The most common type of blood in humans is type O. It is classified based on the presence or absence of specific antigens on the surface of red blood cells. Type O blood lacks A and B antigens, making it compatible with any other blood type in transfusions, hence its designation as the universal donor type. Individuals with type O blood can donate to all other groups—A, B, AB, and O—while they can only receive blood from other O-type donors. This prevalence and versatility make type O an essential component of blood donation practices.