Immunohistological localisation of two hydatid antigens (antigen 5 and antigen b) in the cyst wall, brood capsules and protoscoleces of Echinococcus granulosus (ovine and equine) and E. multilocularis using immunoperoxidase methods.

Cyst wall, brood capsules and evaginated protoscoleces of E. granulosus (ovine and equine) and E. multilocularis were fixed in 10% formol-saline, embedded in paraffin and cut at 8 micrometer. Specific rabbit antisera to antigen 5 and antigen B of hydatid cyst fluid were used with immunoperoxidase methods to localise the antigens in the histological sections. Antigen 5 was found in all parasites and was associated with cells of the subtegumental area of the protoscolex, the brood capsule wall and the germinal membrane. The labelled antigen appeared as distinct granules in all areas. It is suggested that antigen 5 may be synthesised in all of these sites and that a source of the antigen in cyst fluid may be the germinal and brood capsule membranes. The laminated membranes of E. granulosus (ovine and equine) were, except for the superficial layers, free from antigen 5. Antigen B was present in all parasites. It was distributed diffusely throughout the laminated membrane, germinal membrane and brood capsule wall. There were areas of densely labelled antigen B on the surface of the distal cytoplasm of the protoscolex tegument and the surface of calcareous corpuscles. The distribution of antigen B in relation to PAS positive material and possible complement activating substances is discussed. The laminated membrane of E. granulosus was apparently more permeable to antigen B than to antigen 5. It is suggested that differences in the diffusion of these antigens through the laminated membranes of hydatid cysts in the same or different host species may account for variable serological responses during infection.