Antibody Structure and Function – Vocabulary Flashcards

Vocabulary flashcards covering key terms related to antibody structure, isotypes, antigen binding, and effector functions for exam preparation.

Immunoglobulin (Ig)

A glycoprotein produced by B cells that functions as an antibody, composed of two identical heavy chains and two identical light chains.

Antibody

A secreted or membrane-bound immunoglobulin that specifically binds antigens to neutralize or mark them for destruction.

Heavy Chain (H chain)

The larger polypeptide of an antibody; contains one variable (VH) and three or four constant (CH) domains and determines the antibody isotype.

Light Chain (L chain)

The smaller polypeptide of an antibody; contains one variable (VL) and one constant (CL) domain and is of kappa (κ) or lambda (λ) type.

Variable Region (V region)

The N-terminal portion of heavy or light chains that differs between antibodies and forms part of the antigen-binding site.

Constant Region (C region)

The relatively invariant portion of an antibody’s heavy or light chain; mediates effector functions and defines isotype.

Fab Region

The ‘Fragment antigen-binding’ portion of an antibody comprising one constant and one variable domain from each chain; binds antigen.

Fc Region

The ‘Fragment crystallizable’ portion formed by paired heavy-chain constant domains; binds Fc receptors and complement proteins.

Antigen-Binding Site

The pocket formed by a VH–VL pair that specifically recognizes an epitope on an antigen; each Ig has two identical sites.

Complementarity-Determining Regions (CDRs)

Three hypervariable loops within each V domain (CDR1, CDR2, CDR3) that directly contact antigen; CDR3 is most variable.

Immunoglobulin Fold

A stable β-sandwich structure of two β-sheets forming each Ig domain, characteristic of the immunoglobulin superfamily.

Immunoglobulin Superfamily

Proteins sharing the Ig fold, including antibodies, T-cell receptors, MHC molecules, adhesion molecules (e.g., ICAM, VCAM).

Papain Digestion

Proteolytic cleavage that splits an IgG molecule into two Fab fragments and one Fc fragment.

Fab Fragment

Antibody fragment containing a complete antigen-binding site but lacking effector functions.

Fc Fragment

Antibody fragment that crystallizes readily and mediates complement activation and Fc-receptor binding.

Isotype

Antibody class determined by heavy-chain constant region (IgG, IgM, IgA, IgD, IgE).

IgG

The most abundant serum isotype (~80%); monomeric; crosses placenta, opsonizes, activates complement (especially IgG3).

IgM

The first antibody produced; pentameric in serum; efficient complement activator; also exists as monomeric B-cell receptor.

IgA

Major antibody of mucosal secretions; dimeric with J chain and secretory component; provides neutralization at mucosal surfaces.

IgD

Low-abundance serum Ig; mainly membrane-bound on mature B cells alongside IgM; role in B-cell activation.

IgE

Least abundant serum Ig; binds Fcε receptors on mast cells and basophils; mediates allergic reactions and anti-helminth immunity.

J Chain

A small polypeptide that links IgA dimers and IgM pentamers and assists epithelial transport.

Secretory Component

A cleavage product of the poly-Ig receptor that remains attached to secretory IgA, protecting it from proteolysis.

Hinge Region

Flexible segment between CH1 and CH2 domains in IgG, IgA, IgD, allowing Fab movement; absent in IgM and IgE.

Opsonization

Process whereby antibodies (e.g., IgG) coat pathogens, promoting phagocytosis via Fc-receptor engagement.

Complement Activation

Triggering of the classical complement pathway by antibodies (IgM, IgG1-3) bound to antigen.

Neutralization

Antibody function that blocks pathogen attachment or toxin activity without requiring Fc-mediated events.

Antibody-Dependent Cellular Cytotoxicity (ADCC)

Killing of antibody-coated target cells by NK cells via FcγRIII binding to IgG.

Serum Half-Life

Average time an antibody remains in circulation; longest for IgG (~21 days), shortest for IgE (~2 days).

Pentameric

Structure composed of five identical subunits; describes secreted IgM with a valency of 10.

Dimeric

Structure composed of two identical subunits; characteristic of secretory IgA.

Monomeric

Single-unit structure; form of IgG, IgD, IgE, serum IgA, and surface IgM/IgD.

B Cell Receptor (BCR)

Membrane-bound antibody (surface IgM or IgD) associated with Igα/Igβ signaling proteins on B cells.

Antigen

A molecule recognized by an antibody or T-cell receptor; may be protein, polysaccharide, lipid, or small chemical.

Epitope

Specific part of an antigen bound by an antibody’s paratope; also called antigenic determinant.

Paratope

The antigen-binding portion on an antibody, formed by its CDRs.

Hypervariable Region

Another name for CDRs, reflecting high sequence variability that confers antigen specificity.

Two-Gene Theory

Concept that antibody variable and constant regions are encoded by separate gene segments that rearrange in B cells.

Variable (V) Gene Segment

One of many gene segments encoding the antibody variable region, selected during V(D)J recombination.

Constant (C) Gene Segment

Single gene segment encoding the constant region of an antibody heavy or light chain, determining isotype.