IgG has a long half-life and can cross the placenta, thus is ideally suited for passive immunization. IgE is important in the release of pharmacologically active agents from mast cells and thus causes asthma and hayfever. It is also the major mechanism in the elimination of parasites. IgD is primarily a lymphocyte receptor, is the strongest binding antibody, and is important in directing antigen to B cell surfaces to accomplish initial immunization. The paracortical area is an ill-defined band or zone that lies between the cortex and medulla.
T lymphocytes are long-lived and constitute most of the lymphocytes in lymph and blood.
White cells are short-lived scavengers and survive only a few days. Macrophages, however, are long-lived scavengers that engulf cellular debris and foreign matter. Macrophages display specific markers from the invading organisms on their surface known as antigens. Antigens signal helper T cells, which begin reproducing themselves. The B cells begin reproducing themselves and mature into plasma cells.