Learning Objective

By the end of this article, you should be able to describe the basis of blood typing using the ABO and Rhesus (Rh) systems, explain the role of red blood cell surface antigens and plasma antibodies in transfusion compatibility, understand the clinical significance of blood group matching in preventing transfusion reactions, and recognise key investigations and complications associated with blood transfusion, including haemolytic disease of the newborn.

Overview

Blood typing is a method of classifying blood into distinct groups based on the presence or absence of specific antigens on the surface of red blood cells (RBCs). Identifying these blood groups is essential to ensure compatibility between donor and recipient blood, thereby preventing potentially life-threatening transfusion reactions.

ABO Grouping System

Red blood cells possess numerous glycoprotein antigens on their surface. Among these, the ABO antigens are the most clinically significant, determining an individual’s ABO blood group.

Each person inherits one ABO allele from each parent. The A and B alleles are codominant and encode enzymes that produce the corresponding surface antigens.

Group A: A antigen present on RBC surface; anti-B antibodies in plasma
Group B: B antigen present on RBC surface; anti-A antibodies in plasma
Group AB: Both A and B antigens present; no antibodies in plasma
Group O: No A or B antigens present; both anti-A and anti-B antibodies in plasma

ABO antibodies are predominantly IgM and develop naturally during early life. These antibodies recognise and attack foreign antigens, which is why transfusion of incompatible blood can result in rapid and severe haemolytic reactions.

Rhesus (Rh) Grouping System

The second most clinically important blood grouping system is based on Rhesus (Rh) antigens. Although several Rh antigens exist (D, C, c, E, and e), the Rh D antigen is the most immunogenic and therefore the most relevant in transfusion medicine.

Rh-positive (Rh⁺): Rh D antigen present
Rh-negative (Rh⁻): Rh D antigen absent

Unlike ABO antibodies, anti-D antibodies are not naturally occurring. They develop following exposure to Rh⁺ erythrocytes through events such as transfusion or pregnancy.

Rh⁻ individuals should receive only Rh⁻ blood, as exposure to Rh⁺ cells may stimulate the formation of anti-D antibodies, increasing the risk of haemolytic transfusion reactions in future transfusions.

Clinical Relevance – Blood Transfusion

A recipient should not receive blood containing antigens for which they possess corresponding antibodies. Incompatible transfusion results in immune-mediated destruction of donor RBCs.

O⁻ individuals are considered universal donors because their RBCs lack A, B, and Rh D antigens.
AB⁺ individuals are universal recipients as their plasma lacks anti-A, anti-B, and anti-D antibodies.

Clinical Relevance – Haemolytic Disease of the Newborn (HDN)

Anti-D antibodies are responsible for the most severe form of haemolytic disease of the newborn.

Sensitisation may occur when an Rh-negative⁻ mother is exposed to Rh⁺ fetal erythrocytes during pregnancy or delivery. Initial exposure produces IgM antibodies, which do not cross the placenta. However, subsequent pregnancies may trigger IgG anti-D production, which can cross the placenta and cause haemolysis of fetal RBCs.

Administration of anti-D immunoglobulin to unsensitised Rh⁻ mothers reduces the risk of sensitisation and subsequent HDN.