MHC

a) In the MHC Class I, the antigenic proteins which enters the Antigen Presenting Cell will firstly undergo degradation by a protein called protease. After degrading, it will be transported by TAP, and then the antigenic peptide will be loaded onto the Class I MHC Molecule. The MHC then move to the cell surface and display the antigenic peptide. The T killer cell bind onto the MHC molecule, and the killer T cell will then be activated, and then it will be able to kill the infectious foreign particles directly.

b) In the MHC Class II, when the antigens enter the Antigen Presenting Cell, phagosomes are formed, and the phagosomes containing the antigens are merged with the lysosome. Then, the MHC class II molecules migrate into the phagolysosome where the antigen is being degraded, and the invariant chain is removed for peptide binding. The Class II MHC molecule then moves to the cell surface, and present the antigenic peptide to the helper T-cell, and it will help to activate immune cells to fight against the foreign infectious particles.