Ultrastructural evidence for destruction in the halo nevus.

Nine halo nevi in various stages of regression were examined by electron microscopy for fine structural evidence of an immunological mechanism of tumor cell destruction and halo formation. Early regressing lesions (Stage I) showed nevus cells associated with infiltrating lymphocytes, monocytes, and plasma cells, but without nevus cell destruction. In later lesions (Stages II and III), vacuolar cytolysis was commonly observed in nevus cells. In Stage III lesions, portions of nevus cells are found within macrophages. The electron microscopic findings of lymphocyte, monocyte, and plasma cell infiltration of the tumor followed by vacuolar cytolysis support the concept of an immune reaction in regressing halo nevi.