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How Killer and Helper T Cells Recognize Viruses
Two pathways enable different types of T cells to recognize and respond to virus-infected cells.
T cells and B cells are the immune cells of the adaptive immune system. B cells are the antibody-producing type and T cells are the type that can directly kill an infected cell. T cells have a special receptor called the T cell receptor (TCR) and B cells have the B cell receptor (BCR).
Whereas the BCR directly interacts with specific parts of a virus, the TCR does not. Instead, T cells rely on infected cells or cells of the innate immune system to present the viral parts bound to one of two receptors (MHC I or MHC II). Once presented bound to MHC I or MHC II, then the TCR can recognize the viral molecule.
Infected cells and cells of the innate immune system process the viral proteins into little fragments called peptides. When those peptides are displayed on the surface of the infected cell or innate immune cell, they can be recognized by two kinds of T cells, CD8 T cells (called cytotoxic T cells) and CD4 T cells (called helper T cells).
Both types of T cells have TCRs, but the TCRs form a complex with another protein referred to as CD8 or CD4 on the CD8 T cells or the CD4 T cells, respectively. The complex of CD8 and TCR recognizes viral peptides bound by MHC I; the complex of CD4 and TCR recognizes viral peptides bound by MHC II.
Although not all peptides produced from the virus proteins will be recognized by T cells, many of the different peptides made even from a single virus protein can be. The parts of viral proteins (or any foreign protein) that are recognized by the adaptive immune cells are called antigens and the specific part of the antigen that is recognized by the BCR, TCR, or antibody is called an epitope.
Because each viral protein can be broken into multiple peptides, many of which will be antigens, there are many possibilities for training the immune system’s T cells to recognize the infection. Indeed, CD4 and CD8 T cells have custom TCRs that recognize a single epitope. This provides the body with many different populations of CD8 and CD4 T cells. Thus, even if mutations happen between the time a person is first exposed to the virus and the…
