# Type IV Hypersensitivities ___ A **type IV hypersensitivity** is [[hypersensitivity]] that is brought about by activating [[T-cells]] (instead of antibodies, which all the other hypersensitivity reactions do). Two such responces are sseen with exposure to tuberculosis and to poison ivy/oak, but also seen elsewhere like in [[organ transplant|graft rejection]]. ## In Responce to *M tuberculosis* An example of a type IV hyersensitivity involves the body's reaction to the bacteria [[tuberculosis|Mycobacterium tuberculosis]]. During someone's first exposure to *M. tuberculosis* the reaction proceeds as normal, but for whatever reason the [[memory cells|memory T-cells]] that are formed happen to release a *lot* of [[cytokines]] if they are ever to run into that same antigen again. These cytokines are chemoattractants, which means they end up recruiting a whole bunch of [[macrophages]] to the area, which causes lumps. This is actually how the [[tuberculin skin test]] works, because when they give they inject a very small amount of the TB antigen under the skin and wait to see if any lumps from teh hypersensitivity form. ## Poison Ivy & Poison Oak **Poison ivy** **poison oak** have a chemical called **pentadecylcatecol**, which is hydrophopic (due to it's long hydrocarbon tail), and thus able to slip right through the lipid membrane of our [[epithelial tissue|epithelial cells]]. There they interact with some of the proteins that are found inside the cell. Unfortunately when the cell loads one of those on to the [[MHC molecule]] molecule for presentation to a [[cytotoxic T-cells]], they register this as an antigen and activates it. This triggers [[apoptosis]] as well as creates memory cells, effector cells. The signs of poison ivy and poison oak are - blisters - rashes - redness - itching ## Graft vs Host Disease **Graft vs host disease** is an example too. ___