Unstable chromosome aberrations (acentrics, dicentrics, rings) have been observed in tuberculin-stimulated leukocyte cultures from three patients up to six months after therapeutic irradiation. This is considered evidence that human blood contains immunologically committed, long-lived, nondividing small lymphocytes which retain the latent capacity to proliferate when re-exposed to the sensitizing antigen. "Immunologic memory" may reside in such cells, which, because they are not continually dividing, need not have undergone genetic alteration in becoming immunologically committed.

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