Opsonization in vitro of Giardia lamblia trophozoites.
The ability of peritoneal rabbit macrophages from immunized and nonimmunized animals to phagocytose Giardia lamblia trophozoites in the presence of serum was studied and compared in an in vitro system. The rabbits which served as the source of immune serum and macrophages were injected repeatedly at multiple sites (intramuscularly, subcutaneously, and intradermally) with a mixture of G. lamblia trophozoites and Freund complete adjuvant. In the presence of normal rabbit serum, a low level of phagocytosis of Giardia trophozoites by normal and immune macrophages was observed. In the presence of hyperimmune rabbit serum, an increased phagocytic activity of both types of macrophages occurred. The opsonic activity was similar whether whole serum or purified immunoglobulin G was used and whether or not these were heat inactivated. G. lamblia trophozoites in suspension were shown to be agglutinated in the presence of hyperimmune serum. Tests employing serial dilutions of hyperimmune serum resulted in a parallel loss of opsonifying and agglutinating activities. It is suggested that opsonization in vivo may play a role in the ability of the host to limit infection by these organisms.