Blood is classified into four main groups - A, B, AB, and O - based on the presence or absence of antigens on RBCs. These groups are determined by the ABO system and are important for safe blood transfusions. Each group can be Rh positive (+) or Rh negative (–), making eight possible blood types in total.

Quick Chart

• A → A antigen, anti-B antibody
• B → B antigen, anti-A antibody
• AB → Both A & B antigens (universal receiver)
• O → No antigens (universal donor)