From the genetic studies, it has become clear that the specific structures of the O antigens of 13, 22 or 13, 23 (group G) with blood group H activity, 40 (group R) with A, 43 (group U) with B and 4, 12 (group B), that is, the structures of the S-specific side chains, are determined at a gene locus (the O locus) close to the his locus in Salmonella. Phage P22 (iota) can produce the O antigen 1 only in an organism in which the O antigen 12 is manifested.
