Understanding The Immune System

protein mediators known as colony-stimulating factors (CSF); one of its many functions is to nurture the development of immature precursor cells into a variety of mature blood cells. IL-4 helps B cells grow and differentiate; it also affects T cells, macrophages, mast cells, and granulocytes. Natural Killer Cells Natural Killer (NK) cells are yet another type of lethal lymphocyte. Like cytotoxic T cells, they contain granules filled with potent chemicals. They are called "natural" killers because they, unlike cytotoxic T cells, do not need to recognize a specific antigen before swinging into action. They target tumor cells and protect against a wide variety of infectious microbes. In several immunodeficiency diseases, including AIDS, natural killer cell function is abnormal. Natural killer cells may also contribute to immunoregulation by secreting high levels of influential lymphokines. Both cytotoxic T cells and natural killer cells kill on contact. The killer binds to its target, aims its weapons, and then delivers a lethal burst of chemicals that produces holes in the target cell's membrane. Fluids seep in and leak out, and the cell bursts. Phagocytes (literally, "cell eaters") are large white cells that can engulf and digest marauding microorganisms and other antigenic particles. Some phagocytes also have the ability to present antigen to lymphocytes.

Monocytes and Macrophages Important phagocytes are monocytes and macrophages. Monocytes circulate in the blood, then migrate into tissues where they develop into macrophages ("big eaters"). Macrophages are seeded throughout body tissues in a variety of guises. Specialized macrophages include alveolar macrophages in the lungs, mesangial phagocytes in the kidneys, microglial cells in the brain, and Kupffer cells in the liver. Macrophages are versatile cells that play many roles. As scavengers, they rid the body of worn-out cells and other debris. Foremost among the cells that "present" antigen to T cells, having first digested and processed it, macrophages play a crucial role in initiating the immune response. As secretory cells, monocytes and macrophages are vital to the regulation of immune responses and the development of inflammation: they churn out an amazing array of powerful chemical substances (monokines), including enzymes, complement proteins, and regulatory factors such as interleukin-1. At the same time, they carry receptors for lymphokines that allow them to be "activated" into single-minded pursuit of microbes and tumor cells. Macrophages are not the only cells to present antigen to lymphocytes. Other antigen-presenting cells include B cells, as noted above, and dendritic cells, irregularly shaped white blood cells found in the spleen and other lymphoid organs. Dendritic cells typically have long threadlike tentacles that enmesh lymphocytes and antigens. Langerhans cells are dendritic cells that travel about in the skin, picking up antigen and transporting it to nearby lymph nodes. Many other types of body cells, properly stimulated, can also be recruited to present antigens to lymphocytes.