Physiology · Blood, hemostasis and basic immunity

Blood groups, transfusion and transfusion reactions !

Concept
Exam priority: high

Red cells carry sugar and protein labels (A, B, Rh). Giving blood with the wrong label makes the patient's antibodies destroy it.

Updated 2026-11-01

Also called: ABO system, Rh system, Blood typing

See it in 3D: Spleen · Kidneys

Key points

Anti-A and anti-B are 'natural' IgM antibodies made in the first months of life, likely after exposure to similar gut bacterial antigens; they fix complement and cause intravascular hemolysis.
Group O red cells (no A or B) are the emergency universal red cell donor; group AB is the universal plasma donor (no anti-A or anti-B).
Rh antibodies are IgG, appear only after sensitization (transfusion or pregnancy) and can cross the placenta.
Acute hemolytic reaction (usually ABO, clerical error): fever, back pain, hypotension, red urine, DIC and acute kidney injury within minutes.
Anaphylaxis during transfusion: think of IgA deficiency with anti-IgA antibodies.
Delayed hemolytic reaction 3–10 days later: memory IgG against a minor antigen (Kidd, Rh); extravascular, mild jaundice and falling Hb.

What it is

Blood groups are inherited antigens on the red cell surface. In the ABO system, people make antibodies against the A or B antigen they lack, without ever being transfused; the RhD system needs prior exposure. Matching donor cells to the recipient's antibodies prevents hemolytic reactions, the most dangerous and most preventable error in transfusion.

Key facts

A and B antigens are sugars added by glycosyltransferases to the H substance; group O lacks a working transferase.
Cross-match: recipient serum is tested against donor cells; the antibody screen looks for unexpected IgG.
Stored red cells lose 2,3-BPG and K⁺ leaks out; massive transfusion can cause hyperkalemia, hypocalcemia (citrate) and hypothermia.
In Colombia group O RhD-positive is the most common (around six in ten people) and RhD-negative is uncommon.

Oxygen–hemoglobin dissociation curve

P50 16.5 mmHgArterial blood 99 %Mixed venous blood 92 %O₂ given up to the tissues 8 %

Left shift (P50 16.5 mmHg): Hb holds O₂ more tightly and gives up less in the tissues.

Current curve
Normal curve (pH 7.40 · 37 °C)
Arterial blood (PO₂ 100)
Mixed venous blood (PO₂ 40)
O₂ given up to the tissues

The 'stored blood' scenario: low 2,3-BPG shifts the curve left, so freshly transfused cells release O₂ less easily until they regenerate it.

ABO groups

Group	Antigen on red cells	Antibodies in plasma	Can receive red cells from
A	A	Anti-B	A, O
B	B	Anti-A	B, O
AB	A and B	None	AB, A, B, O
O	None	Anti-A and anti-B	O only

Clinical notes

Acute hemolytic reaction: antibody binds donor cells → complement → intravascular lysis → free Hb, hemoglobinuria, cytokine storm, DIC; free Hb and hypotension injure the kidney.
Febrile non-hemolytic reaction: cytokines from donor leukocytes; leukoreduction lowers the risk.
TRALI (lung injury within 6 h, normal filling pressures) versus TACO (volume overload, high filling pressures): both cause dyspnea and hypoxemia.

Test yourself

Think of the answer, then tap the question to see it.

Why can a group A person react to group B blood at the very first transfusion?

Anti-B is a natural IgM present from early infancy, so no prior exposure to human B cells is needed. In contrast, anti-D usually requires prior sensitization.

Why is ABO hemolysis intravascular while most delayed reactions are extravascular?

IgM anti-A/B activates complement up to the membrane attack complex and lyses cells in the vessel. IgG on cells without full complement leads macrophages in the spleen to remove them.

A patient becomes hypotensive with urticaria and wheeze minutes into a plasma transfusion. Which immune defect should be suspected?

IgA deficiency with anti-IgA antibodies: the IgA in donor plasma triggers an anaphylactic reaction.

Why does massive transfusion of stored red cells lower ionized calcium?

Stored blood contains citrate as anticoagulant; citrate binds calcium, and when large volumes are given faster than the liver metabolizes citrate, ionized Ca²⁺ falls.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

In Biochemistry

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 18.6 Blood Typing (OpenStax (Rice University), 2022); Hemolytic transfusion reaction (MedlinePlus (U.S. National Library of Medicine), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 36). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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