In the cellular immune response, cells of the immune system kill cells of the body that have been infected with a virus or that are cancerous. This response relies on the lethal talents of cytotoxic T (TC) cells. TC cells contain molecules, called perforin, that they release onto target cells. The perforin pokes holes in the target cells and thereby kills them.
The cellular immune response occurs in two phases. In the first, called the activation phase, TC cells that have the appropriate T-cell receptors are activated and triggered to divide repeatedly. In the second, called the effector phase, these activated TC cells encounter target cells and kill them.
The cell becomes infected by the virus. The virus uses the cell's own machinery to replicate its genetic material and to produce new viral particles.
During the infection, some of the viral proteins, which are antigens, are broken down by the cell and attached to class I MHC proteins. These complexes of antigens and class I MHC proteins are then presented on the cell's surface.
A cytotoxic T cell participates in the next stage of the cellular immune response. This cytotoxic T cell has T-cell receptors that are specific for the displayed antigen. The T-cell receptors bind to the complexes of antigens and class I MHC proteins. This binding activates the cytotoxic T cell.
The activated cytotoxic T cell proliferates to form a clone of cells, each with T-cell receptors specific for the same antigenic determinant.
In the effector phase, these cytotoxic T cells can now encounter and eliminate other cells of the body that are infected with the virus. Infected body cells present the viral antigens on their class I MHC proteins. Cytotoxic T cells from the activation phase, each with receptors specific for the viral antigen, bind to these complexes.
Upon binding, a cytotoxic T cell is stimulated to release molecules called perforin. Perforin kills the target cell by causing the cell to lyse.
In the cellular immune response, cytotoxic T (TC) cells recognize and kill cancerous or virally infected cells of the body. In the animation, we examined the case of virally infected cells, but the principles of cell recognition and killing are the same in both cases.
The TC cells recognize these abnormal body cells because they display unique antigens, in conjunction with class I MHC proteins, on their cell surfaces. However, this antigen display is not performed only by virally infected or cancerous cells. Virtually all cells of the body continually sample their own cellular proteins (antigens) and display pieces of them on their cell surfaces.
Why, then, don't all cells trigger responses from TC cells? The answer is that early in the development of TC cells (and helper T cells, too), the T cells are in contact with and "screened" by other cells of the body. The body cells provide the developing T cells with an opportunity to test their T-cell receptors. Those that recognize the body's "self" antigens die.
If a body cell becomes cancerous, unique changes occur in the cell. For example, the cancer cell may make a mutant protein that is unique to the body and appears as a "nonself" antigen when it is sampled and displayed on the surface of the cancer cell. Specific TC cells that can recognize this antigen typically exist in the body. These TC cells become activated and proliferate. The activated TC cells can then kill this and other cancer cells that display the same antigen.