Blood groups are another layer of your biological identity, and like genotype, they are something everyone should know. The most commonly used blood group system is the ABO system, which classifies blood into four types: A, B, AB, and O. These letters refer to antigens — specific proteins that sit on the surface of your red blood cells. Blood group A means your red blood cells carry A antigens. Your blood plasma contains anti-B antibodies, which means your immune system will attack type B blood if it enters your body. Blood group B means your red blood cells carry B antigens, and your plasma contains anti-A antibodies. Blood group AB means your red blood cells carry both A and B antigens, and your plasma contains neither antibody. People with AB blood can receive red blood cells from any ABO group. This is why AB is sometimes called the universal recipient. Blood group O means your red blood cells carry neither A nor B antigens. Your plasma contains both anti-A and anti-B antibodies. However, because your cells have no A or B antigens to trigger a reaction in others, O negative blood in particular can be given to almost anyone in an emergency. This is why O negative is often called the universal donor. Why does this matter in practice? In a medical emergency requiring a blood transfusion, receiving the wrong blood type triggers a severe immune reaction where your body attacks the donated blood. This can be fatal. In Nigeria, where blood banks are often under-resourced and donation is critical, knowing your blood group and being ready to donate can directly save lives. Your blood group is fixed from birth and never changes. It takes less than 15 minutes to find out.