Vel Blood Group Antigen
Vel is a high-prevalence RBC antigen that was discovered in 1952. Only 1 in 4000 people are Vel negative. In addition, some people have very weak expression of Vel antigen by serological testing.
The SMIM1 gene codes for the Vel antigen. The Vel negative phenotype is caused by a homozygous 17 base pair deletion in exon 3 of the SMIM1 gene which leads to a frame shift mutation and premature stop codon. No Vel protein is transcribed. The Velweak phenotype is due to a heterozygous deletion in the same exon, resulting one functional copy of the SMIM1 gene.
If a patient who is Vel negative is transfused with Vel positive red blood cells, they may form anti-Vel alloantibodies. These antibodies can be clinically significant. Some antibodies do not cause a hemolytic transfusion reaction, while others cause severe reactions. Likewise, Vel antibodies can cause mild to severe hemolytic disease of the fetus and newborn (HDFN).
