Chapter 12: Effector Responses: Antibody- and Cell-Mediated Immunity

Vocabulary practice flashcards covering humoral effector responses, the six antibody-mediated effector functions, human immunoglobulin classes and subclasses, and specific Fc receptor families.

Neutralization

An antibody effector function in which antibodies bind to the surface proteins of viral particles, bacterial cells, or toxins, blocking their interaction with host receptors and preventing entry or pathological action.

Agglutination

An effector mechanism where multivalent antibodies cross-link multiple particulate antigens into large clumps, enhancing neutralization and facilitating pathogen clearance via mucus trapping, peristalsis, and phagocytosis.

Opsonization

The coating of pathogens with antibodies, allowing their Fc regions to bind Fc receptors ($\text{FcRs}$) on phagocytes (such as macrophages) to promote and accelerate engulfment and lysosomal digestion.

Complement Activation (Classical Pathway)

An antibody-mediated process triggered by antibodies binding to an antigen surface, recruiting $C1q$, $C1r$, and $C1s$ to initiate the cascade that forms the membrane attack complex (MAC) and causes target lysis.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

An effector pathway in which cytotoxic effector cells, such as Natural Killer (NK) cells, recognize antibody-coated infected or tumor cells via Fc receptors and trigger target cell apoptosis.

Antibody-Dependent Degranulation

The release of stored chemical mediators (such as histamine and proteases) from granulocytes (mast cells, basophils, and eosinophils) following the engagement and cross-linking of their surface Fc receptors by antibodies.

IgM

The primary antibody isotype produced by neonates and during a primary immune response; secreted as a decavalent pentamer in serum, it exhibits high avidity, strong agglutination capacity, and high complement activation potential.

IgG

The most abundant and structurally diverse immunoglobulin class in human serum, consisting of four subclasses ($\text{IgG1}$, $\text{IgG2}$, $\text{IgG3}$, and $\text{IgG4}$) that all mediate opsonization via phagocyte Fc receptors.

IgG3

An IgG subclass characterized by an unusually elongated hinge region with numerous interchain disulfide bonds, a shorter serum half-life ($8\text{ days}$), and high potency in classical complement activation and Fc receptor binding.

IgA

The major immunoglobulin isotype found in external secretions ($\text{IgA2}$ as dimers/multimers) and circulating in blood ($\text{IgA1}$ as monomers); it neutralizes toxins and pathogens without activating complement, preventing inflammation.

Secretory Component

A polypeptide fragment derived from enzymatic cleavage of the polymeric immunoglobulin receptor (polyIgR) that remains covalently bound to dimeric IgA upon release into mucosal lumens, conferring resistance against proteolytic digestion.

IgE

A human immunoglobulin isotype present at very low serum concentrations ($0.0003\text{ mg/ml}$) that mediates defenses against parasitic helminths and protozoa, and triggers allergic reactions by cross-linking receptors on mast cells and basophils.

IgD

An immunoglobulin class expressed along with IgM on mature naïve B cells and found in low levels in serum ($0.03\text{ mg/ml}$); it binds basophils and mast cells to trigger the release of antimicrobial peptides, cytokines, and B-cell survival factors.

ITAM and ITIM

Intracellular signaling motifs associated with Fc receptors; Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) transduce activating/positive signals, while Immunoreceptor Tyrosine-based Inhibitory Motifs (ITIMs) deliver inhibitory/negative signals.

FcγR

The most diverse group of Fc receptors, comprising four major families that bind IgG antibodies to mediate phagocytosis by myeloid cells, degranulation by cytotoxic cells, or ADCC by NK cells.

FcγRIIB (CD32)

An inhibitory Fcγ receptor that bears an immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic domain to deliver negative regulatory signals to immune cells.

FcεRI

A high-affinity Fc receptor expressed on mast cells, basophils, and eosinophils that binds monomeric IgE and initiates cellular degranulation upon antigen cross-linking.

FcαR (CD89)

An Fc receptor expressed by myeloid cells (monocytes, macrophages, granulocytes, and dendritic cells) that binds IgA complexes to trigger ADCC, phagocytosis, cytokine production, and superoxide free radical generation.

FcµR

An immunoglobulin Fc receptor expressed selectively by B, T, and NK cells that specifically binds IgM and participates in the regulation of cellular activation.

Fcα/µR (CD351)

An Fc receptor expressed on B cells, macrophages, and dendritic cells that possesses high affinity for IgM and intermediate affinity for IgA, stimulating phagocytosis upon IgM binding.

PolyIgR (Polymeric Immunoglobulin Receptor)

A specialized receptor expressed on the basolateral surface of epithelial cells that facilitates the transcytosis of polymeric IgA and IgM into external secretions such as saliva, tears, milk, and mucus.

FcRn (Neonatal Fc Receptor)

A specialized Fc receptor structurally related to MHC class I and non-covalently complexed with $\beta_2\text{-microglobulin}$ ($\beta_2\text{m}$); it mediates transplacental IgG transfer, absorption of maternal IgG in neonatal intestines, and IgG half-life prolongation via recycling.