What Is the Role of MHC in T-Cell Activity?
The major histocompatability complex (MHC) is a genetic region found in humans and most other vertebrates, which encodes MHC proteins that are essential to immune function. MHC proteins play a vital role in activating specific types of T cells during an important immune process called antigen presentation.
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MHC Classes
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MHC proteins display either "self" or "foreign" antigens to T cells. MHC class I molecules are found on the surface of every cell in the body, and display "self" antigens (e.g., tumor proteins) to cytotoxic T cells.
MHC class II proteins occur on specialized antigen-presenting cells (e.g., macrophages and B cells). They present foreign antigens, like the proteins or DNA from invading viruses or bacteria, to helper T cells.
Professor John W. Kimball suggests that to understand the antigen-presentation process, you should visualize the MHC proteins as a hot dog bun, and the presented proteins or antigens as the hot dog between the buns.
Cytotoxic T Cells
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Cytotoxic T cells are so called because they are capable of bringing about the death of other cells through cytotoxic activity. These cells posses a receptor that can only recognize antigens that are bound to MHC class I molecules on the surface of cells. After recognition, the cytotoxic T cell kills cells that contain the presented antigen.
Helper T Cells
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Helper T cells are only capable of recognizing foreign antigens presented on the surface of cells by MHC class II molecules. Once recognition occurs through the specific receptors on the helper T cell, the helper T cell activates. Activated T cells then go on to increase in number (cell proliferation), and release important molecules that assist the functions of other immune cells.
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