The Sid (Sd(a)) antigen is present on red blood cells of most individuals, but shows considerable variability in the strength of expression. About 96% of Caucasian individuals are Sd(a+), with the antigen strength forming a continuous curve ranging from ordinary (Sd(a+)) to strong (Sd(a++)); the strongest antigen expression is also known as 'Cad.' Most people also have weak anti-Sd(a) antibodies in their serum, which is usually of no clinical importance, but can be a problem if they are transfused with cells showing strong Sd(a++), or Cad, expression; this could result in red cell agglutination and a transfusion reaction (summary by Bird and Wingham, 1976).
Polyagglutination refers to red blood cells that agglutinate upon exposure to almost all human sera, but not to autologous serum or the sera of newborns. The condition becomes apparent during blood typing and cross-matching in the laboratory (summary by Beck, 2000). [from
OMIM]