Ch 7 & 8 Humoral Immune Response and Effector Mechanisms Practice Flashcards

Vocabulary flashcards covering humoral immune responses, effector mechanisms, complement regulatory proteins, and immunoglobulin attributes based on the Chapter 7 and 8 review booklet.

T-dependent antigens

Antibody responses to protein antigens that require antigen-specific $CD4+$ T cell help and display of the antigen on the B cell surface via MHC CII; this leads to B cell proliferation, isotype switching, high-affinity antibodies, and memory cell formation.

T-independent antigens

Antibody responses to nonprotein antigens ($polysaccharides$, $lipids$, $nucleic$ $acids$) that do not require helper T cells; activation is provided by recognition of common microbial constituents or extensive cross-linking of BCRs.

Antibody-dependent cellular cytotoxicity (ADCC)

Process where $CD16$ ($Fc$ receptor) on NK cells recognizes antibody-coated target cells leading to their killing; also involves $IgE$ coating helminths for eosinophils to bind via their $IgE$ $Fc$ receptor.

Complement

A collection of plasma proteins activated by microbes or antibodies; all three pathways (Alternative, Lectin, Classical) lead to the generation of $C3b$, which is deposited on microbes and degraded to $C3d$ for binding to $CR2/CD21$.

Opsonization

The coating of microbes with $C3b$ or antibodies to promote phagocytosis and destruction; $C3b$ is recognized by $CR1/CD35$ expressed on phagocytes.

Neutralization

Antibodies blocking the entry of microbes into the host at epithelial surfaces, preventing the binding of microbes to neighboring cells, and inhibiting the binding of toxins to cells.

FcRn

Neonatal $Fc$ receptor expressed in the placenta for maternal $IgG$ transfer and on endothelial cells to protect $IgG$ from catabolism, thereby prolonging its half-life in the blood.

Poly-IgR

A receptor on the basal surface of epithelial cells that binds $IgA$ and $IgM$ to transport them to the luminal surface; it is cleaved to release $IgA$ as a secretory piece.

$Fc\gamma RIIB$ (CD32)

A low-affinity receptor that negatively regulates B cells, DCs, and mast cells by adjusting the activation threshold and providing feedback inhibition of B cells.

$$Fc\epsilon RI$$

A high-affinity receptor for $IgE$ expressed on mast cells, basophils, and eosinophils; its activation leads to degranulation and the secretion of lipid inflammatory mediators.

$Fc\gamma RIII$ (CD16)

A low-affinity receptor for $Ig$ expressed on NK cells that mediates antibody-dependent cellular cytotoxicity (ADCC).

$Fc\gamma RI$ (CD64)

A high-affinity receptor for $Ig$ gamma heavy chains expressed on macrophages and neutrophils that enables the ingestion and degradation of bacteria bound by $IgG1$ and $IgG3$.

CD23

Also known as $FceRIIB$; a low-affinity $IgE$ $Fc$ receptor that promotes the attachment of eosinophils to worms, leading to ADCC.

C3a

A protein produced by proteolysis of $C3$ that is chemotactic for neutrophils and stimulates the release of inflammatory mediators and leukocyte movement into tissues.

Live-attenuated vaccine

Vaccines consisting of viruses selected for lack of pathogenicity while retaining infectivity; they stimulate neutralizing antibodies to protect against subsequent infection.

Conjugate vaccine

A vaccine where a capsular polysaccharide is coupled with a highly immunogenic protein to allow B cells to recruit helper T cell recognition of different epitopes.

Subunit vaccine

Vaccines composed of microbial proteins or polysaccharides, such as inactivated toxins, which stimulate antibodies to neutralize native toxins and microbes.

Follicular B cells

B cells that reside in lymphoid follicles and primarily contribute to T-dependent, class-switched, and high-affinity antibody responses to protein antigens.

Marginal-zone B cells

B cells that play a greater role specifically in T-independent immune responses.

B cell co-receptor complex

A signaling complex consisting of $CD19$, $CD21$ (which binds to $C3d$), and $CD81$.

C1q INH

A serine protease inhibitor that prevents the assembly of the $C1$ complex ($C1q$, $C1r$, $C1s$), blocking the classical pathway of complement activation.

Factor I

A plasma protein that proteolytically cleaves $C3b$ and $C4b$ into inactive fragments with the help of cofactors like $MCP$ and Factor H.

Factor H

A cofactor for Factor I-mediated cleavage of $C3b$ and $C4b$ that causes the dissociation of alternative pathway $C3$ convertase subunits.

Decay accelerating factor (DAF)

$CD55$; a membrane protein that interferes with the formation of $C3$ convertase by blocking the binding of $Bb$ to $C3b$ or $C4b$ to $C2a$.

CD59

A membrane protein that blocks $C9$ binding, thereby preventing the formation of the Membrane Attack Complex (MAC).

Primary Immune Response (Lag)

The interval after immunization before peak response, usually lasting $\sim 5-10$ days, characterized predominantly by $IgM$ over $IgG$.

Secondary Immune Response (Lag)

The interval after immunization before peak response, usually lasting $\sim 1-3$ days, characterized by higher average affinity and a relative increase in $IgG$.

Follicular DC

Cells in the germinal center that display antigens using $Fc$ receptors or $C3$ receptors (binding $C3b$ and $C3d$) to immune complexes.

IgM

Comprises $\sim 13\%$ of serum antibodies; exists as a monomer on B cells and a pentamer when secreted; functions in complement activation and apoptosis of self-reactive cells.

IgG

Comprises $\sim 80\%$ of serum antibodies; monomeric form that mediates neonatal immunity, $Fc$ receptor-dependent phagocytosis, ADCC, and feedback inhibition of B cells.

IgE

Comprises $< 1\%$ of serum antibodies; functions in mast cell degranulation (immediate hypersensitivity) and defense against helminths.

IgA

Comprises $\sim 6\%$ of serum antibodies; monomeric in serum and dimeric when secreted; critical for mucosal immunity in the GI and respiratory tracts.

IgD

Comprises $\sim 0.2\%$ of serum antibodies; functions by binding to basophils and mast cells.