HLA class II histocompatibility antigen, DR alpha chain (HLA-DRA) is a ~28 kDa protein. An alpha chain of antigen-presenting major histocompatibility complex class II (MHCII) molecule. In complex with the beta chain HLA-DRB, displays antigenic peptides on professional antigen presenting cells (APCs) for recognition by alpha-beta T cell receptor (TCR) on HLA-DR-restricted CD4-positive T cells. This guides antigen-specific T-helper effector functions, both antibody-mediated immune response and macrophage activation, to ultimately eliminate the infectious agents and transformed cells. Typically presents extracellular peptide antigens of 10 to 30 amino acids that arise from proteolysis of endocytosed antigens in lysosomes. In the tumor microenvironment, presents antigenic peptides that are primarily generated in tumor-resident APCs likely via phagocytosis of apoptotic tumor cells or macropinocytosis of secreted tumor proteins. 1. Shams, H. et al. (2004) Journal of immunology (Baltimore, Md. : 1950) 173, 1966-77. PMID: 152659312. Röhn, TA. et al. (2004) Nature immunology 5, 909-18. PMID: 153225403. Deng, L. et al. (2007) Nature immunology 8, 398-408. PMID: 173343684. Kwok, WW. et al. (2012) Journal of immunology (Baltimore, Md. : 1950) 188, 2537-44. PMID: 223270725. Scally, SW. et al. (2013) The Journal of experimental medicine 210, 2569-82. PMID: 241904316. Muehling, LM. et al. (2016) Journal of immunology (Baltimore, Md. : 1950) 197, 3214-3224. PMID: 2759132310. Kropshofer, H. et al. (1997) Immunity 6, 293-302. PMID: 9075930 | 
