The direct antiglobulin test in P. falciparum malaria

The direct antiglobulin test (DAT) was performed on 134 Gambian children with P. falciparum malaria. 52 children had a positive DAT and in 25 this was due to the adherence of C3 to their red cells whilst 13 had sensitization with IgG as well as C3. Sensitization with C4 alone or associated with IgG and/or C3 was only rarely found. The haemoglobin levels and reticulocyte counts were not significantly different in patients with a positive DAT from those with a negative DAT. The children with positive IgG DAT, however, were of a significantly older age group and had significantly higher parasitaemias at presentation than children with complement components only or with a negative DAT. In vitro assay of phagocytosis by peripheral blood monocytes (PBM) showed that PBM from malarious patients were less active in phagocytosis than normal male adult Caucasian PBM. Neither PBM from malarious patients nor normal adult Caucasians PBM showed phagocytosis of cells from DAT positive malarious children. The same in vitro assay was used to test for the presence of opsonizing anti-schizont antibodies in the sera of these children and in adult Gambians. This revealed the presence of a schizont specific opsonizing antibody in the sera of adult Gambians and in the sera of children with a positive DAT due to IgG but not in sera of patients with a negative IgG. These findings indicate that a positive IgG DAT in malaria does not necessarily lead to excess haemolysis of non-parasitized red cells. The presence of a schizont opsonizing antibody, leading to in vitro phagocytosis, in the sera of children with a positive DAT and in sera from adult Gambians indicates a relationship between the development of the positive DAT and acquisition of protective malarial immunity.