The sequence of morphological changes in the rat spleen following SRBC injection associated with hemolysin production has been correlated with estimates of proliferative activity by splenic lymphatic tissue. Formation of new, reactive germinal centers containing macrophages which engulf nuclear debris is a prominent feature of the response. This is prevented by pretreatment of the animal with cortisol. Indirect evidence is provided that short-lived lymphocytes produced in germinal centers may be a necessary component in the induction of other cells to proliferate and differentiate into hemolysin-producing cells. The reasons are discussed for considering short-lived lymphocytes, such as those produced in the thymus, bone marrow, and germinal centers, as differing from long-lived lymphocytes capable of antibody synthesis.

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