Allergies result from immune system hypersensitivity to weak antigens that do not cause an immune response in most people. Allergens, substances that cause allergies, include dust, molds, pollen, cat dander, certain foods, and some medicines (such as penicillin). Up to 10% of the US population suffer from at least one allergy.

After exposure to an allergen, some people make IgE antibodies as well as B and T memory cells. Subsequent exposure to the same allergen causes a massive secondary immune response that releases plenty of IgE antibodies. These bind to mast cells found usually in connective tissues surrounding blood vessels. Mast cells then release histamine, which starts the inflammatory response. In some individuals the histamine release causes life-threatening anaphylaxis or anaphylactic shock.

The immune system usually distinguishes "self" from "nonself". The immune system learns the difference between cells of the body and foreign invaders. Autoimmune diseases result when the immune system attacks and destroys cells and tissues of the body. Juvenile diabetes, Grave's disease, Multiple sclerosis, Systemic lupus erythematosus, and Rheumatoid arthritis are some of the autoimmune diseases.

Myasthenia gravis (MG) is a muscle weakness caused by destruction of muscle-nerve connections. Multiple sclerosis (MS) is caused by antibodies attacking the myelin of nerve cells. Systemic lupus erythematosis (SLE) has the person forming a series of antibodies to their own tissues, such as kidneys (the leading cause of death in SLE patients) and the DNA in their own cellular nuclei. In systemic lupus erythematosus (SLE), the immune system attacks connective tissues and major organs of the body. Rheumatoid Arthritis; sufferers have damage to their joints. Some evidence supports Type I diabetes as an auto immune disease. Juvenile diabetes results from the destruction of insulin-producing cells in the pancreas.

Immunodeficiency diseases result from the lack or failure of one or more parts of the immune system. Affected individuals are susceptible to diseases that normally would not bother most people. Genetic disorders, Hodgkin's disease, cancer chemotherapy, and radiation therapy can cause immunodeficiency diseases.

Severe Combined Immunodeficiency (SCID) results from a complete absence of the cell-mediated and antibody-mediated immune responses. Affected individuals suffer from a series of seemingly minor infections and usually die at an early age. A small group suffering from adenosine deaminase (ADA) deficiency, a type of SCID, are undergoing gene therapy to provide them with normal copies of the defective gene.

Acquired Immunodeficiency Syndrome (AIDS) is currently receiving the most attention among the immunodeficiency diseases. AIDS is a collection of disorders resulting from the destruction of T cells by the Human Immunodeficiency Virus (HIV), a retrovirus. When HIV replicates in the human T cells, it buds from the T cell plasma membrane encased in a coat derived from the T cell plasma membrane. HIV selectively infects and kills T4 helper cells. The viral RNA is converted into DNA by the enzyme reverse transcriptase; this DNA can become incorporated into a human chromosome for months or years.

Structure and replication cycle of HIV. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

When the infected T cell is needed in the immune response, the viral genes are activated and the virus replicates, killing the infected cell and producing a new round on T4 cell infection. Gradually the number of T4 cells, the master on switch for the immune system, decline. The immune response grows less powerful, eventually failing. Premature death results from a series of rare diseases (such as fungal pneumonia and Kaposi's sarcoma, a rare cancer) that overwhelm the body and its compromised immune system.

Information provided by: http://gened.emc.maricopa.edu