OBSOLETE. A major histocompatibility complex class II receptor. These display processed antigens from virally-infected or transformed cells. Class-II-positive cells ('antigen-presenting cells') can take up antigens from outside by endocytosis, degrade them into small peptides, and re-export the peptides (now bound to MHC class II protein) to the cell surface. These peptide-MHC class II complexes can then be recognized by specific CD4+ lymphocytes.