Immunomodulation and allergy.

A major function of the immune system is to protect the body from infection and the diseases caused by infectious agents. The immune system also provides protection against cancer cells, for once they arise, cancers can essentially behave as "foreign" cells capable of causing pathology. In contrast, allergy is a manifestation of the immune response to certain environmental cells or molecules that are usually neither a threat for infection nor cancer. Allergic reactions are generally an annoynance, even life-threatening. I will focus on type I allergy, characterized in part by induction of IgE antibody responses to allergens. It should be noted that not all IgE responses cause allergic symptoms. There is even evidence that IgE responses to tropical helminthic parasites offer a degree of immunity to reinfection. I have three objectives: (1) review T cell differentiation leading to the Th1/Th2 paradigm; (2) evaluate the increased prevalence of atopy, including asthma, as a consequence of a Th2-dominated immune system; (3) relate the high prevalence of asthma in inner city United States black children to the relatively recent migration of their ancestors from tropical regions of Africa, where genetically biased Th2-dependent IgE responses may be important in protection against high burdens of parasitic worms.