Immune thrombocytopenia purpura (ITP), thrombotic thrombocytopenia purpura (TTP), heparin‐induced thrombocytopenia (HIT), and antiphospholipid syndrome (APS) are clinical conditions associated with significant morbidity and mortality. These well‐defined clinical syndromes have in common several properties: (1) their pathogenesis is immune mediated, specifically by autoantibodies; (2) thrombocytopenia is a hallmark in these four conditions; (3) except for the case of ITP, platelet and endothelial cell activation occurs in TTP, HIT, and APS, resulting in a prothrombotic state and an increased risk of thrombosis. Although these four immune‐mediated syndromes are well‐defined diseases, several case reports and studies have documented the association of two diseases in the same patient, illustrating the concept of the kaleidoscope of autoimmunity. Am. J. Hematol. 80:232–242, 2005. © 2005 Wiley‐Liss, Inc.