Cross-sectional and longitudinal studies were performed in a rural population living in The Gambia to examine the relationship between several in vitro assays of the host immune response to asexual stages of Plasmodium falciparum and protection from
malaria in vivo. Assays included an
enzyme-linked
immunosorbent assay for
antibodies to schizont
antigens; an indirect immunofluorescence assay for total antiblood-stage
antibodies; an immunofluorescence assay on
glutaraldehyde-fixed parasites to detect
antibodies to
antigen Pf 155; an assay for serum inhibition of red blood cell invasion; a micro-agglutination assay to detect
antibodies to neo-
antigens on the surface of infected red blood cells; and an assay using polymorphonuclear leucocytes to detect
antibodies capable of opsonizing schizont infected red blood cells. There were marked differences in the age-related pattern of response for different assays performed on sera obtained at a cross-sectional survey of 280 individuals. Examination of the correlation between the various immune responses and malariometric indices at the population level and at the individual level provided no evidence that any of the in vitro assays were related to protective immunity. The relationship between in vitro measurements of the
anti-malarial immune response and protection from clinical episodes of
malaria was examined in a group of 134 children aged 11 years and under who were monitored weekly throughout an entire
malaria transmission season. The only
immune factor to show a consistent protective effect against clinical
malaria was the titre of
antibodies to neo-
antigens on the infected erythrocyte surface (P = 0.01). The same longitudinal techniques were used to examine the effect of two non-
immunological factors,
sickle cell trait and
mosquito net usage, both of which showed significant protection against clinical episodes and
malaria.