The Immune System

Lymphocytes and Lymphoid TissueThe complexity of the cellular interactions that occur during acquired immune responses requires specialized microenvironments in which the relevant cells can collaborate efficiently. Because only a few lymphocytes are specific for a given antigen, T cells and B cells need to migrate throughout the body to increase the probability that they will encounter that particular antigen. In their travels, lymphocytes spend only about 30 minutes in the blood during each cycle around the body.56 Immune responses to blood-borne antigens are usually initiated in the spleen, and responses to microorganisms in tissues are generated in local lymph . . .