The four main types of blood groups, defined by the ABO system, are:

• Blood group A : Has A antigens on the surface of red blood cells and anti-B antibodies in the plasma.
• Blood group B : Has B antigens on red blood cells and anti-A antibodies in the plasma.
• Blood group AB : Has both A and B antigens on red blood cells and no anti-A or anti-B antibodies in the plasma.
• Blood group O : Has no A or B antigens on red blood cells but has both anti-A and anti-B antibodies in the plasma

Additionally, each of these blood groups can be either RhD positive or RhD negative, depending on the presence or absence of the Rh factor (a protein on red blood cells), resulting in a total of eight blood types (e.g., A+, A-, B+, B-, AB+, AB-, O+, O-)

. This classification is crucial for blood transfusions and organ transplants to ensure compatibility and avoid life-threatening reactions