Immunological Side Effects and Illnesses - And Immune deficiency

Immunological Side Effects and Illnesses 
The immune system protects the body from disease be­cause it can differentiate "self" from "nonself." Sometimes, however, the immune system is underprotective, allowing cancer to develop, or is overprotective, preventing an indi­vidual from receiving certain types of blood.
Allergies 
Allergies are caused by an overactive immune system that forms antibodies to substances that are usually not recog­nized as foreign substances. Unfortunately, allergies usu­ally are accompanied by coldlike symptoms, or even at times, by severe systemic reactions, such as anaphylactic shock, a sudden drop in blood pressure and respiratory dif­ficulties that can lead to death.
Of the five varieties of antibodies, IgE antibodies cause allergies. IgE antibodies stick on the membranes of basophils in the blood and of mast cells, which are found in the tissues. When an allergen, an anti­gen that provokes an allergic reaction, attaches to the IgE antibodies on mast cells, the mast cells release histamine and other substances that cause mucus secretion and air­way constriction, resulting in characteristic allergy symp­tom. On occasion, basophils and other white blood cells release these chemicals into the bloodstream. The resulting increased capillary permeability can lead to fluid loss and shock.
Allergy shots sometimes prevent the onset of allergic symptoms. Injections of the allergen cause the body to accumulate large quantities of IgG antibodies, which com­bine with environmental allergens before they have a chance to reach the IgE antibodies located on the mem­branes of mast cells.
Allergic symptoms are caused by the release of histamine and other substances from mast cells. 
Tissue Rejection 
Certain organs, such as the skin, heart, and kidneys, could be transplanted easily from one person to another if the body did not attempt to reject them. Rejection occurs be­cause killer T cells bring about disintegration of foreign tis­sue in the body.
Organ rejection can be controlled in two ways: (1) by careful selection of the organ to be transplanted and (2) by immunosuppression-inactivation of the immune sys­tem. It is best if the transplanted organ has the same type of MHC proteins as those of the recipient because killer T cells treat foreign MHC proteins as antigens.
The immunosuppressive drug cyclosporine has been in use for years. A new drug, tacrolimus (formerly known as FK-506), shows some promise, especially in liver trans­plant patients. Both drugs, which act by inhibiting the pro­duction of interleukin-2, are known to adversely affect the kidneys as a toxic side effect.
When a transplanted organ is rejected, the immune system attacks cells that bear different MHC proteins from those of the individual.
Autoimmune Diseases
Certain human illnesses occur because killer T cells attack an individual's own tissues. Some bacteria are known to produce toxic products called superantigens. A superanti­gen can bind to an MHC protein on a macrophage without first being processed by the macrophage. Subsequently, killer T cells that interact with the affected macrophage learn to recognize and attack the body's own cells. This chain of events is hypothesized to cause at least some au­toimmune diseases.
In the autoimmune disease myasthenia gravis (mi"as­the'ne-ah grah'vis), neuromuscular junctions do not obey nervous stimuli, resulting in muscular weakness. In multi­ple sclerosis (MS), the myelin sheath of nerve fibers is at­tacked, which results in various neuromuscular disorders . A person with the autoimmune disease systemic lupus erythematosus (SLE) has various symptoms, including fever and malaise, and at some time during the course of the illness, signs of kidney failure. In the autoimmune dis­ease rheumatoid arthritis, the joints are affected. Type I di­abetes and heart damage following rheumatic fever are also suspected to be autoimmune illnesses. As yet, there are no cures for autoimmune diseases, but research is progressing.
Autoimmune diseases seem to be preceded by an infection that results in an attack by killer T cells on the body's own organs.
Immune Deficiency 
Immune deficiency represents a breakdown in some aspect of the immune system's ability to protect the body against disease. AlDS is an example of an acquired im­mune deficiency. As a result of a weakened immune sys­tem, AIDS patients show a greater susceptibility to a variety of diseases, including opportunistic infections, such as pneumocystis pneumonia. They also have a higher risk of cancer. One type of cancer, Kaposi's sarcoma (a malignant form of skin cancer), is common among AIDS patients, al­though it is rare in the general population.
Immune deficiency may also be congenital (that is, in­herited). Infrequently, a child may be born with an im­paired B- or T-cell system caused by a defect in lymphocyte development. In severe combined immunodeficiency dis­ease (SCID), both antibody and cell-mediated immunity are lacking or inadequate. Without treatment, even com­mon infections can be fatal. The use of bone marrow trans­plants  containing healthy stem cells has met with some success in the treatment of affected children.
Immune deficiency is caused by a failure of some aspect of tile body's immune system.