Abstract
Giardia lamblia undergoes surface antigenic variation. The ultrastructural location of antigens on four different variants was studied by label fracture and immunocytochemistry with four monoclonal antibodies (MAbs), each of which recognized the predominant variant in a particular clone. Each Giardia clone and its reacting MAb showed similar findings. The entire surface of the organism was covered by a surface coat which contained the variant surface protein. The surface coat was densely and uniformly labeled. Unreacting MAbs failed to label the surface. Label-fractured Giardia trophozoites exposed to reactive MAb revealed a planar distribution of reactivity with no discernible relationship to visualized intramembranous particles. Unexpectedly, some Giardia trophozoites lacked a surface coat and consequently failed to react with the appropriate MAb. The biological relevance of coatless Giardia trophozoites is unknown. These findings localize the variant antigens to the surface coat of the parasite and identify a minority of the population which lacks a surface coat.