Lymphocytes are small white blood cells that bear the major responsibility for carrying out the activities of the immune system; they number about one trillion. The two major classes of lymphocytes are: B cells, which grow to maturity independent of the thymus, and T cells, which are processed in the thymus. Both B cells and T cells recognize specific antigen targets.
B cells work chiefly by secreting soluble substances called antibodies into the body's fluids, or humors. (This is known as humoral immunity.) Antibodies typically interact with circulating antigens such as bacteria and toxic molecules, but are unable to penetrate living cells. T cells, in contrast, interact directly with their targets, attacking body cells that have been commandeered by viruses or warped malignancy. (This is cellular immunity.)
Although small lymphocytes look identical, even under the microscope, they can be told apart by means of distinctive molecules they carry on their cell surface. Not only do such markers distinguish between B cells and T cells, they distinguish among various subsets of cells that behave differently. Every mature T cell, for instance, carries a marker known as T3 (or CD3); in addition, most helper T cells carry a T4 (CD4) marker, a molecule that recognizes class II MHC antigens. A molecule known as T8 (CD8), which recognizes class I MHC antigens, is found on many suppressor/cytotoxic T cells. In addition, different T cells have different kinds of antigen receptors-either alpha/beta or gamma/delta.
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Immunity and Cancer
The immune system provides one of the body's main defenses against cancer. When normal cells turn into cancer cells, some of the antigens on their surface change. These new or altered antigens flag immune defenders, including cytotoxic T cells, natural killer cells, and macrophages.
According to one theory, patrolling cells of the immune system provide continuing bodywide surveillance, spying out and eliminating cells that undergo malignant transformation. Tumors develop when the surveillance system breaks down or is overwhelmed. Some tumors may elude the immune defenses by hiding or disguising their tumor antigens. Alternatively, tumors may survive by encouraging the production of suppressor T cells; these T cells act as the tumor's allies, blocking cytotoxic T cells that would normally attack it.
Blood tests show that people can develop antibodies to many types of tumor antigens (although the antibodies may not actually be effective in fighting the tumor). Skin testing (similar to skin testing for tuberculosis) has demonstrated that tumors provoke cellular immunity as well. Furthermore, studies indicated that cancer patients have a better prognosis when their tumors are infiltrated with many immune cells. Immune responses may underlie the spontaneous disappearance of some cancers.
Tests using antibodies derived from batches of human serum can detect various tumor-associated antigens-including carcinoembryonic antigen (CEA) and alphafetoprotein (AFP)-in blood samples. Because such antigens develop not only in cancer but in other diseases as well, the antibody tests are not useful for cancer screening in the general population. They are however, valuable in monitoring the course of disease and the effectiveness of treatment in patients known to have cancer.
Scientists have developed monoclonal antibodies (Hybridoma Technology) that are targeted specifically at tumor antigens. Linked to radioactive substances, these antibodies can be used to track down and reveal hidden cancer metastases within the body. Monoclonal antitumor antibodies are also being used experimentally to treat cancer-either in their native form or as immunotoxins, linked to natural toxins, anticancer drugs, or radioactive substances.
Other efforts to attack cancer through the immune system center on stimulating or replenishing the patient's immune responses with substances known as biological response modifiers. Among these are interferons (now obtained through genetic engineering) and interleukins. In some cases biological response modifiers are injected directly into the patient; in other cases they are used in the laboratory to transform some of the patient's own lymphocytes into tumor-hungry cells known as lymphokine-activated killer (LAK) cells and tumor-infiltrating lymphocytes (TILS), which are then injected back into the patient. Researchers are even using structures from the tumor cells themselves to construct custom-made anticancer "vaccines."
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