Blood types and transfusion

The glycocalyx is the coat of sugar chains attached to membrane proteins and lipids on the outer surface. It carries a cell's identity markers.

• Correct: The layer of sugar chains on a cell's outer surface:
• The protein skeleton just under the membrane:
• The fluid inside the nucleus:

Each antibody's binding sites fit one antigen's shape, the way a key fits one lock.

• Any foreign cell it meets:
• Correct: One specific antigen:
• Oxygen:

Heme from destroyed red cells is converted to bilirubin. If the liver cannot clear it fast enough, it builds up and yellows the skin and eyes.

• Correct: Bilirubin:
• Ferritin:
• Erythropoietin:

Clumping with anti-A and anti-B means the red cells carry both A and B antigens: type AB. No clumping with anti-D means no D antigen: Rh negative.

• O positive: Type O cells carry neither A nor B, so they would not clump with anti-A or anti-B.
• Correct: AB negative: Correct. A and B present, D absent.
• AB positive: An Rh-positive sample would clump in the anti-D well too.
• O negative: Type O cells would stay smooth in all three wells if Rh negative.

Her plasma holds anti-A and anti-B, so she can receive only red cells with neither antigen: type O. As an Rh-positive person she makes no anti-D, so either Rh-positive or Rh-negative O cells are safe.

• Correct: O negative and O positive only: Correct. Only type O cells, of either Rh type.
• O positive only: O negative is also safe: it lacks the D antigen, which she already has.
• Any Rh-positive type: A, B and AB red cells would meet her anti-A or anti-B.
• O, A and B, as long as they are Rh positive: Her anti-A and anti-B would attack A and B red cells whatever their Rh type.

An ABO-incompatible transfusion sets off an immediate antibody attack on the donor cells. Agglutination and hemolysis release hemoglobin into the plasma and urine, and the reaction causes fever and a fall in blood pressure. The transfusion must be stopped at once.

• Agglutination of the donor red cells: up. The patient's anti-B binds the B antigens on the donor cells and cross-links them into clumps.
• Free hemoglobin in the plasma: up. Antibodies on the donor cells trigger plasma proteins that burst them, releasing hemoglobin into the plasma.
• Hemoglobin in the urine: up. Free hemoglobin in the plasma is small enough to be filtered by the kidneys and spills into the urine, darkening it.
• Body temperature: up. Chemical signals released during the antibody attack cause fever and chills.
• Blood pressure: down. Signals released in the reaction widen blood vessels, so pressure falls and shock can follow.

He has no anti-D yet, so the donor cells are not attacked at once. Over the next weeks his immune system may make anti-D against the D antigen he has met. A later Rh-positive transfusion could then trigger a reaction.

• An immediate, severe transfusion reaction: An immediate reaction needs antibodies already present. Anti-D forms only after exposure.
• Correct: Nothing now, but he may make anti-D for next time: Correct. The first exposure sensitizes; the danger comes later.
• He becomes Rh positive permanently: Transfused cells live about 120 days at most; his own red cells stay Rh negative.
• Nothing now or later: Rh does not matter in men: Rh matters for any future transfusion. In men there is no pregnancy risk, which is why O-positive blood is often used for men in emergencies.

Fetal red cells entered her blood at the first delivery, and she made anti-D. That kind of antibody crosses from mother to fetus. In this Rh-positive fetus, it binds and destroys red cells: hemolytic disease of the newborn.

• The fetus has inherited an enzyme defect from its father: The history of a first Rh-positive baby without treatment points to maternal anti-D, not an inherited enzyme defect.
• The fetus's own antibodies attack its red cells: A fetus does not attack its own antigens; the antibodies come from the mother.
• Her anti-A antibodies cross to the fetus: The problem here is Rh, not ABO, and most anti-A cannot cross to the fetus.
• Correct: Anti-D from her first delivery destroys its red cells: Correct. Maternal anti-D from the first pregnancy.

The injected anti-D clears the fetal Rh-positive cells that entered her blood at delivery before her immune system responds to them, so she never starts making her own anti-D. The injected antibody is gone within months and does not harm later pregnancies.

• To give her newborn baby a supply of antibodies against infection: The injection is for the mother, and it does not protect the baby against infection.
• Correct: To clear fetal Rh-positive cells before she reacts to them: Correct. It prevents her from becoming sensitized.
• To turn her red cells Rh positive: Nothing changes her own red cells' Rh type.
• To destroy any anti-D she already has: Anti-D cannot destroy anti-D. Once she has made her own, the injection no longer helps.

Agglutination is an immune process: antibodies bind antigens on several cells and cross-link them into clumps. Coagulation is thrombin converting fibrinogen into fibrin threads. They can happen together in a severe transfusion reaction, but they are different processes.

• Agglutination needs platelets; clotting does not: Platelets are central to clotting, not to agglutination.
• Agglutination happens outside the body; clotting happens inside it: Agglutination happens inside the vessels in a transfusion reaction as well as in a typing test.
• Correct: Antibodies clump cells in agglutination; fibrin forms a clot: Correct. Different molecules, different mechanisms.
• There is no difference: they are two names for one and the same process: They involve different molecules and are triggered differently.