Which blood type is known as the universal donor?

Type O blood is known as the universal donor because it lacks A and B antigens on the surface of its red blood cells. This means that individuals with Type O blood can donate to any other blood type (A, B, AB, or O) without causing an immune response. When a person receives blood, their immune system reacts to any foreign antigens present; since Type O does not have these antigens, it is compatible with all other blood types.

In contrast, Type A blood has A antigens, Type B has B antigens, and Type AB has both A and B antigens. Each of these types can trigger an immune response in individuals whose blood contains antigens that the donor's blood lacks. For example, if a person with Type B blood receives Type A blood, their immune system will recognize the A antigens as foreign and mount an attack against them. This compatibility issue is what makes Type O the preferred choice for emergency transfusions, allowing greater safety and flexibility in blood donation.