Natural killer cells appear to kill tumor cells and virus-infected cells by processes similar to those employed by CTLs. NK cells bear FasL on their surface and readily induce death in Fas-bearing target cells. The cytoplasm of NK cells contains numerous granules containing perforin and granzymes. Unlike CTLs, which need to be activated before granules appear, NK cells are constitutively cytotoxic, always having large granules in their cytoplasm. After an NK cell adheres to a target cell, degranulation occurs with release of perforin and gran-zymes at the junction of the interacting cells. The roles of per-forin and granzymes in NK-mediated killing of target cells by apoptosis are believed to be similar to their roles in the CTL-mediated process.
Despite these similarities, NK cells differ from CTLs in several significant ways. First, NK cells do not express antigen-specific T-cell receptors or CD3. In addition, recognition of target cells by NK cells is not MHC restricted; that is, in many cases the same levels of NK-cell activity are observed with syngeneic and allogeneic tumor cells. Moreover, although prior priming enhances CTL activity, NK-cell activity does not increase after a second injection with the same tumor cells. In other words, the NK-cell response generates no im-munologic memory.
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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.