Do cells really attack other cells?
They sure do. Depicted here is a group of T cells attacking a cancer cell. When they can, the T cells search out and destroy “bad” cells.
Cell-Mediated Immune Response
In addition to the humoral response, the other type of immune response is the cell-mediated immune response, which involves mainly T cells. It leads to the destruction of cells that are infected with viruses. Some cancer cells are also destroyed in this way. There are several different types of T cells involved in a cell-mediated immune response, including helper, cytotoxic, and regulatory T cells.
T Cell Activation
All three types of T cells must be activated by an antigen before they can fight an infection or cancer. T cell activation is illustrated in the figure below. It begins when a B cell or nonspecific leukocyte engulfs a virus and displays its antigens. When the T cell encounters the matching antigen on a leukocyte, it becomes activated. What happens next depends on which type of T cell it is.
Helper T Cells
Helper T cells are like the “managers” of the immune response. They secrete cytokines, which activate or control the activities of other lymphocytes. Most helper T cells die out once a pathogen has been cleared from the body, but a few remain as memory cells. These memory cells are ready to produce large numbers of antigen-specific helper T cells like themselves if they are exposed to the same antigen in the future.
Cytotoxic T Cells
Cytotoxic T cells destroy virus-infected cells and some cancer cells. Once activated, a cytotoxic T cell divides rapidly and produces an “army” of cells identical to itself. These cells travel throughout the body “searching” for more cells to destroy. the figure below shows how a cytotoxic T cell destroys a body cell infected with viruses. This T cell releases toxins that form pores in the membrane of the infected cell. This causes the cell to burst, destroying both the cell and the viruses inside it.
After an infection has been brought under control, most cytotoxic T cells die off. However, a few remain as memory cells. If the same pathogen enters the body again, the memory cells mount a rapid immune response. They quickly produce many copies of cytotoxic T cells specific to the antigen of that pathogen.
Regulatory T Cells
Regulatory T cells are responsible for ending the cell-mediated immune response after an infection has been curbed. They also suppress T cells that mistakenly react against self antigens. What might happen if these T cells were not suppressed?
- Activated T cells destroy certain cancer cells and cells infected by viruses.
- Memory T cells remain in the body after the immune response and provide antigen-specific immunity to the virus.