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The immune system consists (in part) of cells and their products that protect the body from attack by infectious agents. Without this vital system, the body would soon succumb to one or more of these death-causing agents. Fortunately, the immune system eliminates many of these infectious agents before there is any sign of infection. One of the most important groups of cells in this system are the T lymphocytes, or T cells. A specific subset of T cells called cytotoxic T lymphocytes, or CTLs, are responsible for surveying the cell surfaces for foreign substances. Upon recognition of one of these foreign antigens, the T cell becomes activated and kills the cell expressing the antigen. Cells express foreign antigens when infected by foreign microorganisms, (i.e. pathogens) or when they have turned cancerous. The CTL then, is involved in protecting the body from intracellular pathogens and from the development of tumors.



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