Antigenantibody

An antigen is any substance that can be recognized by the immune system, while an antibody is a large, Y-shaped protein produced mainly by plasma cells that binds specifically to an antigen. The antigen–antibody interaction is driven by non-covalent bonds and is highly specific, occurring at defined regions called epitopes on the antigen and paratopes on the antibody. Affinity describes the strength of a single binding interaction, while avidity describes the overall binding strength when multiple sites interact.

Antibodies, or immunoglobulins, are produced by B cells and organized into a basic Y-shaped structure composed of two identical heavy chains and two identical light chains. The variable regions at the tips form the antigen-binding sites (paratopes) and determine specificity. The rest of the molecule contains a constant region (Fc) that can recruit other immune components and mediate effector functions.

When the antibody binds an antigen, several outcomes may occur: neutralization of toxins or pathogens by blocking functional sites; opsonization to promote phagocytosis; agglutination or precipitation of soluble antigens; activation of the complement system via the classical pathway; and recruitment of effector cells through Fc receptors.

Antigens vary from proteins and polysaccharides to nucleic acids and lipids. Some small molecules (haptens) are antigenic only when conjugated to a larger carrier protein. Antibody responses include monoclonal antibodies, derived from a single B-cell clone, and polyclonal antibodies, derived from multiple clones with broader specificity. Cross-reactivity occurs when an antibody recognizes related epitopes on different antigens.

Antigen–antibody interactions underpin many diagnostic and therapeutic tools. In diagnostics, they enable ELISA, Western blotting, immunohistochemistry, and rapid tests. Therapeutic antibodies are used to treat infections, cancer, and autoimmune diseases and are designed for high specificity and favorable pharmacokinetics.