Easily confused with T activation, Tn polyagglutination is an acquired red blood cell polyagglutination disorder caused by a mutation rather than an infection (in contrast to T activation). This situation results from a somatic mutation of a gene carried on the X chromosome. The mutation results in incomplete synthesis of normal residues on glycophorins A and B, with resultant exposure of a normally hidden antigen (cryptantigen). The red cells are affected in varying proportions, resulting in a characteristic “mixed-field” appearance in tube reactions. Tn polyagglutination can be separated from other forms in the laboratory via characteristic reactions with a panel of lectins. Some epithelial cancers appear to have a worse prognosis when Tn expression is increased.