When any sort of APC(Antigen presenting cell) is infected, this system is used to let the T-cells(the fighters) know to fight the pathogens in the body. Different types of T-cells: Cytotoxic - CD4+ Helper - CD8+ TREGs - CD4+ (and CD25+ in rats) Memory - CD4 and 8 positive
This is what a TCR(T-cell receptor) is and its use. Hover you mouse over the cells to take a closer look
