Function Of The Immune System

The immune system is a complex of organs--highly specialized cells and even a circulatory system separate from blood vessels--all of which work together to clear infection from the body.

The organs of the immune system, positioned throughout the body, are called lymphoid organs. The word "lymph" in Greek means a pure, clear stream--an appropriate description considering its appearance and purpose.

Cells that will grow into the many types of more specialized cells that circulate throughout the immune system are produced in the bone marrow. This nutrient-rich, spongy tissue is found in the center shafts of certain long, flat bones of the body, such as the bones of the pelvis. The cells most relevant for understanding vaccines are the lymphocytes, numbering close to one trillion.

Scientists have gained important new insights into the underlying causes of age-related changes in the immune system which lead to decreased immune function and increased vulnerability to infectious diseases in older people.

T-cells sometimes called "T-helper cells" play a central role in the immune response, signalling other cells in the immune system to perform their special functions. They play an important immune regulatory function and are critical components in defenses against cancer as well as a variety of infectious agents (bacterial, viral, fungal). Age-related defects in T-cell function also can contribute to the diminished response to immunization in older adults.

One process which was found to become abnormal in T-cells from aged mice is called apoptosis. Sometimes referred to as programmed cell death, apoptosis is thought to provide a critical step in the regulation of T-cell response by removing cells with defects and by maintaining appropriate levels of T-cells. Apoptosis is necessary for the removal of inactive or senescent T-cells that may accumulate with age, leading to decreases in immune function.

HIV disease is characterized by a gradual deterioration of immune function. Most notably, crucial immune cells called CD4+ T cells are disabled and killed during the typical course of infection.

Scientists have also discovered that telomerase is present at high levels during both the development and activation stages of immune cells and that this may affect the way the immune system is turned on and regulated. Telomerase, previously thought to be present only in "immortal" cells like sperm, egg, and tumor cells, is also present in normal human immune cells.

Telomerase regulates DNA length by replenishing the ends of DNA strands, called telomeres. These end segments are thought to be responsible, in part, for maintaining the structural integrity of DNA. In most types of cells, telomeres shorten as the cell ages, but in tumor cells, and also in normal immune cells, telomerase might help slow this shortening.

Research has led to improved therapy for individuals with primary immunodeficiency diseases.

Bone Marrow Transplantation

Transplantation of bone marrow cells from a family member with identical human leukocyte antigens (HLA) has led to normal immune function in patients with combined immunodeficiencies. HLA proteins mark the donor tissue as self, which minimizes the potential for rejection by the recipient's immune system. Investigators are working to improve success using bone marrow from an unrelated HLA-matched donor for the two-thirds of patients who lack an HLA-identical family member and to use stem cell transplantation (stem cells can generate all the cells of the immune system).

Replacement of Antibodies

Research has demonstrated the safety and efficacy of replacement therapy with intravenous immunoglobulin (i.e., antibodies) in patients with defects in antibody production. This treatment is life saving, prevents many types of infection, and should be given to all primary immunodeficiency disease patients with defects in antibody production.

Antimicrobials

The use of antibiotics to treat and prevent infections is a key element in the treatment of primary immunodeficiency disease patients. National Institute of Allergy and Infectious Diseases - supported investigators helped identify new antimicrobial agents and new treatment regimens. For example, NIAID researchers showed that chronic granulomatous disease patients receiving daily doses of the antibiotic cotrimoxazole had 65 percent fewer infections.

Immunomodulation

Immune system molecules can be injected into patients to improve immune function. NIAID researchers showed that the immune system molecule interferon gamma improves the function of the white blood cells called neutrophils in patients with chronic granulomatous disease, reducing infections by 70 percent.

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