Upon activation, the T cells multiply and by the release of cytokines are able to set up an immune response that will recognize and destroy cells with this same foreign antigen/HLA complex, when next encountered. The exact mode of activation of HLA class I and HLA class II antigens is different in this process. HLA class I molecules, by virtue of their presence on all nucleated cells, present antigens that are peptides produced by invading viruses. These are specifically presented to CD8 T cells, which will then act directly to kill the virally infected cell. HLA class II molecules have an intracellular chaperone network which prevents endogenous peptide from being inserted into its antigen-binding cleft. They instead bind antigens (peptides) which are derived from outside of the cell (and have been engulfed). Such peptides would be from a bacterial infection. The HLA class II molecule presents this "exogenous" peptide to CD4 T cells which then set up a generalized immune response. Thus , it is apparent that HLA products are an integral part of immunological health, and therefore, it is no surprise to see a wide variety of areas of clinical and genetic implications.
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