MadSci Network: Immunology |
The differentiation of CD4 T cells into TH1 or TH2 determines whether humoral or cell-mediated immunity will predominate. TH1 cells lead to cell-mediated immunity which involves destruction of infected cells by cytotoxic T cells or destruction of intracellular pathogen by activated macrophages. TH2 cells provide humoral immunity which activates B cells to make neutralizing antibody. Thus in general, effective host defense against intracellular pathogens like viruses and mycobacteria requires selective production of TH1 cells, while extracellular pathogens like worms, bacteria, fungi can be best eliminated with a humoral immunity with TH2 cells as the predominant effector cells. TH1 cells can also contribute to humoral immunity by inducing production of strongly opsonizing antibodies. The difference between TH1 and TH2 effector function is critical to the outcome of infection seen in leprosy, a disease caused by infection Mycobacterium leprae growing in macrophage vesicles. If TH1 cells are preferentially induced, few live bacteria are found, little antibody is made, the patient can usually survive the disease. However, if TH2 cells are preferentially induced, antibodies are made but cannot reach the intracellular bacteria, and patients develop leprematous leprosy which is eventually fatal. In many infections, however, both cell-mediated and humoral immunity play a role such as reponse to Pneumocystis carinii, which requires antibody for ingestion by phagocytes and macrophage activaton for effective destruction of the ingested pathogen.
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