Donath Landsteiner antibody is seen in-
PNH
Waldenstrom's macroglobulinemia
Paroxysmal cold hemoglobinuria
Malaria
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