Lawrence in 1954 first reported transfer of CMI in humans by injection of an extract from the leukocytes from the immunized host. The extract from leukocytes contained soluble substance known as transfer factor (TF).
· The transferred immunity is specific in that CMI can be trans-ferred only to those antigens to which the donor is specific.
· Transferred immunity is systemic and not local.
· TF does not transfer humoral immunity.
TF is a nucleopeptide with a low molecular weight of 2000– 4000 Da. It is resistant to trypsin but gets inactivated by heat-ing at 56°C for 30 minutes. It is nonantigenic. The exact mode of action of TF is not known. It is believed to stimulate the release of lymphokines from sensitized T lymphocytes.