Cell contains two different antigen-processing pathways that serve to present peptides to T cells. HLA class I molecules are loaded in the endoplasmic reticulum with peptides derived from degradation of cystosolic proteins by the proteosome, and these HLA class I/peptide complexes are then surface expressed and presented to CD8 T cells. HLA class II molecules exit the endoplasmic reticulum in association with the invariant chain which occupies their-peptide binding groove. In the endocytic pathway, proteolysis results in the degradation of the invariant chain, leaving the residual class II associated invariant chain peptide in the binding groove. In a process catalysed by HLA-DM (in human), class II associated invariant chain peptide is replaced by peptides derived by proteolysis from proteins resident or internalized into the endocytic pathway. After surface expression, these are presented to CD4 T cells. Hence, HLA class I generally serves as a reporter of intracellular infection , while HLA class II sense the antigens present in the extracellular millieu.