Signaling & Blood Typing

4 min read Updated Apr 18, 2026

Cell surface sugars are not passive decoration. They are the fingerprints cells use to identify each other and to catch passing signals. The ABO blood group system is the cleanest MCAT example.

The ABO Blood Group System

Red blood cells display a glycan on their surface built on a core called the H antigen. Different blood types differ in what single extra sugar (if any) is added to the H antigen:

Blood type	Antigen on RBC	Antibodies in plasma	Can receive from
A	H antigen + N-acetylgalactosamine (GalNAc)	Anti-B	A, O
B	H antigen + galactose	Anti-A	B, O
AB	Both A and B antigens	None	A, B, AB, O (universal recipient)
O	H antigen only	Anti-A and anti-B	O only (universal donor of RBCs)

Diagram showing ABO blood group antigens on red blood cells: type A has GalNAc, type B has galactose, type AB has both, and type O has only the H core antigen — ABO blood group antigens. Each blood type has a specific terminal sugar on the core H antigen: GalNAc for A, galactose for B, both for AB, and neither for O. *Credit: Wikimedia Commons, CC BY-SA*

Why Universal Donor and Recipient

The body makes antibodies against whichever A/B antigens it lacks. These anti-A and anti-B antibodies arise naturally in infancy from exposure to environmental sugars on bacteria.

Type A person has anti-B antibodies. If given type B blood, the antibodies clump B cells - a transfusion reaction.
Type O has anti-A and anti-B. Type O cannot receive A or B; Type O is the universal donor of red cells because O cells have neither A nor B to trigger attack.
Type AB has no anti-A or anti-B antibodies. Type AB is the universal recipient.

Rh System

Separate from ABO, the Rh antigen is a protein (not a carb). Someone who expresses Rh is Rh+; absent is Rh-. Critical clinically because an Rh- mother carrying an Rh+ fetus can develop anti-Rh antibodies that attack a subsequent Rh+ pregnancy (hemolytic disease of the newborn). Anti-Rh immunoglobulin (RhoGAM) given prophylactically prevents this sensitization.

Selectins and Lectins

Lectins are proteins that bind specific carbohydrate structures. They are not enzymes - they just grab sugars. Many cellular recognition events use lectins on one side and glycans on the other.

Selectins are a subfamily of lectins on the surface of white blood cells and endothelium. During inflammation, selectins on the endothelial surface grab sugar chains on leukocyte surfaces, causing the leukocytes to slow down and roll along the blood vessel wall. This is the first step of leukocyte extravasation - the process by which white blood cells leave the bloodstream to fight infection.

Integrins (protein-protein binders) later replace the selectin interaction with a firmer attachment, and the white cell squeezes through the endothelium into tissue.

Cell-Cell and Host-Pathogen Recognition

Many viruses and bacteria identify host cells by binding specific surface glycans:

Influenza virus binds sialic acid on respiratory epithelium.
Rotavirus binds histo-blood group antigens on gut epithelium.
H. pylori binds Lewis b antigens on stomach epithelium.

Blocking the sugar-receptor interaction can block infection. Some experimental drugs are sugar mimetics designed to saturate the viral binding protein and prevent attachment.

What is the molecular difference between type A and type B blood?

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Both types start from the H antigen, a core glycan on the red blood cell surface. Type A adds an N-acetylgalactosamine (GalNAc) to the H core. Type B adds a galactose. Type AB has both. Type O leaves the H antigen untouched. Type A people have natural anti-B antibodies; type B have anti-A; type O have both; type AB have neither.

Why is type O the universal RBC donor but type AB the universal plasma donor?

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Universal donor depends on what is being transfused. Type O red cells carry no A or B antigens, so they cannot trigger anti-A or anti-B antibodies in any recipient. But type O plasma contains both anti-A and anti-B antibodies, which would attack a non-O recipient. Type AB plasma has neither anti-A nor anti-B antibodies, so it can be given to any recipient without causing immune attack, making AB the universal plasma donor.

What do selectins do during inflammation?

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Selectins are lectins (sugar-binding proteins) expressed on activated endothelium that bind carbohydrate structures on circulating leukocytes. The binding is transient and triggers "rolling" - the leukocyte slows down and rolls along the vessel wall. This is step 1 of leukocyte extravasation; integrin-mediated firm adhesion and transmigration follow.