Both class I and class II HLA molecules contain four immunoglobulin-like domains, two of which form a peptide-binding groove consisting of two paralel ∝-helix amino chains on an eight ß-pleated polypeptide sheet.
The primary role is to bind small antigenic peptides for presentation to the T-cell receptor, which then lead to specific T-cell activation. Such antigenic peptides may generate from viruses and other microorganisms, and are essential in eliciting cellular immune responses to infections.
HLA molecules with bound peptides on the surface of so-called antigen presenting cells (APC) by two general mechanisms.
The endogenous pathway leads to the expression of class I molecules complexed with endogenously produced peptides that are 8 to 9 amino acid residues long. Such complexes can activate CD8 T cell, including cytotoxic lymphocites.
In the exogenous pathway, APC take up and process exogenous proteins to generate peptides 10 to 25 amino acid residues long that bind to class II molecules, which can lead to activation of CD4 T lympgocytes, including helper T cells.