The<unk>  is a human blood group defined by the presence or absence of the Sd(a) antigen (also known as<unk>  antigen) on a person's red blood cells. About 96% of people are positive for the Sd(a) antigen, which is inherited as a dominant trait. Among Sd(a) positive individuals, the expression of the antigen ranges from extremely weak to extremely strong. Very strong expression of the antigen is referred to as a Sd(a++) phenotype. In addition to being expressed on red blood cells, Sd(a) is secreted in bodily fluids such as saliva and breast milk, and is found in the highest concentrations in urine. Urine testing is considered the most reliable method for determining a person's<unk>  blood type. Antibodies against the Sd(a) antigen are naturally occurring, meaning people produce them without having been exposed to Sd(a) positive blood through transfusion or pregnancy. These antibodies are not usually considered to be clinically significant, but there have been two cases of transfusion reactions associated with transfusion of Sd(a++) blood to people with anti-Sd(a) antibodies.<unk>  was officially designated a blood group in 2019 after its genetic basis was discovered.
Sid blood group system
Lan blood group system
Junior blood group system
Rh blood group system
Augustine blood group system
Vel blood group
Hh blood group
P1PK blood group system