Despite being among the smallest cells in the body, platelets have a critical function - clotting - and many receptors, class I HLA molecules, AB0 blood group antigens and other structures on their surface for communication with other blood cells or tissue cells.

Clinically most important human platelet antigens (HPAs):

HPA 1a/1b

HPA 2a/2b

HPA 3a/3b

HPA 4a/4b

HPA 5a/5b

HPA 15a/15b

Many other platelet antigens are present, but those listed are most likely to cause problems in pregnancies.

Incompatibility in HPAs between the mother and the foetus can lead to alloimmunisation and the development of FAIT or NAIT. This is the platelet equivalent of Morbus hemolyticus fetalis. In rare cases, they can also cause Refraktärzustand in platelet transfusions.

Receptors:

Adhesion

GP Ib-V-IX (HPA-2) - binds to the von Willebrand factor and is thus responsible for initial adhesion in primary haemostasis.

GP VI and GP Ia (HPA-5) - bind to exposed collagen, leading to adhesion.

Aggregation

GP IIb/IIIa (HPA-1,-3,-4) - when the conformation of the cells changes, this receptor changes from inactive to active status. As a result, fibrinogen and collagen can now cross-link adjacent platelets. 

Receptor defects can lead to various bleeding diatheses and thrombocytopenias.

Unlike red cells, platelets need not be transfused blood group matched or compatible. Due to the size of some blood depots, compatibility is often impossible. Platelets are suspended in a nutrient solution and contain only a few isoagglutinin. So if you transfuse a group 0 platelet concentrate to a group A patient, a few of the recipient's red cells will undoubtedly be lysed by the transfused isoagglutinin from the platelet concentrate, but the amount will be minimal. The platelets of blood group 0 do not express blood group antigens, so they are not "noticed" at all by the isoagglutinin of the recipient. However, since there are A and B antigens on platelets in blood groups A, B or AB, there may be a reduced increase if, for example, a patient with blood group 0 receives platelets of blood group A. Otherwise, the patient will not experience any disadvantage as a result.

Last update on 05.08.2023.