The protozoan parasite Leishmania major provides a powerful and illustrative example of how host responses can differ between individuals. These differences can lead to either clearance of the parasite or fatality from the infection. Leish-mania is a protozoan that lives in the phagosomes of macrophages. Resistance to the infection correlates well with the production of IFN-y and the development of a TH1 response. Elegant studies in mice have demonstrated that strains that are resistant to Leishmania develop a TH1 response and produce IFN-y upon infection. Such strains of mice become highly susceptible to Leishmania-induced fatal ity if they lose either IFN-y or the IFN-y receptor, further underscoring the importance of IFN-y in containing the infection. A few strains of mice, such as BALB/c, are highly susceptible to Leishmania, and these animals frequently succumb to infection. These mice mount a TH2-type response to Leishmania infection; they produce high levels of IL-4 and essentially no IFN-y. Thus, one difference between an effective and an ineffective defense against the parasite is the development of a TH1 response or a TH2 response. Recent studies demonstrate that one difference between the resistant strains of mice and BALB/c is that a small restricted subset of BALB/c CD4+ T cells are capable of recognizing a particular epitope on L. major, and this subset produces high levels of IL-4 early in the response to the parasite. This skews the response toward a predominantly TH2 type. Understanding how these different T-helper responses affect the outcome of infection could provide a more rational approach to the design of effective treatments and successful vaccines for other pathogens.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.