Q.Give reason:

Blood Groups: ABO and Rh Systems

Every red blood cell carries identity markers called antigens on its surface. Your immune system uses these markers to tell "self" from "foreign." If a foreign antigen shows up in your bloodstream, your plasma antibodies attack it. Blood grouping is simply a classification of which antigens a person's red cells carry, and the two systems that matter most clinically are the ABO system and the Rh system.

The ABO System

Two antigens define this system: A and B. Depending on which ones a person inherits, they fall into one of four groups.

The governing rule is that a person never carries an antibody against their own antigen — that would mean the immune system attacking its own blood cells. So a person with antigen A makes anti-B antibodies (ready to attack any B cells that enter), and a person with no antigens at all (group O) makes both anti-A and anti-B.

Working Out Transfusion Compatibility

• Group A (anti-B present) cannot accept B or AB blood, but can accept A or O.
• Group B (anti-A present) cannot accept A or AB blood, but can accept B or O.
• Group AB has no antibodies at all, so it can accept blood from any ABO group — the universal recipient.
• Group O carries both antibodies, so it can only accept O blood — but because its red cells carry no antigen for anyone else's antibodies to attack, it can donate to any group — the universal donor.

The Rh System

The Rh system centres on a single important antigen, the Rh (D) factor. A person is either:

• Rh-positive (Rh+) — carries the Rh antigen on the red cells, or
• Rh-negative (Rh-) — does not carry it.

Unlike the ABO system, an Rh-negative person is not born with anti-Rh antibodies. Those antibodies only appear after the person's immune system has actually been exposed to Rh-positive blood, through a transfusion or a pregnancy.

• An Rh-negative person should receive Rh-negative blood.
• An Rh-positive person can safely receive either Rh-positive or Rh-negative blood.

Combining Both Systems …