The M1 and Tm antigens require M and N gene-specified amino acids for expression

It is known that the major red cell sialoglycoprotein (SGP) of M1 + red cells has the same amino acid sequence as M SGP, whereas that of Tm + cells has the same sequence as N SGP. M1 and Tm are serologically demonstrable when a substitution of N-acetyl-D-glucosamine for N-acetyl-neuraminic acid occurs in one or more of the alkali-labile tetrasaccharides bound at positions 2, 3, and/or 4 of M or N SGP, respectively. The authors used two serums containing anti-M and potent anti-M1 and two containing anti-Tm in tests for antibody crossreactivity and cross adsorption and in antibody inhibition studies. The distinct specificities of anti-M1 and anti-Tm, as determined by these studies, show that the M and N gene-specified amino acids are more important than the glycosylation change in the fine structure of M1 and Tm antigens.