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DISORDERS OF THE IMMUNE SYSTEM
The immune system can overreact, causing allergies or autoimmune diseases; on the other side, the immune responses can be attenuated, because the immune system is not present or is not working properly, leading to immunodeficiency diseases.
Allergies and asthma:
Allergic disorders [E], [ES], [I], result from immune system hypersensitivity to weak antigens in the environment that usually do not cause an immune response. Allergens, substances that cause allergies, penetrate into the organism through the respiratory or the alimentary tract, and include dust, pollen, animal hair, certain foods and some medicines or drugs. After exposure to an allergen, some people produce IgE antibodies as well as B and T memory cells. Subsequent exposure to the same allergen causes a huge secondary immune response that releases plenty of IgE antibodies. These bind to mast cells in connective tissues surrounding blood vessels, and these release histamine, which promotes inflammatory response with symptoms like swelling, watery eyes, breathing problems (in case of asthma), itchy rash (associated to eczema) and sneezing; in some individuals the release of histamine causes life-threatening anaphylaxis or anaphylactic shock.
Autoimmune diseases:
Autoimmune disorders [E], [I], [F], result when the immune system fails to learn the difference between cells of the body and foreign invaders, and doesn’t properly distinguish between self and non-self, thus attacking and destroying cells and tissues of the body. Myasthenia gravis, juvenile diabetes, multiple sclerosis, systemic lupus erythematosus, and rheumatoid arthritis are some autoimmune diseases. Myasthenia gravis is a muscle weakness caused by destruction of muscle-nerve connections. Multiple sclerosis is caused by antibodies attacking myelin of nerve cells. In systemic lupus erythematosus, the immune system attacks connective tissues and major organs of the body. Rheumatoid arthritis sufferers have damage to their joints. Juvenile (type I) diabetes results from the destruction of insulin-producing cells in the pancreas.
Immunodeficiency diseases:
Immunodeficiency [E], [ES], results from the lack or failure of one or more parts of the immune system; they can affect B lymphocytes, T lymphocytes, or phagocytes. Affected individuals are susceptible to diseases that normally would not bother most people. Sometimes a person is born with an immunodeficiency (primary or congenital form, as SCID); immunodeficiencies can also be acquired (secondary forms) through infection (as AIDS) or produced by drugs (see below).
• Severe Combined Immunodeficiency (SCID) is a primary immune system disorder resulting from a complete absence of the cell-mediated and antibody-mediated immune responses. Affected individuals suffer from a series of minor infections because of the lack of both B and T lymphocytes, and usually die at an early age.
• Acquired Immunodeficiency Syndrome (AIDS) is a secondary form of immunodeficiency that includes a collection of disorders resulting from the destruction of T cells. It is caused by the Human Immunodeficiency Virus (HIV), a retrovirus that selectively infects and kills T4 helper cells. When HIV replicates in the human T cells, the viral RNA is converted into DNA by the enzyme’s reverse transcriptase; this DNA may become incorporated into a human chromosome for months or years. Without T-helper cells, the immune system is unable to defend the body against normally harmless organisms, which can cause life-threatening infections in people who are affected by AIDS. Newborns can get HIV infection from their mothers while in the uterus, during the birth process, or during breastfeeding. People can contract HIV infection by having unprotected sexual intercourse with an infected person or by sharing contaminated needles for drugs, steroids, or tattoos.
• Immunodeficiencies caused by drugs: There are several medicines that suppress the immune system. One of the drawbacks of chemotherapy treatment for cancer, for example, is that it not only attacks cancer cells, but other fast-growing, healthy cells, including those found in the bone marrow and other parts of the immune system. In addition, people with autoimmune disorders or who have had organ transplants may need to take immunosuppressant drugs in order to prevent self-destruction or rejection of transplanted organs, respectively. These medicines can also reduce the immune system's ability to fight infections and can cause secondary immunodeficiency.
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