling, differentiate as they move to the crypt top. Stem Cell Niche: A microenvironment regulated by other cells and the ECM, crucial for maintaining stem cell function and cell cycle regulation (e.g., Paneth cells in intestine). Embryonic Stem Cells (ES) Characteristics: Pluripotent, originating from the inner cell mass of the blastocyst. Key Factors: Oct4, Sox2, and Nanog; these maintain open chromatin and prevent differentiation. Pioneering Experiments Gurdon Experiment: discovered nuclei of differentiated cells could be reprogrammed to become pluripotent Yamanaka Experiment: Demonstrated that 4 transcription factors (Oct3/4, Sox2, Klf4, sometimes c-Myc) could induce pluripotency in differentiated cells, forming induced pluripotent stem (iPS) cells. Vaccines Type of Immunity: 1. Passive Immunity – protection transferred from another animal or human o E.g. breast milk, trans-placental Ab, blood sera, pooled Ig administration o Effective protection but disappears with time (no memory) ▪ Prevents disease after a known exposure ▪ Lessen symptoms of an ongoing disease ▪ Protection of immune deficient individuals ▪ Block actions of toxins or venoms 2. Active Immunity – Protection produced by the person’s own immune system o E.g. acquired natural infection, vaccination o Immunity lasts for many years, possibly a lifetime (memory formed) o Active Vaccination – exposes individual to immune-activating pathogens (attenuated or killed) or antigens Classification of Vaccines o Live/attenuated – organisms can still grow/replicate, but does NOT cause illness ▪ E.g. MMR, Varicella, Rotavirus, Influenza, etc. o Inactivated – organisms cannot replicate, and are inactivated by heat, chemicals, or purified immunogenic fraction ▪ E.g. toxoid, conjugated, subunit, and recombinant vaccines Live/Attenuated vs. Inactivated Vaccines Property Live Inactivated Dose of antigen Low High Number of doses Single or low Multiple, high Need for adjuvant No Yes Duration of immunity Long-term Short-term Antibody response IgG, IgA (if delivered via oral or IgM, IgG respiratory route) Cell-mediated response Good Poor 4  5 Elements of the Immune System: Histology, Cells, Tissues Lymphatics Overview: o Monitors body surfaces and internal fluid compartments. o Protects against foreign microorganisms, transformed cells, and harmful substances. Lymphocytes: cell type of lymphatic system and effector cells of the immune system responsible for immune responses Primary Lymphatic Organs = where lymphocytes differentiate into immunocompetent cells Thymus (T Cells): o Location: Superior mediastinum, anterior to the heart and great vessels. o Development: Originates from the 3rd pharyngeal pouch; receives stem cells from the bone marrow to develop immunocompetent T cells. o Lifecycle: Fully developed at birth; declines in activity post-puberty as tissue is replaced by adipose. o Structure: (outside→inside) ▪ Capsule with connective tissue. ▪ Trabeculae extending into parenchyma forming lobules that contain: ▪ Cortex: Rich in developing T cells. ▪ Medulla: Contains epithelioreticular cells and Hassall's corpuscles. ▪ Blood-Thymus Barrier: ▪ Tight junctions in endothelial cells prevent macromolecule entry. ▪ Macrophages in connective tissue phagocytose antigens escaping the capillary lumen. ▪ Epithelioreticular cells reinforce the barrier. Bone Marrow (B Cells): Site for B cell differentiation Secondary Lymphatic Organs Lymphatic Nodules: localized concentrations of lymphocytes; NOT ENCAPSULATED o Examples: Tonsils, Peyer’s patches in the small intestine. o Function: Initial immune response to pathogens. o Types: ▪ Primary Nodules: nodule consisting mainly of small lymphocytes. ▪ Secondary Nodules: have seen/are responding to an antigen ▪ Germinal Center: Activated by antigen; light staining. ▪ Mantle Zone: outer ring of small lymphocytes encircling the germinal center Lymph Nodes: o Function: Filter lymph, trap pathogens, and enable immune cell interaction. o Structure: ▪ Capsule: Dense connective tissue surrounding the node. ▪ Trabeculae: Dense tissue extensions into the node. ▪ Reticular Tissue: Meshwork of reticular cells and fibers for support. ▪ Cortex: ▪ Superficial Cortex: Lymphatic follicles rich in B cells. ▪ Deep Cortex: T cell zones without nodules. ▪ Medulla: inner portion consisting of reticular cells, lymphocytes, macrophages, and medullary cords/sinuses for draining. 5  o Lymphocyte Circulation: ▪ Entry into LN: ▪ Afferent Lymphatic Vessels: carry lymph from surrounding tissues into lymphatic sinuses ▪ High Endothelial Venules (HEVs): entry for ~90% of leukocytes into all lymphoid organs (except spleen); also located transiently at site of immune response ▪ Vascular Addressins on lymphoid tissue and homing receptors on leukocytes help guide them to the appropriate location ▪ Exit from LN: ▪ Efferent Lymphatic Vessels: where leukocytes leave LNs after once it has matured/encountered antigen Spleen: o Function: Filters blood and removes old/damaged RBCs; initiates immune responses to blood-borne antigens. o In addition to lymphocytes, it also consists of reticular cells, macrophages, dendritic cells o Structure: ▪ Dense CT capsule from which trabeculae extend into parenchyma ▪ White Pulp: Lymphocytes surrounding an artery ▪ Contains Central arteries: branches of the splenic artery that course through the capsule and trabeculae of the spleen ▪ Periarterial lymphatic sheath (PALS): lymphocytes that aggregate around the central artery ▪ Red Pulp: Splenic sinuses and cords, and macrophages that phagocytose damaged RBCs and recycle iron. Mucosal Lymphoid Tissue o Protect mucosal surfaces from pathogens ▪ Organized = Peyer’s patches, tonsils, adenoids, appendix (gut-associated lymphoid tissue – GALT) ▪ Less organized = aggregates of secondary lymphoid tissue where mucous membranes are found (e.g. bronchiole-associated = BALT) Stages of the Immune Response 1. Innate Immunity Characteristics: o Constant, fixed, and rapid (hours) response with limited specificity. o Keeps infection localized and involves fixed mechanisms like phagocytosis, inflammation, pathogen destruction (MAC) and cytokine secretion. o Pathogen recognition in a broad sense through pathogen associated molecular patterns (PAMPs) o Recruitment of effector mechanisms (e.g. complement, PMNs and other effector cells) to eliminate the pathogen o Damaged tissues and cells signal other cells to trigger innate responses ▪ Cytokines secreted by effector cells and complement factors stimulate vasodilation and an increase in vascular permeability to allow effector cells to leave blood and enter tissue 6  ▪ Cardinal signs of inflammation: redness, heat, swelling and pain Key Effector Cells: o Neutrophils (PMNs): ▪ Most abundant of WBCs ▪ Short-lived but highly effective phagocytes. ▪ Migrate to infected tissues, ingest bacteria, and are degraded by macrophages at the end of their lifespan o Eosinophils: ▪ Defense against intestinal parasites and helminths. ▪ Promote inflammation and allergic response. o Basophils: Regulation of immune responses. Found mainly in blood. o Mast Cells: ▪ Similar to basophils, however are NOT derived from granulocyte precursor ▪ Release histamine and other mediators during inflammation. ▪ Found mainly in tissues. o Monocytes/Macrophages: ▪ Monocytes are the circulating immature precursor to macrophages. ▪ Macrophages are general scavengers that phagocytose pathogens, present antigens to activate T cells, and initiate immune responses. o Natural Killer (NK) Cells: ▪ “Lymphocytes of innate immunity” ▪ Function in anti-viral and anti-tumor responses ▪ Secrete cytokines (e.g. IFN-λ) to enhance immune responses o Dendritic Cells: Bridge innate and adaptive immunity by presenting antigens to lymphocytes, initiating adaptive immunity 2) Adaptive Immunity Characteristics: o Specific, variable, and slow (days-weeks) response that improves over time. o Major difference between innate vs. adaptive immunity is that the lymphocytes in adaptive immunity undergo gene modification to express cell-surface receptors that are specific for individual pathogens o Antigen Presenting Cells (APCs) such as dendritic cells or macrophages capture antigen in the periphery and enter the LNs to present it to lymphocytes ▪ Clonal Selection: lymphocytes that recognize the pathogen are activated ▪ Clonal Expansion: selected pathogen-activated lymphocytes proliferate Key Cells: o B Cells: Produce antibodies (Ig) for neutralization and opsonization. Can also present antigen to T-cells. o T Cells: ▪ Helper T Cells (CD4+): Coordinate responses via cytokine release. ▪ Cytotoxic T Cells (CD8+): Destroy infected cells directly by inducing apoptosis. Types of Responses: o Th1: Targets intracellular pathogens; drives cytotoxic CD8+ T cell and macrophage activity and IgG production. o Th2: Targets extracellular pathogens; promotes humoral immunity (IgG, IgA, and IgE production). 7  o Th17: Fights bacterial infections by recruiting neutrophils. Antibody Types: o IgM: First response; less specific. o IgG: Highly specific, high affinity; dominates later responses. o IgA: Protects mucosal surfaces. o IgE: Defends against parasites and allergens. Innate Immunity Immediate Innate Immune Response (0-4 hours) Key Types of Reactions 1. Inflammation – recruitment and activation of leukocytes 2. Antiviral defense – NK cells and cytokines (primarily type 1 interferons (IFNs)) Complement System Overview Proteins constitutively produced by the liver. Roles: o Recruitment of Inflammatory Cells: done by C3a and C5a ▪ ↑Increased vascular permeability ▪ ↑Increased migration of monocytes and PMNs (C5a mediated) o Opsonization: Tagging pathogens (C3b, iC3b) for phagocytosis. o Pathogen Lysis: Via Membrane Attack Complex (MAC). Activation Pathways: (in order; starts with alternative pathway until inflammatory response develops and acute phase proteins (MBL and CRP) are produced in the liver. 1. Alternative Pathway: ▪ Initiated by pathogen surfaces. ▪ Formation of C3 convertase in this pathway requires Factor B and Factor D; C3 convertase is stabilized by properdin. ▪ Deficiencies/lack of properdin or Factor D → susceptibility to Neisseria. 2. Lectin Pathway: Activated when acute phase reactant MBL (mannose-binding lectin) binds mannose residues on pathogens. 3. Classical Pathway: ▪ Activated when CRP (C-reactive protein) or antibodies (IgG, IgM) binds pathogen surfaces. Complement System Details Pathway Convergence All 3 pathways result in complement fixation (cleavage of C3 → C3a and C3b): o C3a: Inflammatory mediator (anaphylatoxin) o C3b: Fixes to pathogens, aiding phagocytosis and forming C5 convertase. Membrane Attack Complex (MAC) Forms when C5 convertase is formed from the Alternative pathway Cascade of binding and conformation changes between C5-C9 leads to production of a transmembrane pore (MAC pore) on microbe Regulation Mechanisms Regulation of Complement Deposition: o Factors H & I: Limits C3 activation by binding C3b creating iC3b, preventing depletion. Regulation of Complement Activation: 8  o DAF (CD55) and MCP: Protect host cells by disrupting C3 convertase and allowing Factor I to convert Bb into iC3b. Regulation of MAC Formation: o CD59 (protectin) and homologous restriction factor (HRF) prevent assembly of C9 molecules on human cells Clinical Implications: o Genetic deficiencies in Factors H and I cause lack of C3 → increased ear infections, abscesses caused by encapsulated bacteria o Sialic acid (Staph. aureus & Strep. pyogenes) binds factor H and disrupts C3 convertase (C3bBb), providing resistance to complement (similar to MCP on human cells) o C3 deficiency: Susceptible to pyogenic infections. o MAC (C5-C9) deficiency: Susceptibility to Neisseria. o Paroxysmal nocturnal hemoglobinuria = results from DAF, HRF, and/or CD59 (protectin) deficiencies causing unregulated complement activation against RBCs → lysis and hemoglobinuria most pronounced in morning (“cola-colored urine”) Induced Innate Immune Response (4 hours – 4 days) Cellular Receptors Non-clonal, encoded in the germline with limited diversity Recognize PAMPs shared by classes of microbes o Lipopolysaccharide (LPS) receptor = CD14 → stimulates phagocytosis o Toll-like receptors (TLRs) → stimulate cell signaling/cytokine production Examples to remember: o TLR4: recognizes LPS ▪ LPS binding by TLR4 activates transcription factor NFkB (NFkB activates genes for pro-inflammatory cytokine secretion) ▪ TLR4 is crucial for defense against gram-negative bacteria (TLR4 deficiency = ↑susceptibility to septic shock) ▪ X-linked hypohydrotic ectodermal dysplasia and immunodeficiency (NEMO deficiency) = caused by lack of functional IKKλ subunit due to mutation in NEMO gene, leading to an inability to activate NFkB, and thus pro-inflammatory cytokine secretion ▪ Developmental defects, fine/sparse hair, conical or missing teeth o TLR9: recognizes viruses Inflammatory Cytokines Activated macrophages induce local inflammatory state Functions: o IL-1β, TNF-α: Increase vascular permeability (effector cells/molecules can enter tissue) o IL-6, TNF: Enhances metabolism and local temperature. IL-6 also acts on liver to promote acute phase protein production. o CXCL8, IL-8: Recruits neutrophils to site of infection o IL-12: recruits/activates NK cells that in turn stimulate macrophages’ response Cytokines (primarily TNF-α) induce expression of ECAMs (endothelial cell adhesion molecules) that have corresponding leukocyte ligands o Leukocyte Adhesion Deficiency (LAD): ▪ Usually associated with mutation in CD18 (Beta2 Integrin) → impaired neutrophil translocation 9  ▪ Presents with increased neutrophils in periphery (blood) but NOT in the actual infected tissue Phagocytosis and Respiratory Burst Respiratory Burst: the increase in oxidative activity to produce reactive oxygen species (ROS) within immune cells (e.g. PMNs and macrophages) when they phagocytose microbes o NADPH Oxidase = main enzyme involved o Macrophages also produce RNS (reactive nitrogen species) Chronic Granulomatous Disease (CGD) = caused by failure of phagocytes to produce ROS (superoxide and H2O2) due to lack of functional NADPH oxidase o Inability to eradicate catalase positive bacteria and fungi such as Aspergillus o Formation of granulomas containing bacteria/fungi due to other immune mechanisms, but not eradicated Lysosome Granules in PMNs Primary (Azurophilic): Myeloperoxidase Secondary (Specific): Lactoferrin Tertiary: Gelatinase (sequesters iron) Bloodstream Evidence of Inflammation: Leukocytosis: “Left shift” or increased presence of immature PMNs (band forms) mobilized into blood Acute Phase Proteins and Viral Responses Acute Phase Proteins IL-1, IL-6, and TNF-α secretion by macrophages stimulates production of acute phase proteins in the liver (mainly IL-6) and raises body temperature Acute Phase Proteins & Function: o CRP: indirectly binds bacterial LPS of gram-negative bacteria and S. pneumoniae C- polysaccharide; activates classical pathway o MBL: Activates lectin pathway; deficiency → susceptibility to N. meningitidis, respiratory tract infections o Serum amyloid A – Activates cells to produce inflammatory cytokines o Fibrinogen: Aids in coagulation. Responses to Viruses TLRs recognize intracellular pathogens in the cytoplasm and initiate IRF activation and NFkB activation o IRF activation → expression of genes encoding Type I interferons: IFN-α, IFN-β o NFkB activation → expression of genes encoding proinflammatory cytokines Type I Interferons (IFN-α, IFN-β): o Induce antiviral states via autocrine and paracrine secretions ▪ Promote degradation of viral RNA, induce resistance to viral replication ▪ Block protein synthesis. o Activate NK cells to kill virus-infected cells o Cause “Flu-like symptoms” (fatigue, fever, myalgia, anorexia, headache, somnolence) NK Cells: o Recognize lack of self-antigen and destroy virus-infected cells and/or stressed cells (innate counterpart of T cells) o Secrete IFN-γ (type II interferon) to activate macrophages. o CD56 prototypic marker 10  o CD16 marker for ADCC (Antibody Dependent Cellular Cytotoxicity) o Deficiency → recurrent viral infections (especially Herpesviridae) NK Cells Interact with Dendritic Cells: o NK cells are scarce/outnumbered by dendritic cells → drives dendritic cells to mature into the form that initiates adaptive immunity o NK cells are abundant/outnumber the dendritic cells (infection is under control) → NK cells kill the dendritic cells to limit responses Antibodies and B-Cell Diversity B-Cell Development: 1. Hematopoietic Stem Cell → Pro B Cell → Pre-B Cell → Immature B Cell → Mature Naïve OR Memory B Cell 2. Clonal Selection: o Resting naïve B cells have unique receptors, representing potential clones o Pathogens enter and “select” specific clones for activation and proliferation o Antibodies neutralize pathogens, mediate opsonization, and promote clearance Antibody (Ab/Ig) Structure: Composed of heavy (H) and light (L) chains Variable (V) Region: Determines antigen specificity Constant (C) Region (especially Fc): Governs effector functions (e.g. blood antibody vs. secreted across mucosal barrier, etc.) Functional regions: o Fc (Fragment, Crystallizable): Recognized by Fc receptors on immune cells (e.g., eosinophils, NK cells) o Fab (Fragment, Antigen-Binding): Antigen binding region Classes of Immunoglobulins (Ig): 1. IgG, IgM, IgD, IgA, IgE 2. Polyvalent Response: Multiple antibodies bind different epitopes on an antigen 3. Monovalent Response: A single antibody binds repeated epitopes 4. Affinity & Avidity: o Affinity: Strength of individual binding (e.g., IgG > IgM) o Avidity: Strength of multiple bindings (e.g., IgM > IgG) Hypervariable Regions: Regions within the paratope of an antibody generated through somatic hypermutation that increase antibody-antigen affinity Antibodies in Therapy: Agammaglobulinemia Treatment: individual lacks antibodies in general, treat with intravenous immunoglobulin (IVIG) (passive immunity) given regularly based on antibody’s half-life Monoclonal Antibodies: Target single epitopes; used for detection/quantification of antigens for diagnosis, and treatment for autoimmune diseases/cancers Humanization: o Chimeric: Mouse V + Human C region (e.g., "-ximab"). o Humanized: Mouse CDR cloned into human V region (e.g., "-umab"). o Fully Human: Human Ig genes in mice = hRIg Antibody Generation (Before Antigen Encounter): 11  Somatic Recombination: Random recombination of V(D)J gene segments creates diversity o Recombination enzymes produce additional diversity in the antigen-binding site o Developing and naïve B cells use alternative mRNA splicing to display both IgM and IgD o Random recombination of gene segments: 1. Light Chain: VL + JL → VJ 2. Heavy Chain: DH + JH → DJ; DJ + VH → VDJ 3. Successful κ-light chain recombination prevents λ-chain formation o Process is directed by recombination signal sequences (RSS) which are recognized by RAG proteins (e.g. RAG-1 & RAG-2) that carry out the V(D)J recombination Junctional Diversity: Another mechanism for increasing the diversity of specificities in Ig development o RAG produces P-nucleotides, which were originally part of the germline DNA o TdT (terminal deoxynucleotidyl transferase) adds N-nucleotides, which were NOT originally templated in the germline DNA o CDR3 (hypervariable region) has the greatest diversity o Drastically increases antibody diversity, but at cost of increasing damaging changes (e.g. premature or inserted stop codon) Naïve B Cells: Co-express IgM and IgD (each with different C region) via alternative mRNA splicing Display requires Igα and Igβ (signaling molecules) o Without these molecules, surface display does NOT occur and B cell cannot undergo maturation after exposure to an antigen Complete B-Cell Receptor (BCR) = IgM/Igα/Igβ B-Cell Development Phases of B Cell Development In the Bone Marrow: 1. Phase 1 - Repertoire Assembly: Generation of diverse and clonally expressed B-cell receptors (BCRs) to establish potential antigen recognition capabilities. 2. Phase 2 - Negative Selection: Elimination, inactivation, or receptor editing of B-cell receptors that bind to self-antigens to prevent autoimmunity. In the Lymphoid Tissue: 3. Phase 3 - Positive Selection: Encourages immature B cells to transition into mature B cells by interacting with stromal signals in secondary lymphoid tissues. 4. Phase 4 - Searching for Infection: Mature B cells continuously recirculate between lymphatic and circulatory systems, scanning for antigens. 5. Phase 5 - Finding Infection: Antigen-specific activation and interaction with T FH cells leads to clonal expansion in secondary lymphoid tissues. 6. Phase 6 - Attacking Infection: Differentiation into antibody-secreting plasma cells and memory B cells facilitates immune defense. This is also largely dependent on T-cell interactions. Phase 1: Pro-B Cells Develop in the Bone Marrow Pro-B Cell Stage Markers: Progression through markers CD34, CD10, CD127, and CD19 identifies commitment to B-cell lineage. 12  Rearrangements: Heavy chain (HC) gene rearrangement begins with D-J, followed by V-DJ recombination. Fate: Successful rearrangement signals survival and transition to the Pre-B cell stage; failure triggers apoptosis (~50%). Key Regulators: RAG recombinase facilitates recombination; Pax-5 ensures lineage commitment. Pre-B Cell Stage Pre-B-Cell Receptor: Surrogate light chain (λ5 + VpreB) checks heavy chain functionality. Key Events: o Successful HC assembly is paired with Igβ/Igα to mediate survival signals. o LC (κ locus) rearrangement begins, with multiple attempts allowed, resulting in ~85% success rate. o Clinical Correlation: X-linked agammaglobulinemia arises from defective Btk, halting progression at this stage. Outcome: Fully functional pre-B cells progress to immature B cells expressing a BCR. Phase 2: Self-Tolerance Negative Selection Central Tolerance o Process: Immature B cells in the bone marrow are tested against self-antigens. o Outcomes: ▪ Non-reactive cells migrate to peripheral circulation, expressing IgM and IgD. ▪ Reactive cells undergo receptor editing, focusing on LC rearrangements. ▪ Persistent reactivity results in clonal deletion via apoptosis. Peripheral Tolerance: Autoreactive cells that escape the bone marrow and end up in circulation are rendered anergic, characterized by high IgD expression and a short half-life (1-5 days). Phase 3: Maturation and Types of B Cells Immature B cells that have passed negative selection migrate to secondary lymphoid tissues and are selectively promoted to become mature B cells. Mature Naïve B Cells Follicular (FO) B Cells: Predominantly found in lymphoid follicles; long-lived, T-dependent responders to diverse protein antigens. (Express IgM and IgD, LOW level of CD21) Marginal Zone (MZ) B Cells: Localized to the spleen’s marginal zone; specialize in rapid T- independent responses to blood-borne pathogens. (Express IgM only, HIGH level of CD21) B-1 Cells: Found in mucosal and peritoneal sites; express CD5, IgM, and exhibit self-renewal. B-2 Cells: conventional B cells, few express CD5 Phase 4: Recirculation Pathway: Lymph nodes (exits via efferent lymphatic vessels) → thoracic duct (empties into left subclavian vein) → spleen (via white pulp areas, pass into sinusoids of red pulp) → exits spleen via splenic vein → re-enters lymph nodes and MALT via HEVs Phase 5: Fate of the Mature Naïve B Cell Antigen Encounter Overview 13  Naïve B Cells: Interact with antigens presented by follicular dendritic cells (FDCs). Activated B Cells: Present processed antigens via MHC-II to TFH cells, forming cognate pairs. Outcomes: o Immediate IgM production in medullary cords (primary focus). o Germinal center formation (secondary focus) fosters affinity maturation and isotype switching. Activation and Differentiation Requirements for B Cell Activation o Signal 1: Antigen-mediated crosslinking of BCR initiates activation and tyrosine kinase signaling cascade. o Signal 2: B cell complement co-receptor (CD19, CD21, CD81) binds to complement components C3b then C3d. o Signal 1 + Signal 2 = Synergy: BCR and co-receptor cooperate in B-cell activation by a pathogen or soluble antigen and amplifies signaling. o Clinical Correlation: Individuals who lack B cell co-receptor due to defective CD19 or CD81 have: ▪ Low levels of antibody ▪ Almost no isotype switching ▪ Generally poor B cell responses to infection and vaccines T-Independent Antigens o Nature: Multivalent antigens (e.g., polysaccharides) stimulate direct BCR activation (primarily MZ B cells and B-1 cells). o Outcome: Produces short-lived IgM responses with limited memory generation. T-Dependent Antigens o Nature: Protein antigens requiring T cell help to stimulate B cells (primarily follicular B cells; B-2 cells). o Outcome: Produces long-lived B cells that secrete Ig other than IgM due to isotype (class) switching. Also results in affinity maturation and generation of memory B cells. o Patients with complete DiGeorge syndrome lack a thymus and have almost no T cells in circulation, therefore they: ▪ Have normal B cell numbers, but no effective antibody response against most antigens ▪ Suffer from opportunistic infections, usually die from infection within first 2-3 years of life unless given thymus transplant Formation of Germinal Centers Primary Focus: o Activated B cells proliferate in the medullary cords, producing low-affinity IgM antibodies as part of the immediate immune response. Migration to Follicles: o Some activated B cells move to primary lymphoid follicles (secondary focus), transforming them into germinal centers. Germinal Center Reaction Dark Zone: Proliferation and Hypermutation Centroblast Activity: o Rapid proliferation of activated B cells forms a dense population called centroblasts. o Activation-induced cytidine deaminase (AID) introduces mutations in the variable regions of immunoglobulin (Ig) genes. Purpose of Hypermutation: 14  o Generates diversity in the antigen-binding region, enhancing the possibility of producing higher-affinity antibodies. Light Zone: Selection and Differentiation Antigen Presentation: o Centrocytes (mutated B cells) display their newly formed Ig on their surface. o They compete for antigen presented by follicular dendritic cells (FDCs). TFH Cell Support: o Centrocytes with high-affinity Ig receive survival signals from TFH cells through CD40- CD40L interactions and cytokines. o Low-affinity or autoreactive B cells undergo apoptosis, ensuring quality control. Phase 6: Outcomes of Germinal Center Reaction 1. Plasma Cells: o Terminally differentiated B cells that secrete high-affinity antibodies. o May migrate to bone marrow, medullary cords, or mucosal sites for long-term antibody production. o Signaled by IL-10. 2. Memory B Cells: o Long-lived cells capable of rapid reactivation upon antigen re-exposure. o Provide immunological memory essential for secondary immune responses. o Signaled by IL-4. Somatic Hypermutation and Class Switching Mechanism of Hypermutation AID Function: o AID converts cytosine to uracil in single-stranded DNA, leading to mutations during repair. o These mutations are concentrated in the complementary determining regions (CDRs) of HC and LC genes (especially CDR3 of H chain) Affinity Maturation: o Centrocytes with mutations that enhance antigen binding are positively selected, ensuring that antibodies become progressively more effective. Class Switching Process: o AID mediates recombination between switch regions upstream of different constant region genes. o Cytokines from TFH cells dictate the class of antibody produced. Outcome: Allows antibodies to adapt their effector functions (e.g., neutralization, opsonization, complement activation). Clinical Correlations Deficiencies in Germinal Center Activity 1. CD40L Deficiency (Hyper-IgM Syndrome): o X-linked syndrome, occurs only in males. o Prevents class switching, leading to elevated IgM levels and susceptibility to infections. 15  o Symptoms: elevated IgM, lack of germinal centers & swollen lymph nodes 2. AID Deficiency: o Autosomal recessive condition occurring in males & females. o Disables somatic hypermutation and class switching (Hyper-IgM Syndrome), causing poor antibody affinity and response diversity. o Symptoms: Lymphadenopathy (enlarged LN) containing very large germinal centers with no affinity-matured, class-switched centrocytes produced Lymphoma Risk B-Cell Lymphomas: o Germinal centers—sites of high proliferation and mutation—are common origins of malignancies such as follicular lymphoma. Autoimmune Implications Self-Reactive Antibodies: o Errors in the selection process during affinity maturation can lead to the survival of autoreactive B cells, contributing to autoimmune disorders. T-Cell Antigen Recognition Structure of T Cell Receptor (TCR) Vα and Vβ Domains: Contain Complementarity-Determining Regions (CDRs 1, 2, 3). CDR-3 is highly variable and found in Vβ. Antigen Binding: Vα Vβ bind the MHC-peptide ligand complex. Comparison to BCR: Similar to the Fab region of immunoglobulins, but TCRs are not secreted. Rearrangement of Antigen Receptor Genes Involves recombination of gene segments to create diversity and specificity for antigen recognition. Key processes: Somatic recombination of germline gene segments (α and β chains). o V (Variable), J (Joining), C (Constant), and D (Diversity) (D only in TCR β chain) segments. Mechanism of V(D)J Recombination 1. Synapsis: Bringing together recombination signal sequences by RAG-1 & RAG-2. 2. Cleavage: Double strand breaks by RAG enzymes. 3. Coding End Processing: Exonucleases and TdT (Terminal Deoxynucleotidyl Transferase) add or remove bases, generating diversity. 4. Joining: Ligation by double-strand break repair enzymes. Classes of TCRs 1. α:β T Cells: 95% of T cells. 2. γ:δ T Cells: 5% of T cells. TCR Complex Components CD3-ζ Complex: Facilitates intracellular signaling. CD3 Subunits: Non-covalently associated with TCR, homologous subunits (γ, δ, ε) ITAM: conserved sequence motif also in BCR and associated with TCR of NK-T cells MHC Antigen Presentation MHC Molecules and TCR Interaction TCR-MHC Peptide Complex: Essential for antigen recognition. o CD4+ T Cells: Recognize antigens in MHC class II. 16  o CD8+ T Cells: Recognize antigens in MHC class I. Structure of MHC Molecules Class I MHC: o Heterotrimer: α-heavy chain, β2-microglobulin, peptide. o Peptide binding cleft accommodates peptides (8-11 amino acids). Class II MHC: o Heterotrimer: α-chain, β-chain, peptide. o Open cleft allows binding of longer peptides (up to 30 amino acids). CD4 and CD8 Roles in MHC Interaction with TCR CD4: Monomeric receptor on helper T cells; binds β2 domain of MHC class II. CD8: Heterodimer on cytotoxic T cells; binds α3 domain of MHC class I. Antigen Processing and Presentation Pathways 1. Endocytic Pathway (MHC Class II): o Extracellular proteins internalized into vesicles. o Proteins processed in endosomal/lysosomal vesicles. o Peptides bind MHC II and are displayed on the cell surface. 2. Cytosolic Pathway (MHC Class I): o Intracellular proteins degraded by proteasomes. o Peptides transported to ER, bind MHC I, and are expressed on the surface. o Exception: Cross-presentation of exogenous antigens presented via MHC I (e.g., viruses). Professional Antigen-Presenting Cells (APCs) Dendritic Cells (DCs): Activate naïve T cells and facilitate their differentiation into effector cells. Macrophages and B Cells: Engaged by effector T cells to produce cytokines or antibodies. MHC Genes Polymorphism: MHC genes are highly polymorphic, crucial for immune diversity. Expression: Co-dominantly expressed (offspring possess both paternal and maternal components), influencing organ transplant compatibility (siblings usually better donors than parents). o Class I MHC: Expressed on all nucleated cells. o Class II MHC: Expressed on APCs. T-Cell Development Overview: Thymus: Site of T-cell development and involution after year 1 of life; T-cell immunity is maintained by long-lived, self-renewing mature T cells. T-cell Lineage Commitment: Occurs before TCR gene rearrangement; interaction with thymic stromal cells and IL-7 drives differentiation. Positive and Negative Selection: Ensures self-MHC restriction and tolerance. Migration: Precursors travel from bone marrow → thymus; mature T cells exit to secondary lymphoid tissues. Thymocyte Fate: 98% undergo apoptosis due to unsuccessful TCR rearrangement. Successful rearrangement leads to either γδ TCR or pre-TCR (αβ TCR) formation. TCR Gene Rearrangement: Double-negative (DN) thymocytes: Rearrange γ, δ, β loci. o Successful γδ rearrangement halts β chain rearrangement; γδ T cells mature without stringent selection. 17  o Successful β chain rearrangement forms pre-TCR, halts γδ rearrangement, and initiates CD4/CD8 expression. Pre-TCR (β chain + pTα): Assembles with CD3 complex for signaling. o RAG proteins ensure single β chain rearrangement. o Four attempts possible for β chain rearrangement; 80% success rate. α-Chain Rearrangement: Pre-T cells proliferate, become double-positive (CD4+CD8+), and rearrange α chain. Multiple rearrangement attempts possible; eliminates δ chain locus. Selection Processes: Positive Selection: Occurs in thymic cortex. o TCR binds self-MHC-peptide complexes on cortical epithelial cells. o Weak/no binding may be rescued through continued α chain rearrangement. o Outcome: Single-positive cells (CD4 or CD8) depending on MHC class interaction. Negative Selection: Occurs in thymic medulla; eliminates self-reactive T cells. o Moderate binding: T cells survive. o Tight binding: T cells undergo apoptosis. o AIRE transcription factor promotes tolerance by inducing tissue-specific antigen (TSA) expression in medullary epithelial cells. Peripheral Tolerance: Prevents autoimmunity by controlling self-reactive T cells: o Anergic state or activation-induced cell death for self-reactive cells. o Regulatory T cells (Treg) suppress autoreactive T cells. ▪ Express CD25 (IL-2Rα) and FoxP3. ▪ Secrete inhibitory cytokines TGF-β1 and IL-10. Clinical Correlations: Central Tolerance Defects: o AIRE dysfunction: Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy. ▪ Symptoms: Endocrine gland destruction, chronic candidiasis, ectodermal abnormalities (teeth, hair, fingernails). Peripheral Tolerance Defects: o FoxP3 deficiency: Immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome. ▪ Rare, found in boys (no abnormalities at birth) ▪ Symptoms: Intractable diarrhea, type 1 diabetes, eczema within first months of life; fatal without HSC transplant. T Cell-Mediated Immunity Routes of Antigen Entry Microbial antigens enter via skin, GI, and respiratory tracts or bloodstream (captured in the spleen). Antigens are transported to draining lymph nodes for processing and presentation by dendritic cells (DCs). Cross-Presentation: DCs ingest virus-infected or tumor cells and present antigens to CD8+ T cells via MHC class I. Activation of T Cells: Sequence of Events 1. Antigen Recognition → 2. Activation → 3. Clonal Expansion → 4. Differentiation → 5. Effector Functions 18  o CD4+ Helper T Cells (TH) o CD8+ Cytotoxic T Cells (CTL) Ligand-Receptor Pairs in T Cell Activation Functional Groups: o Specificity: CD4, CD8, MHC class I/II o Signaling: CD3, B7-CD28 o Adhesion: LFA-1, ICAM-1 CD4 and CD8 act as co-receptors for signaling and adhesion. Signal Transduction in Naïve T Cell Activation 1. Signal 1: Initiated by TCR complex after antigen recognition. o Requires cross-linking of two TCRs. o CD3 and ζ chains mediate signaling. 2. Signal 2 (Co-Stimulation): B7-CD28 interaction. o Required for T cell proliferation and differentiation. o Negative regulators: CTLA-4 and PD-1 inhibit T cell activation. Formation of the Immunological Synapse Components: Central (cSMAC) and Peripheral (pSMAC) Supramolecular Activation Clusters. Facilitates prolonged signaling, cytokine delivery, and signaling molecule turnover. Intracellular Signaling Pathways Key Events: Receptor clustering, ITAM phosphorylation, adapter protein activation. Pathways: Calcineurin, Protein Kinase C, Ras-MAP Kinase. Outcome: Activation of genes in naïve T cells. Types and Phases of Cell-Mediated Immunity (CMI) CD4+ T Cells: Stimulate macrophage activation and inflammation. CD8+ T Cells: Kill infected host cells and target intracellular microbes. Phases: Induction → Migration → Effector Function. Role of IL-2 in T Cell Proliferation Naïve T cells express low-affinity IL-2 receptors. Activated T cells express high-affinity IL-2 receptors and secrete IL-2. IL-2 promotes clonal expansion and survival. CD4+ T Cell Subsets and Functions TH1 Cells: o Promote macrophage activation (via CD40L-CD40 interactions) and IgG production. o Key cytokine: IFN-γ. TH2 Cells: o Support humoral immunity and IgE production, important for mast cell and eosinophil activation. o Key cytokines: IL-4, IL-5. TH17 Cells: o Induce inflammatory responses; recruit PMNs. o Key cytokines: IL-17, IL-22. Tfh Cells: o Regulate antigen-specific B cell development. o Key cytokines: IL-21, IL-6. Treg Cells: o Includes CD4+, CD25+ (IL-2Rα), FoxP3+ o Inhibit self-reactive T cells; promote tolerance. o Key cytokines: TGF-β, IL-10. 19 Effector Mechanisms of CD8+ T Cells (CTLs) Killing Mechanisms: o Perforin/granzymes: Mediate apoptosis. o Fas/FasL interaction: Alternate apoptosis pathway. CTLs secrete IFN-γ and TNF-α to enhance immune responses. Evasion of CMI by Microbes Strategies include inhibition of phagolysosome fusion (e.g., Mycobacteria), antigen presentation and class I MHC disruption (e.g., HSV, CMV), and decoy molecules (e.g., EBV). Summary of CD4+ T Cell Effector Functions Subset Key Cytokines Functions TH1 IFN-γ Macrophage activation, IgG production TH2 IL-4, IL5 Humoral Immunity, eosinophil activation TH17 IL-17, IL-22 Inflammation, epithelial barrier maintenance TFH IL-21, IL-6 B cell development regulation Treg TGF-β, IL-10 Inhibition of self-reactive T cells, tolerance Memory and the Secondary Response Primary vs. Secondary Immune Response Secondary Response: Occurs faster and with greater magnitude than the primary response, triggered by re-exposure to the same antigen. Can occur weeks or years later. This heightened response is a hallmark of adaptive immunity, unique to B and T cells. Memory T Cells Longevity: Memory T cells retain antigen-specificity for years or a lifetime, enabling rapid and enhanced recall. Rapid Response: A higher number of memory T cells compared to naïve T cells for the same antigen ensures faster clonal expansion. Heterogenous: Derived from both CD4 and CD8 T cells. Maintained through low-level proliferation stimulated by IL-7. Bifurcated Differentiation: o Central Memory T Cells: ▪ Express CCR7 and L-selectin (CD62L) to home in on lymph nodes. ▪ Have limited direct effector functions but proliferate rapidly upon activation. o Effector Memory T Cells: ▪ Lack CCR7 and CD62L, allowing migration to peripheral tissues. ▪ Produce cytokines such as IFN-γ and execute effector functions; show limited proliferation. CD45 Isoforms and Differentiation Alternative splicing of the CD45 gene distinguishes T cell states: o Naïve T Cells: Include specific exons. o Effector and Memory T Cells: Exons are excluded, reflecting their advanced activation state. Primary vs. Secondary B Cell Responses Secondary Response Enhancements: o Increased numbers of antigen-specific B cells. o Elevated antibody production with isotype switching (e.g., from IgM to IgG). o Enhanced affinity maturation, improving antigen binding strength. 20  Antibody Dynamics: o IgM: Initiates the response but remains low-affinity. o IgG: Predominates in secondary responses; affinity and levels increase with repeated antigen exposures. Memory B Cells: o Lack FcRyIIB1, an inhibitory receptor present in naïve B cells, ensuring memory B cells are preferentially activated. Clinical Application: Hemolytic Anemia of Newborns Context: RhD- mother carrying RhD+ fetus. Prevention: Infusion of anti-Rh IgG prevents the maternal immune system from initiating a primary response against RhD antigens. Mechanism: Primary immune response to RhD is inhibited by presence of RhD- specific IgG, protecting fetal erythrocytes. Primary and Secondary Immunodeficiencies Common Features of Immunodeficiencies Increased susceptibility to infections: recurrent, chronic, or caused by unusual microbes. Incomplete response to treatment: adverse live vaccine reactions, failure of immunizations. Cancer risk: higher susceptibility to specific types. Primary Immunodeficiencies Etiology: Congenital defects in critical immune-related genes; inherited as dominant, recessive, or X-linked. Prevalence: Rare (1 in 500 in the U.S.). Key Features Immune System Impact: Can affect maturation, activation, or components of innate/adaptive immunity. Severity & Function: o Single Function Defects: e.g., antibody deficiency (→ extracellular bacterial infections), phagocyte dysfunction (→ bacterial and/or fungal infections). o Multiple Functions: e.g., T-cell defects (→ severe combined immunodeficiency [SCID]). Specific Disorders 1. X-linked Agammaglobulinemia: o Absent B cells, recurrent bacterial (pyogenic, encapsulated microbes) infections, tissue damage (e.g., bronchiectasis). o Missing: Bruton’s tyrosine kinase (Btk); No pre-B cells, no mature B cells, no Ig o Treatment: Lifelong IVIG (passive immunity) 2. Hyper-IgM Syndrome: o Defects in CD40L, CD40, or AID; impaired class switching. o X-linked Hyper-IgM Syndrome = CD40L deficiency on T cells Loss of T cell help for IgM class switching Treatment: Lifelong IVIG o AID (AICDA) Deficiency in B cells = Autosomal Recessive Hyper IgM Syndrome = decreased Ab class switching Treatment: IVIG, GM-CSF to mobilize WBCs, HSC transplant. 21  3. Selective IgA Deficiency: o B cells expressing surface IgA are present, but fail to develop into plasma cells or memory B cells o Often asymptomatic; recurrent infections, autoimmune disorders, allergic disorders. o Treatment: Manage comorbid conditions, IVIG with caution. 4. Common Variable Immunodeficiency (CVID): o Impaired B cell differentiation with defective Ig production o Reduced IgG and IgA/IgM; recurrent infections, autoimmune diseases. o Treatment: IVIG, antibiotics. 5. SCID: o Both T and B cell dysfunction; life-threatening in infancy. o Multiple Causes: X-linked SCID (IL-2Rγ deficiency) = 50% of all cases Adenosine deaminase (ADA) deficiency = 20% of all cases JAK3 tyrosine kinase mutation = ~12% of all cases Numerous other mutations (ILRA mutation, DCLRE1C (Artemis) mutation, RAG1 and/or RAG2 mutation (classic or leaky Omenn Syndrome), ZAP-70 mutation, complete DiGeorge Syndrome) o Typical SCID: Recurrent severe infections, chronic diarrhea, failure to thrive Absence of thymic shadow, low T cell count o Treatment: HSC transplant, isolation, IVIG, enzyme replacement (depends on cause) 6. DiGeorge Syndrome: o Thymic dysgenesis; multiple organ defects (heart, head and neck, parathyroids). o Treatment: Thymus transplant. Innate Immune Defects 1. Complement Deficiencies: o Immune Complex Disease Deficiency/defective C1, C2, or C4 → decreased C3b and C4b → decreased removal of immune complexes (ICs) → deposition of ICs in tissue → autoimmunity (via subsequent complement activation and damage from phagocytes) o Complement Regulatory Proteins (CRP) Defects Decreased DAF, CD59, HRF Defects/loss of PIG-A where CRPs are anchored preventing MAC assembly ▪ Results in Paroxysmal nocturnal hemoglobinuria 2. Phagocyte Dysfunction: o Leukocyte Adhesion Deficiency Disease-1 (LAD-1) = CD18 Integrin Deficiency Prevents PMNs, monocytes and macrophages from leaving circulation to enter the target tissues o Chronic Granulomatous Disease = Loss of NADPH Oxidase Activity (No oxidative burst) o Chediak-Higashi Syndrome = mutation in lysosomal trafficking regulatory protein Results in defective endosomal-lysosomal fusion, defective phagocytosis 3. Cytokine Signaling Defects: 22  o Loss of IFNγ Receptor 1 = major cytokine to activate macrophages to phagocytose and kill bacteria (produced by NK cells, Th1 CD4+ T cells and CD8+ cytotoxic T cells) o IL-12 Receptor Deficiency = Loss of activation of NK cells, macrophages and T cells Secondary Immunodeficiencies Causes: o Non-infectious: Malnutrition, cancer, immunosuppressive therapies (iatrogenic). o Infectious: HIV/AIDS (loss of CD4+ T cells), severe sepsis. Principles of Hypersensitivity and Autoimmunity Type I Hypersensitivity: IgE-Mediated Immunity and Allergy The Good: IgE and Helminths IgE effectively defends against helminth infections by collaborating with mast cells, basophils, and eosinophils. Antigen-mediated cross-linking of IgE on these cells induces degranulation and initiates inflammatory responses that help expel parasites. The Bad: Allergies in Absence of Parasites In the absence of parasites, harmless antigens (allergens) may trigger IgE-mediated responses, causing allergies. Increased allergy prevalence is associated with improved hygiene and reduced exposure to parasitic infections Major Players in Allergies IgE, Mast Cells, Eosinophils, Basophils: Work synergistically in allergen response. Tfh Cells: Specialized CD4+ T cells produce IL-4 and IL-13, driving IgE production. Reactions: o Immediate Phase (Minutes): Rapid release of vasoactive amines and lipid mediators from activated mast cells. o Late Phase (2-4 Hours): Mediated by inflammatory cytokines. Sensitization and Activation First exposure: Mast cells irreversibly bind IgE via FcεRI, becoming sensitized. Subsequent allergen exposure triggers degranulation, releasing mediators like histamines, proteases, and TNF-α. These induce vascular permeability, smooth muscle contraction, and mucosal secretion. Common Allergens Dust mites (Derp1 allergen) and genetic predisposition (atopy) significantly contribute to allergic diseases. Maternal IgE in amniotic fluid influences fetal immune responses. Phases of IgE-Mediated Reactions Immediate: Characterized by wheal-and-flare reactions, managed with antihistamines. Late: Prolonged swelling due to mast cell synthesis of leukotrienes and cytokines. Severe Reactions Systemic anaphylaxis involves widespread mast cell activation. Epinephrine counteracts symptoms by stabilizing vascular permeability, reducing swelling, and relaxing airway smooth muscles. Allergic asthma involves chronic inflammation driven by Th2 cells and eosinophils, leading to airway hyperreactivity. Treatments 23  Preventive Measures: Avoid allergen exposure through behavioral modifications. Pharmacological Interventions: For milder reactions, use antihistamines, corticosteroids, and bronchodilators (e.g., albuterol). For anaphylaxis, use epinephrine. Immunotherapy: Desensitization and therapeutic antibodies like omalizumab mitigate allergic responses. Type II Hypersensitivity: Antibody-Mediated Mechanisms Autoantibodies (IgG, IgM) target specific cell surface or extracellular matrix antigens, leading to complement activation, phagocytosis, or interference with cell functions. Selected Examples Autoimmune Hemolytic Anemia: RBCs are destroyed by complement or splenic macrophages due to antibody binding. Goodpasture’s Syndrome: Autoantibodies against type IV collagen damage kidneys and lungs. Graves’ Disease: Thyroid-stimulating antibodies over-activate thyroid hormone production. Myasthenia Gravis: Antibodies block acetylcholine receptors, impairing muscle contraction. Contributing Factors Genetic Susceptibility: MHC (HLA) alleles are associated with increased risk. Environmental Triggers: Infections and tissue damage expose antigens and activate autoreactive immune cells (“molecular mimicry”). Type III Hypersensitivity: Immune Complex-Mediated Mechanisms Immune complexes form between antibodies and soluble antigens, depositing in tissues and activating complement. This leads to inflammation and tissue damage (e.g. glomerulonephritis in kidneys, arthritis in joints, systemic vasculitis in skin) Selected Examples Serum Sickness: Complexes form after injection of foreign proteins, causing vasculitis, nephritis, or arthritis. Post-Streptococcal Glomerulonephritis: Complexes deposit in kidney glomeruli, inducing inflammation and complement activation. Type IV Hypersensitivity: T Cell-Mediated (Th1) Mechanisms Th1 and Th17 responses drive inflammation and tissue damage through cytokine release and macrophage activation. Referred to as DELAYED type hypersensitivity (in contrast to Type 1) Selected Examples Celiac Disease: Gluten-reactive CD4+ T cells initiate gut inflammation (HLA-DQ2/DQ8 linked). Hashimoto’s Thyroiditis: Th1-mediated destruction of thyroid tissue leads to hypothyroidism. Multiple Sclerosis: Myelin-reactive T cells cause CNS demyelination and sclerotic plaque formation. Type 1 Diabetes Mellitus: CD8+ T cells destroy pancreatic beta cells over time, resulting in insulin deficiency. Contact Dermatitis: CD8+ T cell responses to urushiol lead to skin inflammation. 24