Résumé
IgG4 has been proposed to act as a 'blocking antibody' due to its ability to compete for the same epitopes as IgE thus preventing IgE-dependent allergic responses. IgG4 and IgE are both elevated in helminth infections and strong anti-parasite IgE responses are associated with resistance to infection. We wished to determine the relationship between anti-parasite IgG4 and IgE and
Ascaris lumbricoides infection status. We examined anti-parasite responses, including antibody levels to recombinant
Ascaris allergen-1A (rABA-1A), a target of serum IgE in endemic populations. Worm burden was indirectly estimated by measuring parasite egg output in a cross-sectional human population (
N
=
105). Levels of anti-parasite IgG4 and IgE in patients' plasma were quantified by immunoassay. Global anti-parasite antibody responses did not bear any significant relationships with intensity of
Ascaris infection. Individuals who had detectable levels of IgE but not IgG4 to rABA-1A (11%) had lower average levels of infection compared with individuals who produced anti-rABA-1A IgG4 (40%) and sero-negative individuals (49%) (
P
=
0.008). The ratio of IgG4/IgE in rABA-1A responders positively correlated with intensity of infection (
P
<
0.025). IgG4 levels positively correlated with infection level in younger children (age 4–11) where average levels of infection were increasing (
P
=
0.038), whereas allergen specific IgE emerged as a correlate of immunity in older children and adults (age 12–36) where infection levels were decreasing (
P
=
0.048). Therefore, in a gastrointestinal helminth infection, differential regulation of anti-allergen antibody isotypes relate to infection level. Our results are consistent with the concept that IgG4 antibody can block IgE-mediated immunity and therefore allergic processes in humans.