The most common blood type in humans is O, specifically type O positive. Blood types are classified based on the presence or absence of specific antigens on the surface of red blood cells. Type O blood does not have A or B antigens, making it a universal donor, which can be transfused to individuals with any blood type. O positive, in particular, is the most prevalent blood type worldwide, found in approximately 37-40% of the population. Understanding blood types is critical for safe blood transfusions, organ transplants, and other medical procedures where compatibility is essential.