Platelet antibodies can be divided into two categories: autoimmune and alloimmune. Autoimmune antibodies are part of an autoimmune condition, such as autoimmune thrombocytopenic purpura (ITP) or SLE, or they may develop following administration of certain drugs. Alloimmune antibodies develop as the result of immunization of transfused, incompatible, platelets.
The development of platelet antibodies may result in shortened platelet survival and refractoriness to platelet transfusions (lack of adequate and sustained increment in platelet number). Therefore, from 20 to 70% of multitransfused thrombocytopenic patients become refractory to transfused platelets. Platelet antibodies in pregnant women may cause neonatal alloimmune thrombocytopenia. Platelet antibodies react with several antigenic groups on the platelet surface: ABO antibodies, HLA antibodies.
The most common platelet antigen is known as HPA-1, also known as PlA1, present in 98% of the Caucasian population. Anti-HPA-1 are the most common clinically significant antibodies. The HPA-1b (PLA2) antigen occurs in 27% of the Caucasian population. Both reside on the platelet membrane protein GPIIIa.
Use
In refractory, multitransfused patients, the common approach is to determine the HLA type of the patient (ideally to be done before treatments that predictably result in the need for repeated platelet transfusions) and transfuse platelets from the best HLA-matched, ABO compatible, donor. Platelet cross-matching may also be used to select the best cross-matched compatible donors. Unfortunately, cross-matched platelets are effective in only 50% of transfused patients.
Many hematologists used the platelet antibody assays to diagnose ITP. Because of the low specificity, this assay is presently not recommended.
Limitations
Attachment of antibodies to platelets is difficult to measure because platelets have normally cell-bound immunoglobulins attached to them. In addition, platelets do not lend themselves to agglutination methodology, as used for RBC antibody detection (DAT). The use of different proposed methodologies remains difficult to standardize, and practicality is limited. Solid-phase methodologies, such as those using ELISA immunoassays, are used by some laboratories to detect IgG antibodies against HLA, ABO, and HPA antigens.