Immune response

Humoral, cellular, and macrophage-mediated mechanisms are each important components of the host response (den Hollander et al. 1988). The critical site for this response is the gut. Therefore the secretory antibodies are the most important (Taylor and Wenman 1987; Ljungstrom and Castor 1992). IgA is the predominant antibody class detected, although these antibodies do not appear to be cytotoxic (Heyworth 1992). Cellular mechanisms, particularly T-helper cells, are necessary for the development of secretory antibody response (Heyworth et al. 1987). Macrophages may also act as effector cells, their phagocytic activity for Giardia trophozoites being increased in the presence of specific antibodies. Current evidence suggests that anti-Giardia sIgA acts in clearing Giardia from the gut lumen by trophozoite agglutination and/or inhibition of the flagellar motility (Char et al. 1992). Individuals with hypo- or agammaglobulinemia are at risk of chronic giardiasis although individuals with HIV and AIDS do not seem to develop symptomatic disease at a higher number. The secretory immunity in the intestinal lumen is more important for clearance than the cell-mediated responses within the mucosa. Despite the limits of the humoral immune response there is some evidence that antibodies provide protection against newly acquired infection or reinfection. Human milk appears to be protective by nonimmune mechanisms. It is cytotoxic against Giardia trophozoites by the activity of free fatty acids (Reiner et al. 1986) although it also may contain anti-Giardia antibodies, which may be protective to breast-fed infants (Nayak et al. 1987).

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