Question

IMMUNOLOGY ONLY

13. Mycobacterium tuberculosis is an intracellular bacterial pathogen. An individual’s exposure to this pathogen can be tested by injecting it into the skin. What type of reaction would you expect in an individual who had been exposed to M. tuberculosis? What is the principle behind this test?

Answer 1

For testing tuberculosis infection, a purified protein derivative called tuberculin is extracted from M.tuberculosis culture and then injected onto the skin. This protein causes Type IV hypersensitivity reaction or delayed type hypersensitivity reaction. This is not an antibody response but a cell mediated immunity response where the T-lymphocytes interacts with the antigen presenting cells such as monocytes, macrophages etc.

The principle of this test is based on the fact that if the person is already infected by M.tuberculosis bacteria , the T-cells in the person's body will be sensitized to the M.tuberculosis bacterial proteins. Now when the extracted Tubercuulin is administered in the skin , the sensitized T cells will be attracted to the skin site and will release Lymphokines which is a chemical massenger. This lymphokines causes induration (a hard inflammation with distinct margins around the injection site) throuh local vasodilation which causes fluid deposition leading to edema.

The reaction is at its peak after 24 hours of administration of the injection, and the reading for the result is taken after 48 or 72 hours when the diameter of the induration is checked. A certain diameter is indicative of infection with M.tuberculosis bacteria.