Antibody-induced membrane fusion in Paramecium.

Immobilization of cells by specific immune serum involves crosslinking between immunoglobulin G (IgG) and the i-antigen in the cell membrane. Globular material is seen to accumulate at the ciliary tips by phase-contrast and fluorescence microscopy in a manner analogous to 'capping' in more typical eukaryotes. When immobilized cells of Paramecium are examined by scanning electron microscopy, the fused ciliary tips are seen to be distended, discoidal membranes. Transmission electron microscopy often reveals several ciliary axonemes enclosed within a single, enlarged membrane that is oriented with the ferritin-labelled second antibody directed against the i-antigen antibody on the outer surface only. Fixed cells or living cells treated with immune Fab do not show membrane changes, but do bind antibody. Membrane fusion occurs only if cells are alive and the i-antigen is directly or indirectly cross-linked by intact immune IgG.