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
PNH
Waldenstrom's macroglobulinemia
Paroxysmal cold hemoglobinuria
Malaria