Transport in Humans

A blood transfusion is the medical process of giving a patient blood from a donor (or blood products such as plasma or platelets). Transfusions save lives during surgery, after major injuries, and in conditions like leukaemia. But blood from a random donor is not always safe to give; it depends on the blood group.

The ABO blood group system

Every person's red blood cells carry one of four combinations of two possible antigens on their surface, called A and B:

The plasma contains antibodies against the antigens that the person does not have on their own red blood cells.

Why blood groups matter for transfusion

If a patient with blood group A is given blood group B, two things go wrong:

1. The recipient's anti-B antibodies bind to the B antigens on the donor's red blood cells.
2. The bound antibodies clump (agglutinate) the donor cells together.

The clumps block small blood vessels and can cause shock, kidney failure or death. So compatible blood groups have to be matched before transfusion.

Compatibility rules:

• Group O is the universal donor: people with group O have no A or B antigens on their red blood cells, so the recipient's antibodies have nothing to bind to. Group O blood can be given to anyone.
• Group AB is the universal recipient: people with group AB have no anti-A or anti-B antibodies in their plasma, so they cannot mount an agglutination response. They can receive any blood group.
• For all other groups, donor and recipient must match exactly (A → A, B → B, AB → AB).

The Rh (rhesus) system

Independently of the ABO system, red blood cells either do or do not carry another antigen called the rhesus (Rh) antigen:

• Rh-positive (Rh+) people have the Rh antigen.
• Rh-negative (Rh−) people do not.

About 85% of people are Rh-positive. The Rh system matters mainly during pregnancy and for repeated transfusions; once an Rh-negative person has been exposed to Rh-positive blood, their immune system can make anti-Rh antibodies that cause problems in any later exposure.

A patient is described by combining their ABO group with their Rh status, giving the familiar labels like A+, O−, AB+ and so on.

A safe match is one where the donor blood has no antigens that the recipient has antibodies against. The fully cross-matched table looks like this:

Each row shows who can receive each blood group. AB+ is the universal recipient (every row entry filled); O− is the universal donor (top row, only matched by itself, but its column is empty, meaning O− can be given to anyone).