Alloimmunization to human platelet antigens (HPAs) may occur either during pregnancy, when a HPA‑negative mother gives birth to a newborn who inherits HPAs from the father, or following blood transfusion or stem cell transplantation. Antiplatelet alloantibodies do not cause thrombocytopenia in a patient, but their detection must always be recorded in medical records because they may induce fetal and neonatal alloimmune thrombocytopenia in present and all subsequent pregnancies, platelet refractoriness, posttransfusion purpura, or prolonged thrombocytopenia with engraftment failure after stem cell transplantation. Passive transfer of platelet alloantibodies through transfused blood components may trigger thrombocytopenia and severe posttransfusion reactions in the recipient. In a Caucasian population, such clinical outcome of platelet alloimmunization is mostly due to anti‑HPA‑1a antibodies, less frequently to anti‑HPA‑5b, anti‑HPA‑1b, and others. Information on anti‑HPA alloantibodies is crucial for the prevention and treatment of their consequences.