Somatic hypermutation

Somatic hypermutation is the process by which the antibody variable region genes of activated B cells are mutated at extraordinarily high rates — roughly one mutation per thousand base pairs per cell division, a millionfold above background mutation rates. The enzyme AID (activation-induced cytidine deaminase) deaminates cytosine to uracil in the immunoglobulin genes, triggering error-prone repair mechanisms that introduce point mutations. These mutations alter the shape and charge of the antibody binding site, generating a cloud of variant receptors around the original. The process is not targeted to improve binding; it is purely random, and improvement emerges only through the subsequent filter of clonal selection in the germinal center.

Somatic hypermutation is evolution accelerated and compressed: what natural selection accomplishes over millennia in populations, the immune system accomplishes in days within a single lymphoid organ. But this compression comes with risk. AID is not perfectly specific to immunoglobulin genes, and its off-target activity contributes to lymphomagenesis and autoimmune disease. The same mechanism that produces protective antibodies can, in the wrong context, produce self-reactivity or genomic instability.