Acquired immunity is the product of the body’s lymphocytes. In people who have a genetic lack of lymphocytes or whose lymphocytes have been destroyed by radiation or chemicals, no acquired immunity can develop. Within days after birth, such a person dies of fulminating bacterial infection unless he or she is treated by heroic measures. Therefore, it is clear that the lymphocytes are essential to the survival of the human being.
The lymphocytes are located most extensively in the lymph nodes, but they are also found in special lymphoid tissues such as the spleen, submucosal areas of the gastrointestinal tract, thymus, and bone marrow. The lymphoid tissue is distributed advantageously in the body to intercept invading organisms or toxins before they can spread too widely.
In most instances, the invading agent first enters the tissue fluids and then is carried by lymph vessels to the lymph node or other lymphoid tissue. For instance, the lymphoid tissue of the gastrointestinal walls is exposed immediately to antigens invading from the gut. The lymphoid tissue of the throat and pharynx (the tonsils and adenoids) is well located to intercept antigens that enter by way of the upper respiratory tract. The lymphoid tissue in the lymph nodes is exposed to antigens that invade the peripheral tissues of the body, and the lymphoid tissue of the spleen, thymus, and bone marrow plays the specific role of intercepting antigenic agents that have succeeded in reaching the circulating blood.
T and B Lymphocytes Promote “Cell-Mediated” Immunity or “Humoral” Immunity. Although most lymphocytes in normal lymphoid tissue look alike when studied under a 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 common lymphoid progenitor cells 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 lymphoid progenitor cells 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 mid fetal 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 1 shows the two lymphocyte systems for the formation, respectively, of (1) the activated T lymphocytes and (2) the antibodies.
Fig1. Formation of antibodies and sensitized lymphocytes by a lymph node in response to antigens. This figure also shows the origin of thymic (T) and bursal (B) lymphocytes that, respectively, are responsible for the cell-mediated and humoral immune processes.
