Serum antibodies to giardial surface antigens: lower titres in persistent than in non-persistent giardiasis.

Antisera to two antigens of Giardia lamblia--plasma membrane (PM) protein and an affinity-purified surface antigen (SA56)--were raised in rabbits, and shown to agglutinate and kill trophozoites in vitro. These antibodies were also demonstrated by ELISA in the sera of paediatric patients with giardiasis. The titres of both antibodies were significantly higher in non-persistent (acute) and asymptomatic cases than in patients with persistent infection; and the latter group did not respond to anti-giardial therapy. The inability of this group to clear G. lamblia infection, in spite of therapy, may result from the low level of antibodies which mediate the killing of trophozoites.