A local police encounters a most wanted criminal and puts him behind bars. The criminals’

Alocal police encounters a most wanted criminal and puts him behind bars. The criminals’ information is now sent to the FBI for further investigation. It is the FBI which will take appropriate actions against the criminal.

Now, imagine the criminal is the antigen which has been encountered by a cell - local police. The cell represents a part of antigen - criminals information on their cell surface so that the T-cells - the FBI can identify the antigen and destroy it!

T-cells can bind to only those antigens which are present on cell membrane proteins called Major histocompatibility complexes. The FBI can trust the police only if they come with a badge.

There are 2 class of these MHC Complexes :
• Class I (present on the surface of nucleated cells)
• Class II (present on the surface of Antigen Presenting Cells)

Also, there are 2 major types of T-cells:T-Helper cellsandT-Cytotoxic cells.

The TH-cells recognizes antigens on MHC Class II Complex present only on Antigen Presenting Cells and TC-cells cells recognizes antigens on MHC Class I Complex present only on Nucleated cells.