Paroxysmal nocturnal haemoglobinuria with coexisting deficiency of the ninth component of complement: lack of massive haemolytic attack.

A 47-year-old woman with paroxysmal nocturnal haemoglobinuria (PNH) was found to have an inherited deficiency in the ninth complement component (C9). In complement-sensitivity lysis tests, 80% of her erythrocytes were markedly complement-sensitive (PNH-III). Laser cytofluorimetry with a monoclonal antibody against decay-accelerating factor (DAF) revealed that 95% of her erythrocytes were DAF-negative.
Surprisingly, she has suffered only mild haemolysis and has never experienced massive spontaneous haemolysis.
Gross haemoglobinuria and jaundice occurred only after receiving postoperative transfusion of whole blood.
In her serum, C9 was not detectable either by immunological or by functional assays.
Both the Ham test and the sugar water test using normal human serum or plasma yielded marked haemolysis of the patients erythrocytes.
When the patients serum or plasma was used, only a trace of lysis was detected.
Addition of purified human C9 to her plasma fully restored haemolysis.
These observations indicated that C9 may play a critical role in haemolytic attacks in patients with PNH and that characteristic haemolysis in PNH may be tempered by coexisting C9 deficiency.. 
