The human body can respond to antigen in many different ways. These fall into two major categories:
- antibody-mediated immunity. Antibodies — dissolved in blood, lymph, and other body fluids — bind the antigen and trigger a response to it. (This form of immunity is also called humoral immunity.)
- cell-mediated immunity (CMI). T cells (lymphocytes) bind to the surface of other cells that display the antigen and trigger a response. The response may involve
- other lymphocytes
- any of the other white blood cells (leukocytes)
Examples of Cell-Mediated Immunity
Delayed-Type Hypersensitivity (DTH): the tuberculin test
Many states in the United States require that professors and teachers (among others) be checked periodically for tuberculosis. This chronic disease, caused by Mycobacterium tuberculosis, evokes an immune response that, unfortunately, does not cure the patient, but does provide an inexpensive test for the disease called the tuberculin test (or Mantoux test). A tiny amount of protein, extracted from the bacteria, is injected into the skin. If the subject is currently infected, or has ever been infected, with the bacteria, a positive test results. In 24 hours or so, a hard, red nodule develops at the site of the injection. This nodule is densely packed with lymphocytes and macrophages.
In Europe, most people produce a positive tuberculin reaction, not because they have had the infection, but because earlier they had been vaccinated against tuberculosis with a preparation of a related (but harmless) bacterium called BCG.
The response to tuberculin is called "delayed" because of the time it takes to occur (in contrast to the "immediate" responses characteristic of many antibody-mediated sensitivities like an allergic response to a bee sting).
DTH is a cell-mediated response (in fact, anti-tuberculin antibodies are rarely found in tuberculin-positive people). The T cells responsible for DTH are members of the CD4+ subset.
Many people develop rashes on their skin following contact with certain chemicals. Nickel, certain dyes, and the active ingredient of the poison ivy plant are common examples. The response takes some 24 hours to occur, and like DTH, is triggered by CD4+ T cells. The actual antigen is probably created by the binding of the chemical to proteins in the skin. After the antigen is engulfed by dendritic cells in the skin, they migrate to nearby lymph nodes where they present fragments of the antigen to CD4+ T cells. The activated T cells migrate from the lymph nodes to the skin to elicit the inflammatory response.
Killing intracellular parasites
Some human pathogens avoid exposure to antibodies by taking up residence within cells. These include all viruses (discussed in the next section), and some bacteria such as
- the bacterium that causes Legionnaires's disease
- Listeria monocytogenes, that humans sometimes acquire from contaminated food and even some protozoans.
These microorganisms are engulfed by phagocytic cells, like macrophages, but evade the normal intracellular mechanisms that should destroy them. However, the macrophages can present fragments of antigens derived from these parasites. These are displayed in the class II histocompatibility molecules of the macrophages. CD4+ T cells responding to these epitopes release lymphokines that stimulate the macrophages sufficiently that they can now begin to destroy the organisms.