A calibrated cell transfer system allows detection of the anamnestic response to albumin without interference from the host's immune machinery; it was used to study the immunological memory of mouse spleen cell populations. The secondary antibody-forming capacity of the transferred cells was measured by challenging them at periods up to 6 months after transfer. The peak levels attained show a declining pattern in two phases: during the first month with a half-life of 15 days; thereafter, with a half-life of 100 days. The corresponding half-lives of the cellular memory are 26 and 190 days.
In the light of these and of radioinactivation data, immunological memory is defined as the persistence of a specifically determined stem cell line, along which the information necessary to give rise to an antibody-forming cell population is transmitted from mother to daughter cells.