Donath Landsteiner antibody is seen in-

Paroxysmal cold hemoglobinuria (PCH) PCH is characterized by intermittent massive intravascular hemolysis frequently with hemoglobinuria at low temperature. The autoantibodies are of IgG class, also known as Donath-Landsteiner antibody. These antibodies are directed against P-antigen on RBC. Antigen-antibody complex on the red blood cell activates complement. As a result, a large amount of membrane attack complex forms which destroys red cells directly → intravascular hemolysis. Causes are: - Syphilis, Mycoplasma, Mumps, Measles, Flu syndrome