Antigens and antibodies


Antigens are carbohydrate or proteins found on the surface of all blood cells (red cells, white cells and platelets) but can also be found on body tissue, body fluids and on surfaces of bacteria and viruses.

Antigens can trigger the immune system to produce antibodies directed against the donor’s blood group antigens if the recipient does not have that antigen.

These antibodies (or alloantibodies) can be clinically benign or cause severe transfusion reactions such as acute or delayed haemolytic reactions and haemolytic disease of the fetus and newborn (HDFN).


Antibodies are immunoglobulins (Ig); there are five main subtypes and each have a unique role within the immune system:

  • IgG 
  • IgM 
  • IgA 
  • IgD, and  
  • IgE.

Almost all antibodies to blood cells are IgG or IgM subtypes.

IgM antibodies are usually naturally occurring following exposure to environmental substances that have a similar structure to a red cell antigen. These IgM antibodies are generally non-reactive at 37 °C and are therefore of little clinical significance. However, IgM anti-A and anti-B are potent haemolysins, capable of causing intravascular haemolysis by binding complement and are therefore classified as clinically significant antibodies.

IgG antibodies are formed following exposure to foreign red cell antigens through transfusion or during pregnancy.

These IgG antibodies are reactive at 37 °C and are usually considered clinically significant as they’re capable of mediating destruction or sequestration of transfused allogeneic red cells IgG antibodies can also cross the placenta and can cause haemolytic disease of the fetus and newborn (HDFN).

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Red cell antigens and antibodies

The presence or absence of antigens on the surface of red cells allows laboratories to determine an individual’s blood group. Antibodies are formed following exposure to foreign red cell antigens during transfusion or pregnancy.

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Human platelet antigens (HPA)

Platelets express both platelet specific antigens such as human platelet antigens (HPA) and antigens which are shared with other cells such as ABO and HLA class I antigens. Currently there have been 35 HPA identified on 6 platelet membrane glycoproteins.

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Human neutrophil antigens (HNA)

Human neutrophils express five neutrophil specific glycoproteins on their surface known as human neutrophil antigens (HNA).

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Human leucocyte antigens (HLA)

Human leucocyte antigens (HLA) are polymorphisms in cell surface molecules that distinguish 'self' from 'non-self'. They may also be involved in the pathogenesis of certain autoimmune and infectious diseases.

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Further information