Immunoglobulins (also called antibodies), produced by B cells, are the best-understood of the molecules that confer adaptive immunity. An individual human has the capacity to make a limitless number of different antibodies, but any given specific antibody is typically made only when the individual has been exposed to the antigen (immunized) to which the antibody will bind specifically—hence antibody production is an adaptive immune response. In this section, we describe the structural organization of immunoglobulins, their diversity, and how they bind to antigens. The mechanisms that generate diverse antibodies are described in Section 23.3.