Hypersensitivity is a harmful overreaction of the immune system, commonly known as allergy. In cases of allergy, a person is more sensitive to a particular antigen than the average individual. Common allergens are pollen, animal dander, dust, and foods, but there are many more. A seasonal allergy to inhaled pollens is commonly called "hay fever." Responses may include itching, redness or tearing of the eyes (conjunctivitis), skin rash, asthma, runny nose (rhinitis), sneezing, urticaria (hives), and angioedema, a reaction similar to hives but involving deeper layers of tissue.
An anaphylactic reaction is a severe generalized allergic response that can lead rapidly to death as a result of shock and respiratory distress. It must be treated by immediate administration of epinephrine (adrena-line), maintenance of open airways, and antihistamines. Common causes of anaphylaxis are drugs, especially penicillin and other antibiotics, vaccines, diagnostic chemicals, foods, and insect venom.
A delayed hypersensitivity reaction involves T cells and takes at least 12 hours to develop. A common example is the reaction to contact with plant irritants such as those of poison ivy and poison oak.
The term immunodeficiency refers to any failure in the immune system. This may be congenital (present at birth) or acquired and may involve any components of the system. The deficiency may vary in severity but is always evidenced by an increased susceptibility to disease.
AIDS is acquired by infection with HIV, which attacks certain T cells. These cells have a specific surface attachment site, the CD4 receptor, for the virus. HIV is spread by sexual contact, use of contaminated needles, blood transfusions, and passage from an infected mother to a fetus. It leaves the host susceptible to opportunistic infections such as pneumonia caused by the protozoon Pneumocystis carinii; thrush, a fungal infection of the mouth caused by Candida albicans; and infection with Cryptosporidium, a protozoon that causes cramps and diarrhea. It also predisposes to Kaposi sarcoma, a once-rare form of skin cancer. It may also induce autoimmunity or attack the nervous system.
AIDS is diagnosed and followed by CD4+ T lymphocyte counts, which measure the number of cells that have the HIV receptor. A count of less than 200/µL of blood signifies severe immunodeficiency. Antibody levels to HIV and direct viral counts in the blood are also used to track the course of the disease. At present there is no vaccine or cure for AIDS, but some drugs can delay progress of the disease.
A disease that results from an immune response to one’s own tissues is classified as an autoimmune disorder. The cause may be a failure in the immune system or a reaction to body cells that have been slightly altered by mutation or disease. The list of diseases that are believed to be caused, at least in part, by autoimmunity is long. Some, such as systemic lupus erythematosus (SLE), systemic sclerosis (scleroderma), and Sjögren syndrome, affect tissues in multiple systems. Others target more specific organs or systems. Examples are pernicious anemia, rheumatoid arthritis, Graves disease (of the thyroid), myasthenia gravis (a muscle disease), fibromyalgia syndrome (a musculoskeletal disorder), rheumatic heart disease, and glomerulonephritis (a kidney disease).
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