Two Types of Lymphocytes Promote “Cell-Mediated” Immunity or “Humoral” Immunity—the T and the B Lymphocytes.
Although most lymphocytes in normal lymphoid tissue look alike when studied under the microscope, these cells are distinctly divided into two major populations. One of the populations, the T lymphocytes, is responsible for forming the activated lymphocytes that provide “cell-mediated” immunity, and the other population, the B lymphocytes, is responsible for forming antibodies that provide “humoral” immunity. Both types of lymphocytes are derived originally in the embryo from pluripotent hematopoietic stem cells that form lymphocytes as one of their most important offspring as they differentiate. Almost all of the lymphocytes that are formed eventually end up in the lymphoid tissue, but before doing so, they are further differentiated or “preprocessed” in the following ways. The lymphocytes that are destined to eventually form activated T lymphocytes first migrate to and are preprocessed in the thymus gland, and thus they are called “T” lymphocytes to designate the role of the thymus. They are responsible for cell-mediated immunity.
The other population of lymphocytes—the B lymphocytes that are destined to form antibodies—are preprocessed in the liver during midfetal life and in the
bone marrow in late fetal life and after birth. This population of cells was first discovered in birds, which have a special preprocessing organ called the bursa of Fabricius. For this reason, these lymphocytes are called “B” lymphocytes to designate the role of the bursa, and they are responsible for humoral immunity. Figure 34–1 shows the two lymphocyte systems for the formation, respectively, of the activated T lymphocytes and the antibodies.