Cell-Mediated Immunity

The various arms of the immune system can be categorized in several different ways. One of these is in terms of antibody-mediated and cell-mediated immunity.

During a cell-mediated immune response, the principal players are helper T cells and cytotoxic T cells, in contrast to antibody-mediated immunity (see previous articles in this series) in which the main participants are B cells and the antibody they produce.

During a cell-mediated immune response (also known as a type 1 response), B cells—and antibody production—are not activated. Instead, a range of other cells, which are capable of either directly or indirectly killing intracellular bacteria and infected host cells, are activated.

These include phagocytes such as macrophages, as well as natural killer cells, which are capable of directly killing bacterial cells as well as infected host cells.

As the adaptive immune response kicks into gear, type 1 helper T cells are activated, and these help control and direct the immune response to ensure it is capable of eliminating the pathogen causing the infection.

In addition, cytotoxic T lymphocytes are activated. These are antigen-specific cells which, once activated, are capable of killing host cells which have been infected with intracellular pathogens.

Cell-mediated immunity protects the body against infection and invasion in three main ways:

• Activation of the innate immune system – non-specific natural killer cells and macrophages

Both of these cell types are able to kill host cells which have become infected with intracellular pathogens (such as viruses, parasites, and certain types of bacteria). Both can also kill extracellular pathogens.

• Activation of the adaptive immune system – antigen-specific cytotoxic T cells

In an ongoing immune response, antigen-specific cytotoxic cells become activated after they recognize antigen presented by macrophages and other antigen-presenting cells. These also kill infected host cells, but do so with much greater specificity, and greater efficiency than any other kind of immune cell.

• Stimulation of cells which secrete cytokines that direct and regulate the immune response to produce a response which is tailored towards eliminating a specific pathogen.

This type of immune response is most effective at killing intracellular pathogens such as viruses and certain types of bacteria. In addition, the cell-mediated immune response plays a role in eliminating cancer cells (this role is being explored in cancer therapy), and is an important factor in transplant rejection.