Understanding The Immune System

reactions responsible for the symptoms of allergy (see Allergy). IgD is almost exclusively found inserted into the membranes of B cells, where it somehow regulates the cell's activation. Antibodies can work in several ways, depending on the nature of the antigen. Antibodies that interlock with toxins produced by certain bacteria can disable them directly (and are known as antitoxins). Other antibodies, by coating (or opsonizing) bacteria, make the microbes highly palatable to scavenger cells equipped to engulf and destroy them. More often, an antigen-antibody combination unleashes a group of lethal serum enzymes known as complement (see Complement). Yet other antibodies block viruses from entering into cells (a quality that is exploited in making vaccines). And, in a phenomenon known as antibody-dependent cell-mediated cytotoxicity (ADCC), cells coated with antibody become vulnerable to attack by several types of white blood cells. T Cells and Lymphokines T cells contribute to the immune defenses in two major ways. Regulatory T cells are vital to orchestrating the elaborate system. (B cells, for instance, cannot make antibody against most substances without T cell help.) Cytotoxic T cells, on the other hand, directly attack body cells that are infected or malignant. Chief among the regulatory T cells are "helper/inducer" cells. Identifiable by the T4 cell marker, helper T cells are essential for activating B cells and other T cells as well as natural killer cells and macrophages. Another subset of T cells, carrying the T8 marker, acts to turn off or "suppress" these cells. Cytotoxic T cells, which also carry the T8 marker, are killer cells. In addition to ridding the body of cells that have been infected by viruses or transformed by cancer, they are responsible for the rejection of tissue and organ grafts. T cells work primarily by secreting substances known as cytokines or, more specifically, lymphokines. Lymphokines (which are also secreted by B cells) and their relatives, the monokines produced by monocytes and macrophages, are diverse and potent chemical messengers. Binding to specific receptors on target cells, lymphokines call into play many other cells and substances, including the elements of the inflammatory response. They encourage cell growth, promote cell activation, direct cellular traffic, destroy target cells, and incite macrophages. One of the first cytokines to be discovered was interferon. Produced by T cells and macrophages (as well as by cells outside the immune system), interferons are a family of proteins with antiviral properties. Interferon from immune cells, known as immune interferon or gamma interferon, activates macrophages. Two other cytokines, closely related to one another, are lymphotoxin (from lymphocytes) and tumor necrosis factor (from macrophages). Both kill tumor cells; tumor necrosis factor (TNF) also inhibits parasites and viruses. Many cytokines are initially given descriptive names but, as their basic structure is identified, they are renamed as "interleukins"-messengers between leukocytes, or white cells. Interleukin-1, or IL-1, is a product of macrophages (and many other cells) that helps to activate B cells and T cells. IL-2, originally known as T cell growth factor, or TCGF, is produced by antigen activated T cells and promotes the rapid growth of mature T cells and B cells. IL-3 is a T-cell derived member of the family of