ABSTRACT
The cell type responsible for mediating the inducible immune response is the lymphocyte. Morphologically, the lymphocyte usually appears in the peripheral blood as a small, round uninucleate cell approximately 7 to 8 µm in diameter. Despite the fact that all lymphocytes look similar when examined by light or electron microscopy, lymphocytes can be divided into subsets that have a wide range of immunological functions. The spleen, thymus, and bone marrow are connected to the rest of the immune system by the blood. Lymphocytes within the immune system are fixed within tissues but constantly migrate throughout the body during their lifetime in search of foreign material in a process called lymphocyte trafficking. The basic pattern of the immune system found in the adult animal is developed during embryogenesis. During the formation of the lymphoid organs, cells of endodermal and mesodermal origin form a loose matrix of connective tissue that becomes filled with developing lymphocytes.
