Tuberculin purified protein derivative

When exposed to M. tuberculosis antigen, the sensitization initiates in the regional lymph nodes where T lymphocytes proliferate in response to the antigenic stimulus to give rise to specifically sensitized lymphocytes which may exist in the circulation up to many years. Antigen is presented to T cells by being ingested by antigen presenting cells (APC), which then present it on their surface to lymphocytes in combination with various MHC molecules once they reach local lymph nodes. Tuberculin PPD most likely interacts with toll-like receptor 2 expressed on APCs that initiates an inflammatory response. Subsequent restimulation of these sensitized lymphocytes with the same or a similar antigen, such as the intradermal injection of tuberculin PPD, evokes a local reaction mediated by these cells. This reaction is referred to as a delayed-type hypersensitivity response that includes vasodilation, edema, and the infiltration of lymphocytes, basophils, monocytes, and neutrophils into the site of antigen injection. The sensitized antigen-specific T lymphocytes proliferate and release lymphokines, which mediate the accumulation of other cells at the site. In vitro studies show that Tuberculin PPD promotes the upregulation of vascular endothelial growth factor (VEGF) expression in T lymphocytes through major histocompatibility (MHC) class II interaction with CD4+ T lymphocyte interaction. The reactions are evident after 5-6 hours following administration.