The ABO blood group isn't the only one. In fact, there are about 34 different human blood groups. Now, scientists in Nature Genetics have explained the genetic basis behind a rare blood group called Vel. Vel Blood Group Most people are Vel+ (Vel-positive), meaning that they possess the Vel antigen on the surface of their red blood cells. However, about 1 in 5,000 individuals are Vel- (Vel-negative), and thus do not carry the antigen. Their immune system recognizes the Vel antigen as foreign, and if given Vel+ blood, they can experience kidney failure or possibly death. A screen currently exists for this blood type, but it can be inaccurate; therefore, identifying the blood type's genetic basis will allow doctors to more easily screen patients.