Immunology: Antibody Structure, Isotypes, Effector Functions, and Mucosal Immunity

Comprehensive vocabulary flashcards covering antibody isotypes, classical complement fixation, Fc receptors, transcytosis pathways, and non-canonical animal antibodies.

Mucins

Extremely large polymeric glycoproteins secreted at mucosal surfaces that hydrate to form mucus, acting as a physical and chemical barrier to protect epithelial layers from pathogen invasion.

Transcytosis

The receptor-mediated active transport of macromolecules, such as dimeric $\text{IgA}$ or microorganisms, from one side of a polarized cell to the other through membrane-bound intracellular vesicles.

M Cells (Microfold Cells)

Specialized mucosal epithelial cells overlying organized lymphoid follicles that sample luminal antigens and pathogens and deliver them across the epithelial barrier to underlying antigen-presenting cells via transcytosis.

Polymeric Immunoglobulin Receptor (poly-IgR / pIgR)

A basolateral epithelial receptor that binds the $\text{J chain}$ of dimeric $\text{IgA}$ and pentameric $\text{IgM}$, mediates their transcytosis to the mucosal apical lumen, and is cleaved to leave behind a protective secretory piece.

J Chain (Joining Chain)

A dedicated structural polypeptide that covalently joins monomeric antibody subunits to assemble dimeric $\text{IgA}$ and pentameric $\text{IgM}$, providing the critical binding site recognized by $\text{poly-IgR}$.

Neonatal Fc Receptor (FcRn)

An endothelial transport receptor that binds circulating monomeric $\text{IgG}$ at acidic $\text{pH}$ in endosomes, rescues it from lysosomal proteolysis, and releases it into extracellular fluid at basic physiological $\text{pH}$.

Passive Transfer

The acquisition of preformed humoral immunity from an external source without active endogenous immune activation, exemplified by transplacental transfer of maternal $\text{IgG}$, ingestion of colostrum, or administration of antivenom.

Transient Infant Susceptibility Window

The vulnerable developmental interval between $3$ and $12\,\text{months}$ of age when passively acquired maternal $\text{IgG}$ has degraded and the infant's own newly synthesized antibody production has not yet fully matured.

Fab Fragment (Fragment Antigen-Binding)

The monovalent variable arm of an antibody molecule generated by proteolytic cleavage, composed of one constant and one variable domain of each of the heavy and light chains, responsible for antigen recognition and specificity.

Fc Fragment (Fragment Crystallizable)

The constant base stem region of an immunoglobulin formed by paired heavy-chain constant domains that crystallizes readily and determines the antibody's biological effector class functions and $\text{Fc}$ receptor interactions.

Pentameric IgM

The first secreted antibody produced in a primary adaptive immune response; contains $10$ antigen-binding sites and multiple $\text{C1q}$ binding sites, enabling potent classical complement fixation despite relatively low individual affinity.

Monomeric IgG

The most abundant antibody isotype in blood and extracellular fluids, possessing high antigen affinity and a compact monomeric structure that permits ready extravasation across vascular barriers into inflamed peripheral tissues.

IgG Subclasses

Four distinct structural variants ($\text{IgG1}$, $\text{IgG2}$, $\text{IgG3}$, and $\text{IgG4}$) differentiated primarily by the length and flexibility of their hinge regions, the number of interchain disulfide bonds, and their complement-fixing capacities.

Dimeric IgA

The predominant immunoglobulin isotype operating at mucosal surfaces (gut, respiratory tract, urogenital tract, tears, and saliva) and within breast milk, specialized in mucosal pathogen neutralization and immune exclusion.

IgE

An antibody isotype secreted in low quantities that acts primarily as a cell-bound surface receptor by rapidly associating with high-affinity $\text{Fc}\epsilon\text{RI}$ on mast cells, basophils, and eosinophils to trigger anti-parasite and allergic responses.

FcεRI

The high-affinity receptor for the $\text{Fc}$ domain of $\text{IgE}$ constitutively expressed on mast cells, basophils, and eosinophils whose multivalent cross-linking stimulates immediate cytoplasmic degranulation and histamine release.

IgD

A monomeric immunoglobulin co-expressed alongside $\text{IgM}$ via alternative splicing on mature naïve $\text{B cells}$ used as a developmental marker, found at very low circulating concentrations in serum with largely unknown effector roles.

Complement Fixation

The antibody-dependent triggering of the classical complement cascade; pentameric $\text{IgM}$ and $\text{IgG3}$ exhibit the highest relative capacity ($+++$), requiring at least one multimeric $\text{IgM}$ or two neighboring $\text{IgG}$ molecules to bind $\text{C1q}$.

CR1 (Complement Receptor 1 / CD35)

A surface receptor expressed on circulating erythrocytes that captures $\text{C3b}$-coated soluble antigen:antibody immune complexes, escorting them safely through the bloodstream to hepatic and splenic macrophages for disposal.

FcγRI (CD64)

A high-affinity, activating $\text{Fc}$ receptor expressed on monocytes, macrophages, and neutrophils that binds the lower hinge and $\text{C}_\text{H}2$ domains of $\text{IgG1}$ and $\text{IgG3}$ to trigger antibody-dependent cellular phagocytosis.

FcγRIII (CD16)

An activating $\text{Fc}$ receptor expressed predominantly on natural killer ($\text{NK}$) cells that binds clustered, surface-bound $\text{IgG}$ molecules on infected or malignant cells, delivering cytotoxic signals.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

An immune mechanism where effector natural killer ($\text{NK}$) cells recognize antibody-coated target cells via $\text{Fc}\gamma\text{RIII}$ ($\text{CD16}$), resulting in directional degranulation of perforin and granzymes that causes target cell apoptosis.

Neutralizing Antibodies (NAbs / BNAbs)

Antibodies that bind critical surface epitopes of viral particles, bacterial adhesins, or microbial toxins, physically preventing host receptor binding and cellular entry independent of innate immune effector cells.

Camelid Heavy Chain-Only Antibodies (HCAbs)

Naturally evolved functional immunoglobulins produced by camelids (alpacas and llamas) that lack light chains entirely, possessing antigen-binding regions composed solely of a single heavy chain variable domain ($\text{V}_\text{H}\text{H}$).

Nanobody (VHH)

A small ($\sim 15\,\text{kDa}$) single-domain recombinant antigen-binding fragment derived from camelid heavy-chain-only antibodies that is readily synthesizable in vitro, highly stable, and capable of targeting occluded epitopes.

Cormac

The specific llama resident at Triple J Farms in Bellingham, Washington, from which NIH researchers isolated therapeutic single-domain neutralizing nanobodies targeting the spike protein of $\text{SARS-CoV-2}$.

Bovine Ball and Stalk Antibody Structure

An unusual antibody architecture found in cattle featuring an exceptionally long $\text{CDR H3}$ loop that folds into a rigid, extended beta-strand stalk terminating in a disulfide-stabilized ball domain capable of binding recessed viral epitopes such as those on $\text{HIV}$.

Peyer's Patches

Organized secondary lymphoid aggregates within the gut-associated lymphoid tissue ($\text{GALT}$) containing $\text{B-cell}$ follicles and $\text{T-cell}$ zones that lack afferent lymphatics, relying exclusively on $\text{M cells}$ for luminal antigen intake.

Gut Mucosal Homing Axis (α4β7 / MAdCAM-1 / CCL25)

The coordinated molecular homing mechanism wherein primed lymphocytes expressing integrin $\alpha_4\beta_7$ bind to mucosal vascular addressin $\text{MAdCAM-1}$ guided by epithelial secretion of chemokine $\text{CCL25}$.

ACE2 (Angiotensin-Converting Enzyme 2)

The functional host cell-surface receptor utilized by $\text{SARS-CoV-2}$ for viral entry; its widespread expression on epithelial cells of both the pulmonary and intestinal tracts explains the dual respiratory and digestive clinical symptoms of $\text{COVID-19}$.

Gnotobiotic Animals

Germ-free research animals or animals purposefully colonized solely with known, defined microbial species, used experimentally to analyze the symbiotic immunological effects of commensal microbiota.

Thymus-Independent Antigens (TI Antigens)

Non-protein antigens featuring highly repetitive identical epitopes (such as bacterial capsular polysaccharides) that extensively cross-link $\text{B-cell}$ receptors, stimulating low-affinity $\text{IgM}$ secretion in the absence of helper $\text{T cells}$.