English: There are three basic variants of immunoglobulin antigens in humans that share a very similar chemical structure but are distinctly different. Red circles show where there are differences in chemical structure in the antigen-binding site (sometimes called the antibody-combining site) of human immunoglobulin. Notice the O-type antigen does not have a binding site. 1. Hara, Atsushi; Imamura, Akihiro; Ando, Hiromune; Ishida, Hideharu; Kiso, Makoto (31 December 2013). "A New Chemical Approach to Human ABO Histo-Blood Group Type 2 Antigens". Molecules 19 (1): 414–437. doi:10.3390/molecules19010414.