The Immune System / The Thymus Gland

submitted by Dr. Gary Farr Last Updated July, 22, 2003

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A Look at the Immune System

"The immune system is the most important homeostatic system for defending the body against foreign substances. We can define its action as a defense against non-self substances which are potentially harmful." (Chaffee, page 304)

"The immune response can be defined as the body's ability to recognize materials that are foreign to it and the ability to make specific responses to these materials.

The Two Types of Immunity within the Immune System:

"The type of immunity known as humoral immunity is due to a specialized group of plasma proteins called antibodies. Specific antibodies are formed against foreign materials called antigens. The humoral immune system is mobile. Antibodies are found in the blood and lymph and may travel to the surface of the body to become the secretory immune system. Another type of immunity, provided by the phagocytic cellular system, is known as cell-mediated immunity. (Chaffee, pages 304-305)

1) Humoral Immunity:

"The B cells, which are responsible for humoral immunity, are so named because it was found that , in the chick, lymphoid tissue of the lower intestinal tract, called the bursa of Fabricus, is responsible for the development of humoral immunity. Since humans lack this bursa, it is postulated that bursal equivalent tissue exists in the bone marrow and possibly in the liver and spleen." (Chaffee, page 306)

"The body makes millions of antibodies." (Thomas, M.D., page 186)

"All the humoral antibodies are found in the globulin fraction of the blood proteins and, thus, are referred to as immunoglobulins (Ig)....The presence of five classes of immunoglobulins...IgM, IgG, IgA, IgD, and IgE...as well as many subclasses indicates that the different immunoglobulins probably react with different types of antigens...IgG is the smallest molecule and constitutes about 85 percent of plasma antibody." (Chaffee, page 307)

"The IgG and IgM antibodies use the enzymatic activity of a group of plasma globulins referred to as the complement system to accomplish their final goal. The complement system consists of eleven proteins which become active enzymes through a sequence of steps activated by the presence of an antigen-antibody complex. The antibody recognizes the antigen (foreign cell) and binds with it to form a complex which then fixes complement on the surface of the foreign cell where the enzymatic reactions take place. The result usually is lysis of the invading cell and probably phagocytosis." (Chaffee, pages 306-307)

2) Cell-Mediated Immunity

"The T cells, responsible for cell-mediated immunity, are so named from the determination that activity of the thymus and its secretory product, thymosin, is essential for the normal development of cell-mediated immunity. (Chaffee, page 306)

"The lymphocytes responsible for cell-mediated immunity are designated T cells. Like B cells, T cells originate from primitive stem cells and go through stages of maturation...The most noteworthy step is the transformation of pre-T cells into mature activated T cells by the action of the thymus hormone thymosin. This step may be carried out within the thymus gland or elsewhere in the body by the action of the circulating thymosin. These activated T cells may circulate in the blood and lymph or they may come to rest in the inner cortex of the lymph nodes where they appear to form subgroups of T cells with different functions in the immune reaction." (Chaffee, page 308)

"The primary activated T cell is the effector cell. This is the cell that determines the responses we recognize as cell-mediated immunity. This cell can produce and release a series of factors (lymphokines). A very important lymphokine for the final and tissue destruction, is migration-inhibitory factor (MIF). MIF attracts monocytes and macrophages to the site of foreign tissue where the monocytes are transformed into so-called angry or killer macrophages which nonspecifically destroy all cells in their way by lysis and phagocytosis." (Chaffee, page 308)

"Helper T cells appear also to arise from activated T cells. These cells produce a soluble factor that is required, in some cases, for antibody formation by B cells. Such helper action from T cells appears to be most important for IgE and IgG production." (Chaffee, page 308)

"Other T-cells regulate the strength of the immune response. Those known as helper cells secrete substances that turn on antibody production and stimulate phagocytes and other T-cells in times of need..." (Thomas, M.D., page 196)

"Suppressor T cells are cells that appear to regulate or suppress the activity of the B cells in the production of antibodies. There is also evidence that these suppressor T cells are active againsthelper T cells and other aspects of cellular immunity." (Chaffee, page 308)

"Other T-cells regulate the strength of the immune response...those known as suppressor cells produce chemicals that turn off antibody production and suppress the action of other T-cells." (Thomas, M.D., page 196)

T Lymphocytes From The Thymus Gland

"There are two main types of immune cells in the body and they are both different sorts of lymphocytes. The T or 'thymus' cell lymphocytes are under the control of the thymus and are responsible for the recognition of foreign substances and for many of the ways in which the body attacks them. The other sort of immune cell-the B lymphocyte- is responsible for actually manufacturing anti-bodies to foreign substances." (Weston, M.D., page 99)

"Thymus...Stem cells are produced in bone marrow. They then migrate to the thymus, replicate, and develop into T cells." (Clayman, M.D., page 124)

"B lymphocyte...These cells begin life as stem cells in bone marrow. They develop in the lymph nodes...B lymphocytes recognize the foreign proteins, or antigens, of disease organisms since they differ from natural body proteins. Antigens trigger B cells to multiply. Some develop into plasma cells, which secrete antibodies--proteins that attack and destroy only the antigens." (Clayman, page127)

"The thymus gland, then, prepares the T cell to do its work--to distinguish self from not self, friend from foe, and to destroy foreign cells. This role, called immunological surveillance, is directly concerned with resistance to infections and cancer." (Diamond, M.D., page 10)

"T lymphocytes develop inside the thymus gland. 'Killer' T cells react to the remains of destroyed specific antigens, attacking them, as well as any infected cells, with powerful proteins called lymphokines. 'Helper' T cells activate B and T cells, while 'suppressor' T cells inhibit the response of other cells to the invading antigens." (Clayman, M.D., page 127)

"...activated...('killer') T lymphocyte...become(s) attached to an infected target cell after having recognized antigens on its surface. As well as attacking specific antigens, T cells also attack cancer cells, slowing tumor growth." (Clayman, M.D., page 127)

"Memory T cell...These cells may survive for many years to respond to an attempted second invasion by the same antigen. They mobilize very quickly." (Clayman, M.D., page 127)

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