Ch. 8 Effector Mechanisms of Humoral Immunity

Flashcards covering the effector mechanisms of humoral immunity, including antibody functions, the complement system, mucosal/neonatal immunity, and microbial evasion strategies.

Humoral Immunity

The type of adaptive immunity mediated by antibodies, produced by B cells, which destruction extracellular microbes, toxins, and prevents the spread of intracellular infections.

Fab region

The antigen-binding fragment of an antibody that contains hypervariable/CDRs regions providing specificity of binding to microbes and toxins.

Fc region

The heavy-chain portion of an antibody that contains binding sites for Fc receptors ($FcRs$) on phagocytes and for complement proteins to activate diverse effector mechanisms.

Isotype switching

A process resulting in the production of antibodies with distinct heavy-chain regions and effector functions to optimize the clearance of specific microbes.

Affinity maturation

The process that increases the ability of antibodies to bind to and neutralize or eliminate microbes during an immune response.

Centroblasts

Antigen-stimulated B cells that migrate into the dark zone of germinal centers, reduce expression of surface immunoglobulin, and undergo rapid cell division.

Centrocytes

B cells that have migrated to the light zone of the germinal center, increased surface immunoglobulin expression, and must compete for antigen on Follicular Dendritic Cells ($FDCs$) to survive.

FcRn (Neonatal Fc receptor)

A receptor expressed in the placenta, endothelium, and phagocytes that sequesters $IgG$ in endosomal vesicles ($pH \thicksim 4$) and recycles it to the surface ($pH \thicksim 7$), prolonging the antibody's half-life to about 3 weeks.

Opsonization

The process of coating microbes with antibodies or complement fragments to promote their ingestion and destruction by phagocytes.

Fc$\gamma$RI (CD64)

A high-affinity receptor for $IgG1$ and $IgG3$ heavy chains expressed on macrophages and neutrophils that triggers phagocytosis and increases bactericidal activity.

Antibody-Dependent Cellular Cytotoxicity (ADCC)

A process where NK cells use their Fc receptor $Fc\gamma$RIIIA (CD16) to recognize and kill $IgG1$ or $IgG3$ coated target cells by discharging cytotoxic granules.

Mast cells

Large cells in connective tissue expressing $Fc\epsilon$RI that, when cross-linked by $IgE$ and antigen, release inflammatory mediators and cytokines through degranulation.

Eosinophils

Leukocytes that attack helminths by binding to $IgE$ via $Fc\epsilon$RI; their granule release is enhanced by $IL-5$ secreted by Th2 cells.

Alternative pathway

A complement activation pathway initiated by the presence of a bacterial surface and the spontaneous activation of $C3$ in plasma.

Lectin pathway

A complement activation pathway initiated by mannose-binding lectin ($MBL$) or ficolins recognizing carbohydrates on microbial surfaces.

Classical pathway

A complement activation pathway initiated by the binding of the $C1$ complex to either CRP, $IgM$, or at least two $IgG$ antibodies on a pathogen surface.

C3 convertase

An enzyme complex, such as $C4bC2a$ (classical/lectin) or $C3bBb$ (alternative), that cleaves factor $C3$ into $C3b$ and $C3a$.

Complement fixation

The covalent attachment and deposition of $C3b$ onto the surface of a pathogen during complement activation.

Membrane Attack Complex (MAC)

A cytolytic pore formed on the pathogen surface by the sequential binding of $C5b$, $C6$, $C7$, $C8$, and polymerized $C9$, leading to osmotic lysis.

C1 inhibitor (C1 INH)

A regulatory protein that prevents the assembly of the $C1$ complex and inhibits plasma kallikrein; its deficiency causes hereditary angioedema.

Decay-accelerating factor (DAF or CD55)

A lipid-linked cell surface protein that interferes with $C3$ convertase formation by blocking binding of $Bb$ to $C3b$ or $C4b$ to $C2a$.

Atypical hemolytic uremic syndrome

A condition caused by mutations in Factor H, which normally acts with Factor I and MCP to cleave and inactivate $C3b$.

Poly-Ig receptor (poly-IgR)

A receptor on the basal surface of epithelial cells that binds $IgA$ and $IgM$ to transport them through vesicles to the luminal surface.

Antigenic drift

Genetic variation in viruses caused by error-prone replication that generates point mutations leading to new strains and yearly epidemics.

Antigenic shift

A major change that occurs when a viral genome from a strain infecting nonhumans recombines with a human-infecting strain, causing pandemics.

Latency

A state in which a virus (such as Herpes simplex) persists in the body without replicating or displaying viral peptides, thus evading immune detection.

Conjugate vaccine

A vaccine composed of microbial polysaccharides chemically coupled to proteins to activate Th cells and produce high-affinity antibodies against the polysaccharides.