A− → AB− red cells — compatible

Why

The recipient's antibodies must not attack the antigens on the donated red cells.

The other direction

AB− giving the same component back to A− is not compatible. This pair flips when you swap the direction.

Same pair, other components

Change the component and the answer can flip. Of the 64 donor–recipient pairs, 39 give a different answer for red cells than for plasma.

Where this donor can give

Where this recipient can draw from

Where the rule flips

The antigens sit on the red cells; the antibodies float in the plasma. So when you move red cells, the recipient’s antibodies are the problem; when you move plasma, the donor’s antibodies are. The whole condition reverses, and so does the table — fold the red cell chart along its diagonal and you get the plasma chart.

Rh only matters for red cells

ABO antibodies are there from birth; anti-D is not. It only appears after an Rh− person has received Rh+ red cells. That is why the first such transfusion can pass uneventfully and the next one is the dangerous one — the same mechanism that matters in pregnancy. Plasma carries almost no red cells, so Rh is not checked for it.

How to read it

• The name of a blood type is its antigen list — A has the A antigen, AB has both, O has none.
• Antibodies attack what you do not have — O carries both anti-A and anti-B.
• To move red cells, check whether the recipient’s antibodies attack the donated antigens.
• To move plasma, check the reverse: whether the donated antibodies attack the recipient’s antigens.
• Rh is checked only for red cells — there is no natural anti-D; it appears after exposure.
• Whole blood stacks both conditions, so only same-ABO pairs survive.

This chart does not decide a real transfusion

Beyond ABO and RhD there are hundreds of other red cell antigens, and someone who has been transfused repeatedly may carry antibodies to some of them. A real transfusion is therefore decided by a crossmatch — the lab mixing those two people’s blood directly. This chart explains why the rules have the shape they do.