Immune System Diseases and Disorders

Often referred to as hypersensitivity, allergies are caused by immune responses to some types of antigens, called allergens. There are two major categories of allergic reactions, based on the two major types of immune responses. These are rapid and delayed.

Rapid allergic reactions are caused by humoral immune responses. They result from the production of IgE antibodies. These become attached to mast cells, which are abundant in the lungs and intestine. Mast cells are macrophages that contain histamine, which triggers sneezing, runny nose, and watery eyes. When allergen fills the antigen-binding sites of mast cells, they release histamine. Some allergens that cause this are pollen, bee venom, and animal fur.

Delayed-type hypersensitivity (DTH) is much slower. This can be caused when allergens that cannot be easily destroyed by macrophages enter the body. Because of this, T-cells are activated, causing inflammation of the affected body tissue. This inflammation remains for as long as the T-cells are activated. Allergens that can cause such reactions are poison ivy and poison oak.

Immune deficiencyy can be either inherited or acquired. They reflect the failure of a gene important to the production or function of the immune system.

Some inherited diseases make macrophages incapable of ingesting invading organisms, making the individuals affected extremely susceptible to infections. Some of these are called opportunistic infections, which are organisms that are normally harmless that can flourish in a person's body with a weakened immune system. Severe Combined Immunodeficiency (SCID) and DiGeorge's syndrome are also inherited immune disorders. Those suffering from SCID have no T or B-cells, and those with DiGeorge's syndrome have no thymus.

Acquired immune deficiencies can be caused by infection and other agents. Environmental factors, poor nutrition, and stress can affect the immune system. Radiation therapy and some types of drugs can reduce lymphocyte production. Patients undergoing such treatment are susceptible to infections.

The Human Immunodeficiency Virus (HIV) is an infectious agent that is fatal. It causes Acquired Immune Deficiency Syndrome (AIDS) by infecting and destroying helper T-cells. This results in the inability to make adaptive immune responses, since helper T-cells are in charge of all immune responses. People with AIDS are extremely susceptible to all infectious agents due to their complete lack of immune function.

Because the immune system attacks anything even slightly different than the substances normally present within an individual, it often will turn on transplanted tissues and organs. To prevent this, the recipient is given transplants that share as many MHC genes as possible, as well as drugs that suppress the immune system to prevent rejection. If transplanted tissue contains T-cells of the donor, the donor T-cells are removed, since they may attack the recipient's tissues as foreign substances, called graft-versus-host (GVH) reaction.

Autoimmunity is the immune response of the body turned against it's own cells and tissues. These may involve either cell-mediated responses, humoral responses, or both. The basis for autoimmunity is unclear, since the mechanisms of autoimmune diseases are poorly understood. There are several diseases that cause autoimmunity. Lupus Erythematosus causes the body to produce antibodies that attack against different bodily tissues. In myasthenia gravis, antibodies attack normal molecules that control neuromuscular activity, often resulting in paralysis. People with type I diabetes make immune responses against cells which produce insulin. This results in the body's inability to produce sugars. Rheumatic fever causes the production of antibodies that bind to valves in the heart, leading to permanent heart damage.