This study in Ghanaian children is one of a series of studies designed to assess, using standardized methods, the association of antibody levels to four leading asexual blood-stage malaria antigens (MSP119, MSP3, AMA1 and GLURP) with the incidence of clinical malaria in different epidemiological settings. Previous results have been difficult to interpret due to different study protocols and analytical methods having been used [5–7, 9, 14, 29, 38–40]. In this study, the prevalence of asymptomatic malaria parasitaemia was relatively high and stable, while incidence of clinical malaria fluctuated in parallel with the intensity of transmission and seasonal rainfall pattern. These patterns are typical of this area and have been reported in previous studies [6, 41–43]. The variation in the incidence of clinical malaria during the study period may be due to the introduction of new parasites with different antigenic presentation into the population leading to clinical malaria in susceptible individuals. The risk of malaria decreased with age, while isotype and IgG subclass levels to the four antigens generally increased with age. This is consistent with the hypothesis that immunity to malaria is largely effected through antibody-mediated mechanisms and that protective antibody levels to relevant antigens increase with age-related exposure to the parasites . Increasing IgG and IgM levels with age may reflect greater cumulative exposure of older children but may also be due to older children having a more mature immune system . The association of IgM responses with reduced malaria incidence indicates a possible role in immunity in Ghanaian children. Although much emphasis has been placed on IgG as the important isotype in immunity against malaria, IgM, which has lower affinity but is multivalent, may afford protection via other mechanisms such as the blocking of merozoite invasion of erythrocytes, complement activation, agglutination of merozoites .
The association with malaria incidence of IgG responses to MSP3, MSP119 and GLURP is consistent with data from several other immuno-epidemiological studies [5, 7–9, 12, 29, 30, 38, 47–49] indicating that these antigens may be targets of protective antibodies [1, 2, 4]. There was no evidence that IgG levels to AMA1 were associated with malaria incidence, and there was no evidence of an interaction with baseline parasitaemia in contrast with similar studies conducted in Kenya . As shown in other studies, the cytophilic antibody levels to the four antigens tested in this study were higher than the non-cytophilic ones, emphasizing their importance in anti-malaria immunity [9, 29, 38, 50, 51]. IgG and cytophilic antibody levels were highest for AMA1, while the levels were relatively low for MSP3. These differences in specific antibody levels may be related to the number of immunogenic B-Cell epitopes exposed to the immune system and could also be related to the structure, location and function of the particular antigen(s). With the exception of IgG2 levels to AMA1, IgG2 levels to GLURP, MSP119 and MSP3 increased with age which may suggest that IgG2 is involved in immunity against malaria. In recent studies, malaria antigen specific IgG2 have been shown to bind with high affinity to mutant Fcγ RII H131->R receptors  on monocytes, granulocytes and B cells, thus affording protection against malaria through monocytes and or neutrophil mediated mechanisms in subjects expressing the mutant CD32 form . There was however, no evidence found that IgG2 was associated with malaria incidence for any of the four antigens tested.
The IgG subclasses, IgG1, IgG2 and IgG3 for MSP3, MSP119, GLURP and IgG4 to MSP3 were associated with a reduced risk of malaria in un-adjusted analysis but of these only IgG1 to MSP119 was independently associated with malaria incidence after adjustment for age. Other studies have shown the importance of IgG1 in clearing parasitaemia in children [5, 47, 54] In a previous cohort study conducted in the same area, there was no association between antibody levels to MSP119 and malaria incidence . This may be due to differences in antigen and antibody reagents used in the two studies; the MSP119 used in this study was a Baculovirus product that included a synthetic G-C enriched PfMSP1 gene that coded for the 43 N-terminal MSP1 precursor residues and 16 amino acid residues upstream of the "classical" MSP-119 (NIS---FCS)  compared to the one produced in E. coli, which had been used in the previous study. Although antibodies to MSP119 have been shown to be associated with both exposure and protection from disease, the fine specificities of such responses may contribute to protection . The antigen used in this study may have assessed antibodies of fine specificities that are protective [40, 55, 56], whereas the antigen used in the previous study did not. It may, therefore, be important to assess in a standardized way the various MSP119, and other antigens, that are produced in different expression systems, in order to select the most appropriate antigen/expression system for malaria vaccine development. In another study in this series, in Burkina Faso, the same antigens, reagents, ELISA procedure and analytical methods were used; none of the isotypes and subclasses to MSP119 was associated with the incidence of clinical malaria. Since the same laboratory methods were used, the different outcomes of these two studies may be attributed to differences in malaria transmission or to the age of the children . In Burkina Faso, the malaria transmission season is much shorter, which may influence the induction of differing antibody types for controlling malaria as shown in recent studies conducted in areas with different malaria endemicities in Tanzania [9, 44]. Although total IgG to GLURP and MSP3 were associated with the risk of malaria, none of the constituent subclasses was identified to be associated with protection. When the effects of all the immunological variables were considered simultaneously, only IgG1 and IgM to MSP119 were independently associated with the incidence of clinical malaria, which may indicate the importance of MSP119 in malaria vaccine development. Parasite growth inhibition assays would be required to confirm if this association reflects a functional role of MSP119 in immunity.