How can an immune response produce antibodies that bind a target more strongly?

An immune response can produce antibodies that bind a target more strongly over time, even though no B cell knows which DNA change would improve its receptor. Early responding B cells differ in how effectively they recognize an antigen, a molecular feature of the target. Some enter organized regions of lymphoid tissue called germinal centers, where variation and selection interact.

As B cells divide, a specialized process introduces mutations into DNA regions encoding the antibody's binding surface. This process, called somatic hypermutation, generates variants. A change may improve binding, weaken it or have little useful effect. The mutations are not instructions written in response to a cell's knowledge of the correct answer.

Selection follows through interactions with antigen and other cells. B cells whose receptors capture antigen effectively can display antigen-derived material to helper T cells. Appropriate T-cell help supplies signals supporting survival and further division. Cells receiving stronger support can contribute more descendants, some of which undergo additional rounds of variation and selection. Stronger antigen capture can thereby lead to a larger contribution to the next round of cells.

Repeated cycles can raise the average binding strength, called affinity, among selected lineages. Some descendants become antibody-secreting cells or memory cells. Stronger binding can help a response, but the target and the antibody's other functions also matter, and it does not guarantee future protection. Affinity maturation connects changes in a molecular binding surface to unequal survival and reproduction of cells within one body.