HLA class II proteins have constitutive expression limited to immune active cells, including B cells and other antigen-presenting cells (APC). In addition, expression can be upregulated to other cell types in an inflammatory environtment. Each HLA class II protein comprises an α- and ß- chain both of wich are encoded within the MHC (eg. HLA-DRA with HLA-DRB1). Each chain has two extra-cellular domain and again, those most distal to the cell membrane form a peptide-binding groove which always carries a self- or non-self-protein-derived peptide sourced from outside the cell by endocytosis. Class II proteins and bound peptides interact with CD4 T cells (often helper) and their receptors. The affinity with wich peptides are bound to HLA molecules will influence allogenicity as presented to and recognized by T cell receptor.
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