Preprocessing of the T and B Lymphocytes
Although all lymphocytes in the body originate from lymphocyte-committed stem cells of the embryo, these stem cells themselves are incapable of forming directly either activated T lymphocytes or antibodies.
Before they can do so, they must be further differentiated in appropriate processing areas as follows.
Thymus Gland Preprocesses the T Lymphocytes. The T lym-phocytes, after origination in the bone marrow, first migrate to the thymus gland. Here they divide rapidly and at the same time develop extreme diversity for reacting against different specific antigens. That is, one thymic lymphocyte develops specific reactivity against one antigen. Then the next lymphocyte develops speci-ficity against another antigen. This continues until there are thousands of different types of thymic lym-phocytes with specific reactivities against many thou-sands of different antigens. These different types of preprocessed T lymphocytes now leave the thymus and spread by way of the blood throughout the body to lodge in lymphoid tissue everywhere.
The thymus also makes certain that any T lympho-cytes leaving the thymus will not react against proteins or other antigens that are present in the body’s own tissues; otherwise, the T lymphocytes would be lethal to the person’s own body in only a few days. The thymus selects which T lymphocytes will be released by first mixing them with virtually all the spe-cific “self-antigens” from the body’s own tissues. If a T lymphocyte reacts, it is destroyed and phagocytized instead of being released. This happens to as many as 90 per cent of the cells. Thus, the only cells that are finally released are those that are nonreactive against the body’s own antigens—they react only against anti-gens from an outside source, such as from a bacterium, a toxin, or even transplanted tissue from another person.
Most of the preprocessing of T lymphocytes in the thymus occurs shortly before birth of a baby and for a few months after birth. Beyond this period, removal of the thymus gland diminishes (but does not eliminate) the T-lymphocytic immune system. However, removal of the thymus several months before birth can prevent development of all cell-mediated immunity. Because this cellular type of immunity is mainly responsible for rejection of transplanted organs, such as hearts and kidneys, one can transplant organs with much less like-lihood of rejection if the thymus is removed from an animal a reasonable time before its birth.
Liver and Bone Marrow Preprocess the B Lymphocytes. Muchless is known about the details for preprocessing B lymphocytes than for preprocessing T lymphocytes. In the human being, B lymphocytes are known to be pre-processed in the liver during midfetal life and in the bone marrow during late fetal life and after birth.
B lymphocytes are different from T lymphocytes in two ways: First, instead of the whole cell developing reactivity against the antigen, as occurs for the T lym-phocytes, the B lymphocytes actively secrete antibod-ies that are the reactive agents. These agents are largeprotein molecules that are capable of combining with and destroying the antigenic substance. Second, the B lymphocytes have even greater diversity than the T lymphocytes, thus forming many millions of types of B-lymphocyte antibodies with dif-ferent specific reactivities. After preprocessing, the B lymphocytes, like the T lymphocytes, migrate to lym-phoid tissue throughout the body, where they lodge near but slightly removed from the T-lymphocyte areas.
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