The Immune System
Lesson 10, Page 4 of 5

The most common cell of the lymphatic system is the lymphocyte.

Humoral Immunity: Antigens and Antibodies

An antigen is a substance that, when introduced into the body, induces an immune response consisting of the production of a circulating antibody. An antibody is a molecule that is responsible for recognizing and marking an antigen for destruction by the white blood cells. This type of immunity is known as humoral immunity and is provided by the B cells. Within a few days after an infection, an antigen results in large amounts of the antibody capable of interacting with it.

Cell-Mediated Immunity

T cells are involved in the attacking of certain bacteria, viruses, and fungi, in some skin reactions resulting from contact with chemicals (contact dermatitis), and in immunity to cancer cells. Because the immunity associated with T cells does not involve the secretion of antibodies but requires direct physical contact with antigens, it is called cell-mediated immunity.

Scientists have recently discovered that there are four kinds of T cells. One kind, the cytotoxic T cell, defends the body by destroying foreign, infected, and cancerous cells. Helper T cells regulate immune responses, enabling the other T cells and B cells to perform their functions, by secreting messenger proteins or by direct contact with other cells. It is this cell that is destroyed by the HIV virus in patients with Acquired Immune Deficiency Syndrome (AIDS). Destruction of helper T cells results in a depressed immune response allowing infection by a variety of microorganisms and the growth of certain kinds of tumors. The third kind of T cell, suppressor T cells, dampen the immune response of B and T cells to keep them in check. The fourth kind of T cell is involved in certain kinds of hypersensitivity reactions.

Under normal circumstances the immune system responds to foreign organisms by the production of antibodies and the stimulation of specialized cells which destroy the organisms or neutralize their toxic products. In a normal healthy individual there is a balance of all the different cell types of the immune system providing an effective defense against all foreign invaders. When the immune response is not working properly (immunodeficient), as in AIDS, the individual will become more susceptible to repeated infections.

Another important function of the immune system is the removal of damaged or dying cells. When the immune system is out of balance, this function may be misdirected resulting in an immune response against the body's own cells producing a condition known as an autoimmune disease.

The function of the immune system most recently discovered is the system's ability to recognize and eliminate the abnormal (mutant) cells that frequently arise within the body. These mutant, or cancer, cells may occur spontaneously, or they may be induced by certain viruses (oncogenic viruses) or chemicals (mutagens). An immune system that is functioning properly can recognize and dispose of such cancer cells by means of a process called immune surveillance. The malfunction of this process may result in cancer.

One function of the immune system is to recognize and eliminate cancer cells that arise within the body. This process is known as. . .
Immune Surveillance
Cell-Mediated Immunity
Humoral Immunity