Antibody structure, diversity, and applications

Antibody definition and antigen specificity

An antibody (immunoglobulin) is a large immune-system protein that binds to specific antigens with high precision. Each antibody has antigen-binding sites (paratopes) located at the tips of its “Y”-shaped structure, formed by variable regions on both heavy and light chains. The paratope recognizes a particular epitope, which is a specific part of the antigen, in a lock-and-key–like interaction that allows antibodies to tag pathogens for immune attack or neutralize them directly. Antigen specificity is determined by the variable-domain hypervariable regions, especially the complementarity-determining regions (CDRs), whose shapes complement the epitope. The immune system generates vast diversity of these binding sites through mechanisms such as V(D)J recombination (creating unique variable regions) and somatic hypermutation with affinity maturation (introducing mutations that can increase or decrease binding strength). Specificity can be described in terms of how many antigens/epitopes an antibody recognizes, such as monospecific (one), bispecific (two), or polyvalent (many).

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Sources of antibodies from B cell differentiation

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