Paroxysmal cold hemoglobinuria is a hemolytic syndrome. Hemolysis most commonly occurs intravascularly, after the antibody has passed through a cell attachment phase in the lower temperatures of the peripheral circulation.
The Donath-Landsteiner cold hemolysin is an unusual IgG antibody with anti-P specificity that was originally noted in cases of congenital or acquired syphilis. The disease it causes is termed "paroxysmal cold hemoglobinuria".
The intravascular hemolysis is due to the unusual complement-activating efficiency of this IgG antibody. As its name implies, this antibody is associated with cold hemoglobinuria. This antibody, although uncommon, is most frequently found in children with viral infections. Hemolysis, even though sometimes severe, is usually mild and tends to resolve as the infection clears.
Intravascular hemolysis + direct Coomb test positive for complement only and active at 4°C.