they become trapped in, and damage, the tissues of the kidneys, as seen here, or the lungs, skin, joints, or blood vessels. disorders of the immune system : aids when the immune system is lacking one or more of its components, the result is an immunodeficiency disorder. these disorders can be inherited, acquired through infection, or produced as an inadvertent side effect of drugs such as those used to treat cancer or transplant patients. aids is an immunodeficiency disorder caused by a virus that destroys helper t cells. the virus copies itself incessantly and invades helper t cells and macrophages, the very cells needed to organize an immune defense. the aids virus splices its dna into the dna of the cell it infects ; the cell is thereafter directed to churn out new viruses. human tissue typing for transplants although mhc proteins are required for t cell responses against foreign invaders, they can pose difficulty during transplantation. every cell in the body is covered with mhc self - markers, and each person bears a slightly unique set. if a t lymphocyte recognizes a non - self mhc scaffold, it will rally immune cells to destroy the cell that bears it. for successful organ or blood stem cell transplantations, doctors must pair organ recipients with donors whose mhc sets match as closely as possible. otherwise, the recipient ' s t cells will likely attack the transplant, leading to graft rejection. to find good matches, tissue typing is usually done on white blood cells, or leukocytes. in this case, the mhc - self - markers are called human leukocyte antigens, or hla. each cell has a double set of six major hla markers, hla - a, b, and c, and three types of hla - d. since each of these antigens exists, in different individuals, in as many as 20 varieties, the number of possible hla types is about 10, 000. the genes that encode the hla antigens are located on chromosome 6. a child in the womb carries foreign antigens from the father as well as immunologically compatible self - antigens from the mother. one might expect this condition to trigger a graft rejection, but it does not because the uterus is an " immunologically privileged " site where immune responses are somehow subdued. immunity and cancer when normal cells turn into cancer cells, some of the antigens on their


to trigger a graft rejection, but it does not because the uterus is an " immunologically privileged " site where immune responses are somehow subdued. immunity and cancer when normal cells turn into cancer cells, some of the antigens on their surface change. these cells, like many body cells, constantly shed bits of protein from their surface into the circulatory system. often, tumor antigens are among the shed proteins. these shed antigens prompt action from immune defenders, including cytotoxic t cells, natural killer cells, and macrophages. according to one theory, patrolling cells of the immune system provide continuous bodywide surveillance, catching and eliminating cells that undergo malignant transformation. tumors develop when this immune surveillance breaks down or is overwhelmed. a new approach to cancer therapy uses antibodies that have been specially made to recognize specific cancers. when coupled with natural toxins, drugs, or radioactive substances, the antibodies seek out their target cancer cells and deliver their lethal load. alternatively, toxins can be linked to a lymphokine and routed to cells equipped with receptors for the lymphokine. dendritic cells that attack cancer another approach to cancer therapy takes advantage of the normal role of the dendritic cell as an immune educator. dendritic cells grab antigens from viruses, bacteria, or other organisms and wave them at t cells to recruit their help in an initial t cell immune response. this works well against foreign cells that enter the body, but cancer cells often evade the self / non - self detection system. by modifying dendritic cells, researchers are able to trigger a special kind of autoimmune response that includes a t cell attack of the cancer cells. because a cancer antigen alone is not enough to rally the immune troops, scientists first fuse a cytokine to a tumor antigen with the hope that this will send a strong antigenic signal. next, they grow a patient ' s dendritic cells in the incubator and let them take up this fused cytokine - tumor antigen. this enables the dendritic cells to mature and eventually display the same tumor antigens as appear on the patient ' s cancer cells. when these special mature dendritic cells are given back to the patient, they wave their newly acquired tumor antigens at the patient ' s immune system, and those t cells that can respond mount an attack on the patient ' s cancer cells. the immune system and the nervous system biological links between the immune system and the central nervous system exist


newly acquired tumor antigens at the patient ' s immune system, and those t cells that can respond mount an attack on the patient ' s cancer cells. the immune system and the nervous system biological links between the immune system and the central nervous system exist at several levels. hormones and other chemicals such as neuropeptides, which convey messages among nerve cells, have been found also to " speak " to cells of the immune system - - and some immune cells even manufacture typical neuropeptides. in addition, networks of nerve fibers have been found to connect directly to the lymphoid organs. the picture that is emerging is of closely interlocked systems facilitating a two - way flow of information. immune cells, it has been suggested, may function in a sensory capacity, detecting the arrival of foreign invaders and relaying chemical signals to alert the brain. the brain, for its part, may send signals that guide the traffic of cells through the lymphoid organs. a hybridoma is a hybrid cell produced by injecting a specific antigen into a mouse, collecting an antibody - producing cell from the mouse ' s spleen, and fusing it with a long - lived cancerous immune cell called a myeloma cell. individual hybridoma cells are cloned and tested to find those that produce the desired antibody. their many identical daughter clones will secrete, over a long period of time, millions of identical copies of made - to - order " monoclonal " antibodies. thanks to hybridoma technology, scientists are now able to make large quantities of specific antibodies. genetic engineering allows scientists to pluck genes - - segments of dna - - from one type of organism and to combine them with genes of a second organism. in this way, relatively simple organisms such as bacteria or yeast can be induced to make quantities of human proteins, including interferons and interleukins. they can also manufacture proteins from infectious agents, such as the hepatitis virus or the aids virus, for use in vaccines. the scid - hu mouse the scid mouse, which lacks a functioning immune system of its own, is helpless to fight infection or reject transplanted tissue. by transplanting immature human immune tissues and / or immune cells into these mice, scientists have created an in vivo model that promises to be of immense value in advancing our understanding of the immune system.