Antibodies undergo three phases of genetic alteration. Researchers know that perhaps one billion different specificities of human antibodies are created from just 1,000 genes in the germline by shifting and reshifting various segments, called variability (V), diversity (D), and joining (J). These antibodies together recognize nearly every pathogen and toxin the body could encounter. Researchers know, too, that an antibody's affinity for its antigen tends to increase over the course of a response--a phenomenon known as affinity maturation--through a process called somatic hypermutation. SHM takes place in the lymph node germinal centers as B cells compete for limited antigen, with the ability to survive conferred only on those with the highest affinity.
A third genetic process, which also occurs in the germinal centers, is class switch recombination (CSR). In CSR, the molecule's constant (C) segment--encoding the stem of the Y-shaped antibody--is changed, conferring different functions on the antibody.
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