BIOL 201 – Chapter 16 Specific Defense: The Immune Response Study Notes
Adaptive (Specific) Immunity
Highly specialized, antigen-specific arm of host defense that develops after exposure to a particular pathogen or foreign molecule.
Distinguishing hallmarks:
Specificity: Each lymphocyte clone recognizes a single epitope; receptors generated by random gene rearrangement.
Diversity: possible B-cell receptors (BCR) or T-cell receptors (TCR) can be produced, allowing recognition of virtually any organic molecule.
Clonal Expansion:
Upon first encounter with its cognate epitope, a naïve lymphocyte receives activation signals, enters rapid mitosis, and produces a large population of identical effector & memory cells.
Significance: amplifies a rare, antigen-specific cell into a force large enough to influence host physiology; accelerates response upon re-infection.
Memory: All adaptive responses leave behind long-lived memory B & T cells that survive in lymphoid tissues; these drive the much faster secondary immune response.
Self-tolerance: Autoreactive clones are deleted (central tolerance) or suppressed (peripheral tolerance) to minimize autoimmunity.
Two inter-linked arms:
Humoral Immunity: mediated by antibodies secreted from B-cell–derived plasma cells; best at neutralizing extracellular microbes/toxins.
Cell-Mediated Immunity: mediated by T lymphocytes (CD4, CD8) and NK cells; handles intracellular pathogens & tumor surveillance.
Lymphatic System & Lymphoid Organs
Structural Overview
Lymph: plasma-derived interstitial fluid that drains into lymphatic vessels; carries antigens, APCs, cytokines.
Lymphatic Vessels: thin-walled, blind-ended capillaries merge into progressively larger trunks that empty into the right lymphatic duct & thoracic duct → venous circulation.
Lymph Nodes: bean-shaped, encapsulated organs positioned along lymphatics; cortex rich in B-cell follicles, paracortex rich in T cells, medulla with plasma cells & macrophages.
Spleen: filters blood (not lymph); white pulp = lymphoid tissue around arterioles, red pulp = RBC recycling.
Thymus: site of T-cell maturation & clonal deletion; cortex densely packed with immature thymocytes, medulla with mature cells & Hassall’s corpuscles.
Mucosa-Associated Lymphoid Tissue (MALT)
Diffuse lymphoid aggregates monitoring mucosal surfaces (e.g., tonsils, adenoids, bronchial-, nasal- and genital-associated tissue).
Gut-Associated Lymphoid Tissue (GALT): specialized subset within intestines (Peyer’s patches, appendix, isolated lymphoid follicles).
Peyer’s Patches: domed lymphoid nodules in ileum; M cells sample intestinal lumen → deliver antigen to underlying dendritic cells.
Functional Themes: concentration of antigen, APCs, and lymphocytes in same microenvironment boosts likelihood of productive interactions.
Antigens & Antigen Presentation
Antigen: any molecule that can be bound by an antibody or TCR; immunogen if it elicits an immune response.
Epitopes (Antigenic Determinants): discrete 3-D regions recognized by lymphocyte receptors; proteins usually contain dominant epitopes.
Exogenous Antigens: originate outside host cells (bacteria, toxins), processed in endosomes → presented on MHC II.
Endogenous Antigens: synthesized within infected or neoplastic cells (viral proteins, mutated self proteins), processed by proteasome → presented on MHC I.
Autoantigens: self molecules capable of triggering disease when tolerance fails (e.g., insulin in Type I diabetes).
Antigen-Presenting Cells (APCs)
Professional APCs: dendritic cells, macrophages, B cells; constitutively express MHC II + costimulatory molecules (CD80/86).
Antigen Processing
MHC I Pathway: cytosolic proteins → ubiquitination → proteasome → peptide transport via TAP into ER → loaded onto MHC I → surface display → recognized by CD8 T cells.
MHC II Pathway: phagocytosed proteins → endosomal proteases → CLIP removal by HLA-DM → peptide-MHC II complex → surface → recognized by CD4 T cells.
Cross-Presentation: dendritic cells can load exogenous antigens onto MHC I, permitting CD8 activation against tumors/viruses that do not infect APCs directly.
B Lymphocytes & Antibodies (Humoral Response)
B-Cell Activation
T-Dependent Antigens (proteins): require CD4 T cell help; produce class switching, affinity maturation, memory.
T-Independent Antigens (polysaccharides, LPS): extensive cross-linking of BCRs; primarily IgM, little memory.
Antibody Structure
Two identical heavy (H) & light (L) chains linked by disulfide bonds.
Each chain has variable (V) & constant (C) domains; Fab region binds antigen, Fc region binds Fc receptors & complement.
Hinge gives flexibility; J chain links pentameric IgM & dimeric IgA.
Immunoglobulin Classes
IgM: pentamer; first made; excellent agglutinator; activates complement.
IgG: monomer; most abundant in serum; crosses placenta (passive immunity); opsonization & ADCC.
IgA: dimer in secretions (tears, milk, mucosa); key for MALT immunity.
IgE: binds FcεR on mast cells/basophils; mediates allergy & anti-helminth responses.
IgD: membrane receptor on naïve B cells; initiates activation.
Effector Mechanisms
Neutralization: antibodies block microbe attachment sites or toxin active sites.
Agglutination: cross-linking leads to clumping, enhancing phagocytosis.
Opsonization: Fc-mediated coating promotes phagocyte recognition; complement fragment serves as additional opsonin.
Complement Activation: classical pathway begins when IgM or IgG Fc binds .
Memory B Cells: long-lived, surface Ig with high affinity; rapidly differentiate into plasma cells upon re-exposure.
T Lymphocytes, NK Cells & Cell-Mediated Responses
Helper T Cells (CD4 )
TH1: driven by IL-12; secrete IFN-γ, TNF-β; activate macrophages, support cytotoxicity, important vs. intracellular bacteria (e.g., Mycobacterium).
TH2: driven by IL-4; secrete IL-4, IL-5, IL-13; stimulate B-cell class switching to IgE/IgA, eosinophil activation, anti-helminth immunity.
Regulatory/Suppressor (Treg): express CD25 & FoxP3; produce IL-10, TGF-β to dampen immune reactions, enforce tolerance.
Cytotoxic T Cells (CD8 )
Recognize MHC I–peptide complexes on infected or malignant cells.
Induce apoptosis via:
Perforin/Granzyme Pathway: perforin forms pores; granzymes enter cytosol → activate caspases → DNA fragmentation.
Fas–FasL Interaction: triggers death receptor pathway.
Natural Killer (NK) Cells
Innate-like lymphocytes lacking antigen-specific receptors.
Detect “missing self”: low MHC I expression → activation.
Use same perforin/granzyme arsenal; produce IFN-γ to shape TH1 bias.
Primary vs. Secondary Immune Responses
Primary Response
Lag phase days; IgM peaks first, followed by modest IgG.
Affinity of antibody relatively low; effector T cells peak and contract.
Secondary (Memory) Response
Lag phase days; rapid, high-titer IgG (or IgA/IgE depending on prior class switching).
Higher affinity due to somatic hypermutation & selection.
Clinically correlated with milder or asymptomatic reinfection (basis of vaccination).
Types of Acquired Immunity
Naturally Acquired Active Immunity: infection → own immune response; durable memory.
Naturally Acquired Passive Immunity: maternal IgG across placenta & IgA in breast milk; immediate but temporary ( months).
Artificially Acquired Active Immunity: vaccination (e.g., whole-cell killed vaccine); induces primary response without disease.
Artificially Acquired Passive Immunity: injection of preformed antibodies/antitoxins (e.g., rabies Ig, anti-venom); rapid, short-lived.
Glossary of Key Terms (Alphabetical)
Agglutination: antibody-mediated clumping of particles; enhances clearance.
Apoptosis: programmed cell death involving caspase cascade & membrane blebbing; minimizes inflammation.
Autoantigen: self antigen that can trigger autoimmunity when tolerance fails.
Chemokines: chemotactic cytokines directing cell migration (e.g., CXCL8).
Clonal Deletion: elimination of self-reactive lymphocytes during development (central tolerance).
Clonal Selection: activation & proliferation of lymphocytes whose receptors bind a specific antigen.
Cytokines: soluble proteins (interleukins, interferons, TNFs) that mediate cell communication.
Endogenous/Exogenous Antigen: generated within vs. outside host cells.
Fc Region: constant region of antibody heavy chain; engages Fc receptors & complement.
Granzyme: serine protease from CTL/NK granules inducing apoptosis.
Growth Factors: cytokines that drive cell proliferation (e.g., IL-2 for T cells).
Immunity: state of protection against infectious disease.
Interferons (IFNs): antiviral cytokines (Type I IFN-α/β, Type II IFN-γ).
Interleukins (IL): cytokines acting between leukocytes.
Lymph: fluid in lymphatic system.
Lymphocytes: B, T, NK cells; key for adaptive & innate immunity.
Macrophages: phagocytes derived from monocytes; APCs; secrete cytokines.
Major Histocompatibility Complex (MHC): polymorphic loci coding for antigen-presenting molecules; termed HLA in humans.
Memory Responses: heightened, rapid responses upon re-exposure.
Natural Killer Cells (NK): innate lymphocytes killing virus-infected/tumor cells without prior sensitization.
Neutralization: antibody prevents pathogen/toxin interaction with host targets.
Opsonization: coating that enhances phagocytosis via Fc or complement receptors.
Perforins: pore-forming proteins secreted by CTL/NK cells.
Plasma Cells: terminally differentiated B cells secreting large quantities of antibodies.
Primary/Secondary Immune Response: first vs. subsequent encounters with same antigen.
Suppressor T Cells (Treg): inhibit immune responses to maintain tolerance.
T-Cell Receptor (TCR): heterodimeric protein (αβ or γδ) recognizing peptide–MHC.
Tumor Necrosis Factors (TNF): cytokines mediating inflammation & apoptosis; TNF-α key in septic shock.
Practical & Ethical Implications
Vaccination campaigns exploit memory responses to confer herd immunity; ethical debates revolve around mandates vs. autonomy.
Understanding clonal deletion informs therapies for autoimmunity (e.g., inducing tolerance in type 1 diabetes).
NK- and T-cell cytotoxic mechanisms are harnessed in cancer immunotherapy (CAR-T, checkpoint blockade).