Abstract
It is known that following immunization with agents which cause delayed hypersensitivity, T cells proliferate in the draining lymph nodes, become large pyroninophilic cells with a peak about day 4 and then revert to small lymphocytes. In parallel with these changes, cells appear which have the ability to move to sites of inflammation and to show non-specific cytotoxic killing. These cells are large, theta positive and synthesize DNA. Their activity reaches a peak four days after immunization with contact sensitizing agents. It is postulated that movement to sites of inflammation and non-specific cytotoxic killing are the properties of large pyroninophilic cells (T blasts) or their immediate descendents, and that these cells represent a single sequence or line of T cells. It is suggested that the biological role of these properties is to ensure that the cells which mediate delayed hypersensitivity arrive at the sites where they are required.