Immune System : Autoimmune Diseases, Cancers, Transplants

by National Institute of Health

(Page 5 of 6)

Immune Complex Diseases

Immune complexes are clusters of interlocking antigens and antibodies. Normally, immune complexes are rapidly removed from the bloodstream. Sometimes, however, they continue to circulate, and eventually become trapped in the tissues of the kidneys, the lungs, skin, joints, or blood vessels. There they set off reactions with complement that lead to inflammation and tissue damage.

Immune complexes work their mischief in many diseases. These include malaria and viral hepatitis, as well as many autoimmune diseases.

Immunodeficiency Disorders

When the immune system is missing one or more of its components, the result is an immunodeficiency disorder. Immunodeficiency disorders can be inherited, acquired through infection, or produced unintentionally by drugs such as those used to treat people with cancer or those who have received transplants.

Temporary immune deficiencies can develop in the wake of common virus infections, including influenza, infectious mononucleosis, and measles. Immune responses can also be depressed by blood transfusions, surgery, malnutrition, smoking, and stress.

Some children are born with poorly functioning immune systems. Some have flaws in the B cell system and cannot produce antibodies. Others, whose thymus is either missing or small and abnormal, lack T cells. Very rarely, infants are born lacking all of the major immune defenses. This condition is known as severe combined immunodeficiency disease or SCID.

AIDS is an immunodeficiency disorder caused by a virus (HIV) that infects immune cells. HIV can destroy or disable vital T cells, paving the way for a variety of immunologic shortcomings. HIV also can hide out for long periods in immune cells. As the immune defenses falter, a person with AIDS falls prey to unusual, often life-threatening infections and rare cancers.

A contagious disease, AIDS is spread by intimate sexual contact, transfer of the virus from mother to infant during pregnancy, or direct blood contamination. There is no cure for AIDS, but newly developed antiviral drugs can slow the advance of the disease, at least for a time. Researchers also are testing HIV vaccines in clinical studies.

Cancers of the Immune System

The cells of the immune system, like other cells, can grow uncontrollably, resulting in cancer. Leukemias are caused by the proliferation of white blood cells, or leukocytes. The uncontrolled growth of antibody-producing plasma cells can lead to multiple myeloma. Cancers of the lymphoid organs, known as lymphomas, include Hodgkin's disease.

Immunology and Transplants

Each year thousands of American lives are prolonged by transplanted organs - kidney, heart, lung, liver, and pancreas. For a transplant to "take," however, the body's natural tendency to rid itself of foreign tissue must be overridden.

One way, tissue typing, makes sure markers of self on the donor's tissue are as similar as possible to those of the recipient. Each cell has a double set of 6 major tissue antigens, and each of the antigens exists, in different individuals, in as many as 20 varieties. The chance of 2 people having identical transplant antigens is about 1 in 100,000.

A second way is to lull the recipient's immune system. This can be done with powerful immunosuppressive drugs such as cyclosporine A, or by using laboratory-manufactured antibodies that attack mature T cells.

Bone Marrow Transplants

When the immune response is severely depressed - in infants born with immune disorders or in people with cancer - one possible remedy is a transfer of healthy bone marrow. Introduced into the circulation, transplanted bone marrow cells can develop into functioning B and T cells.

In bone marrow transplants, a close match is extremely important. Not only is there a danger that the body will reject the transplanted bone marrow cells, but mature T cells from the bone marrow transplant may counterattack and destroy the recipient's tissues. To prevent this situation, known as graft-versus-host disease, scientists use drugs or antibodies to "cleanse" the donor marrow of potentially dangerous mature T cells.

Immunity and Cancer

When normal cells turn into cancer cells, some of the antigens on their surface may change. If the immune system notices the foreign antigens, it launches the body's defenders, including killer T cells, NK cells, and macrophages. But the immune system cannot patrol everywhere to provide bodywide surveillance, flushing out and eliminating all cells that become cancerous. Tumors develop when the system breaks down or is overwhelmed.

Scientists are shaping immune cells and substances into ingenious new anticancer weapons. They are using substances known as biological response modifiers, including lymphocytes and lymphokines, to bolster the patient's immune responses. In some cases, biological response modifiers are injected directly into the patient. They can also be used in the laboratory to transform some of the patient's own lymphocytes into tumor-hungry cells, which are then injected back into the patient so they can attack the cancer cells.

Antibodies specially made to recognize specific cancers can be coupled with drugs, toxins, or radioactive materials, then sent off like "magic bullets" to deliver their lethal cargo directly to the target cancer cells. Alternatively, toxins can be linked to a lymphokine and routed to cells equipped with receptors for the lymphokine. Radioactively labeled antibodies can also be used to track down hidden nests of cancer cells (metastases).

Still other researchers are testing therapeutic cancer vaccines. These differ from traditional vaccines, which are given before disease onset to protect a person from future infections. Cancer vaccines are used after the cancer has arisen, and are designed to help the immune system fight off the illness.

The immune system often responds weakly or not at all to cancer cells. Cancer vaccines try to improve on the natural anticancer response by stimulating strong killer T-cell responses against a tumor. Although such vaccines are generally not able to destroy a tumor if given as the only form of treatment, research suggests they can be effective partners if administered along with other forms of treatment.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.