Oo

Anti-A and anti-B

A, B, or AB

basis of blood type was described in Section 6.1.1. The gene for blood type has three alleles: A, B, or O. Alleles A and B code for the production of a corresponding surface sugar. The allele for O does not produce a surface sugar. Thus, a person with alleles AA or AO have only type A sugars on the surface of their RBCs, and those people have type A blood. Unlike standard antibody response, people with type A blood always have anti-B antibodies even if they have never been exposed to the type B antigen. Thus, if blood of types B or AB is transfused into type A persons, their antibodies will produce a reaction that involves hemolysis (RBC rupture) or clumping of RBCs. Table 9.4 shows the similar relationships for other blood types. Note that type AB persons can accept transfusions of any blood type. Thus, they are called universal recipients. Type O persons can donate blood to anyone and are called universal donors.

Another type of T cell, the helper T (TH) cell, is produced by a mechanism similar to the production of cytotoxic T cells. The TH cell produces cytokines that stimulate the maturation of cytotoxic T cells and the B cells. The human immunodeficiency virus (HIV), which causes the disease known as acquired immune deficiency syndrome (AIDS) infects and kills a type of TH cell. These T cells slowly disappear over a period of years in infected individual, eventually disabling the entire specific defense system.

Vaccines consist of inactivated antigens, such as killed infectious bacteria or virus. When injected into a human, the person develops antibodies for that bacteria or virus just as if it were faced with a real infection. If a real infection occurs subsequently, circulating antibodies and memory T cells and B cells will be ready to respond more quickly to stop the infection.

There are several types of diseases of the immune system. AIDS is a type of immunodeficiency disease. Besides the infectious type, this problem can be caused by radiation or congenital problems. Autoimmune disorders occur when the immune system treats part of the body as if it were a foreign substance. Examples are rheumatoid arthritis, lupus erythematosis, psoriasis, and diabetes mellitus. Allergies are excessive or inappropriate immune responses. A first exposure to an antigen stimulates the production of antibodies, which mediate an exaggerated response on subsequent exposures.

As described earlier, stress produces the release of glucocorticoids, which inhibit inflammation responses and ultimately, the healing of wounds. This makes the stressed person more susceptible to disease.

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