1. Lymphocytes.

Lymphocytes are white blood cells uniform in appearance but varied in function and include T, B, and natural killer cells. These cells are responsible for antibody production, direct cell-mediated killing of virus-infected and tumor cells, and regulation of the immune response. Advances in immunology have led to the characterization of newly appreciated effector populations such as IL-17-producing T cells, T cells with regulatory function, and natural killer T cells, thus revising established paradigms. This chapter provides an overview of the major lymphocyte populations with emphasis on their development, distinguishing characteristics, and functions.