Antibodies

Core Concepts

Structure: the Y that binds and the Y that acts

Each antibody is a tetramer of two heavy (H) chains and two light (L) chains held together by disulfide bonds. Each chain has a variable (V) region at its tip — unique to that antibody — and a constant (C) region. The two variable regions (one on each arm) form the antigen-binding site (Fab); because there are two, most antibodies are bivalent — they can bind two epitopes at once, which enables clumping. The Fc region (the stem) interacts with receptors on phagocytes and NK cells, and with complement protein C1q. The hinge region gives the arms flexibility. Millions of different antibodies exist, each produced by a different clone of B cells — the product of the same gene rearrangement that creates unique B cell receptors.

The five classes of antibodies

• IgG: the most abundant antibody in blood. A monomer that dominates the secondary response, crosses the placenta (protecting the fetus and newborn), opsonizes pathogens, and activates complement.
• IgM: a pentamer (five Y-shaped units) that is the first antibody produced in a primary response and the main B cell receptor on naive B cells. Its many binding sites make it excellent at agglutinating pathogens, though it stays in the blood.
• IgA: a dimer found in mucus, saliva, tears, and breast milk (colostrum) — the frontline defense at mucosal surfaces.
• IgE: binds mast cells and basophils by its Fc region; when antigen cross-links it, the cells release histamine — the basis of allergic reactions — and it also defends against parasitic worms.
• IgD: a membrane-bound receptor on naive B cells whose signaling role is still not fully understood.

Class matters because the variable region stays the same when a B cell switches classes — specificity is unchanged; only the effector function changes. This is class switching, directed by cytokines from helper T cells.

Effector functions: what antibodies actually do

Binding antigen is only half the job; the Fc region recruits the killing machinery:

• Neutralization: antibodies cover a virus or toxin, blocking its ability to bind host cells — the virus can no longer infect, the toxin can no longer act.
• Opsonization: the Fc region of a bound antibody is recognized by Fc receptors on phagocytes, which engulf the antibody-coated invader far more efficiently.
• Complement activation: antibody–antigen complexes trigger the classical pathway — C1q binds the Fc regions — leading to opsonization by C3b and lysis by the membrane attack complex.
• Agglutination and precipitation: because antibodies are bivalent (and IgM multivalent), they cross-link many pathogens or soluble antigens into clumps that phagocytes clear more easily.
• Antibody-dependent cell-mediated cytotoxicity (ADCC): NK cells bind the Fc regions of antibodies on target cells and kill those cells — a way of eliminating antibody-marked infected or tumor cells.

Production and memory

Antibodies are produced by plasma cells, terminally differentiated B cells specialized for secretion — a single plasma cell can release thousands of antibodies per second. Memory B cells from the same clone persist and, on re-exposure, rapidly produce more, better-matched antibodies (the secondary response, dominated by IgG). Laboratory-made monoclonal antibodies are identical antibodies from a single clone, giving one precise specificity; natural immune responses produce polyclonal antibodies — many clones, many specificities, which is why serum from an immune person recognizes many epitopes of a pathogen.

Common Confusions