Cell-mediated Immune Responses

Cell-mediated immunity is an adaptive cellular response to prevent the spread of infection.

When a foreign pathogen, an infectious particle like a bacterium is detected in the body, specific white blood cells called macrophages are called in as the first line of defense. Via phagocytosis they engulf and digest the harmful pathogens that contain unique proteins, known as antigens, on their surface.

The antigens are then fragmented within the phagolysosome and transported to the macrophage's surface, which is why they are referred to as antigen-presenting cells or APCs. Other proteins, major histocompatibility class or MHC molecules embed the antigen fragments for presentation on the surface of the APC.

The complex can now be detected by T lymphocytes or T cells, another type of white blood cell, that rapidly multiplies by mitosis to generate specialized T cells to combat the infection.

The main ones, helper T cells, secrete chemicals to stimulate the growth and differentiation of cytotoxic T cells which kill damaged cells. They can also remain in the host, memory T cells, after the infection has been cleared, just in case the same pathogen is re-encountered at a later time. Once the infection is under control, suppressor T cells inhibit the immune system to prevent further distraction to the host tissue.