The ultrastructure of binding sites in rosette-forming cells of mice after immunization with sheep red cells was studied by means of scanning and transmission electron microscopy. It was found that the red cells were bound to the lymphocyte surface in circumscribed, immunoglobulin-containing areas, consistent with a spotlike or patchy distribution of antigen-binding immunoglobulin receptors. In these contact areas the cell membranes formed a gap of 80 Å (range 75–90 Å) which exhibited electron-opaque bridges at high magnification. These results are discussed in the light of the recent recognition of the formation of immunoglobulin spots on the lymphocyte surface after antigen contact. Morphological details suggest that the same mechanism is operating in rosette formation, possibly including the movement of the contact areas on the lymphocyte membrane.

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