Antibodies are Y-shaped tags that memorize molecular enemies
Each free antibody is a roughly 150-kilodalton protein about ten nanometers across, built like a Y with two matching tips. Those tips—paratopes—fit specific antigen epitopes so the immune system can neutralize a virus or flag it for destruction.
Also called immunoglobulins, antibodies belong to a protein superfamily used to identify and neutralize antigens on bacteria, viruses, and infected cells. Each Y-shaped molecule bears paratopes that bind particular epitopes with precision, tagging targets for immune attack or blocking invasion directly. Membrane-bound forms act as B-cell receptors; the word antibody usually means the secreted version freed into body fluids. Humans deploy five isotypes—IgA, IgD, IgE, IgG, IgM—with IgG and IgA further subdivided—each tuned for different effector jobs, locations, and stages of a response.
A typical mammalian unit joins two identical heavy chains and two identical light chains via disulfide bonds into three globular regions. Variable domains at the tips hold three hypervariable loops apiece; folded together, the complementarity-determining regions sculpt pockets, flat faces, or protruding shapes that match antigen, though only a few residues often supply most binding energy. Dual identical binding sites let one molecule grip repeating bacterial sugars and form dimers or larger complexes. The Fc trunk recruits immune partners; a flexible hinge lets the arms span distant epitopes. In blood-protein electrophoresis, antibodies largely migrate with the gamma-globulin fraction.
Secretion comes from plasmablasts and plasma cells after B cells differentiate—not from resting B cells lacking the transcription factor BLIMP-1 that switches heavy chains to the secreted form. Short-lived, rapidly dividing plasmablasts surge early from extrafollicular reactions; long-lived plasma cells can persist for a lifetime in bone-marrow or mucosal niches, pouring antibody even without fresh antigen. Memory B cells wait decades for a secondary response, then class-switch, affinity-mature, and become secretors again. Though often filed under adaptive immunity for their specificity and genetic rearrangement, natural IgM from B-1 cells can behave more like innate scouts with complement.
Source: Antibody
