Developments in delayed-type hypersensitivities: 1950-1975.

Significant developments during the last 25 years are discussed and interpreted. The following areas of delayed hypersensitivity are included: the mode of active sensitization to simple allergenic chemicals; evidence for anamnestic responses; cell types and cell-cell interactions via lymphokines; function of skin and lymphatics, and the role of the carrier in initial sensitization to allergenic chemicals; acquired tolerance; transfer factor. Some prognostications for the future are attempted.