The direct and indirect plaque technique for the detection of antibody-forming cells against sheep erythrocytes was utilized for the investigation of a number of biological parameters of the primary and secondary immune response on a cellular level.

The sequential pattern of 19S followed by 7S antibody formation was elicited in the primary response after a latent period of at least 1–2 days and 2–3 days respectively. The secondary response initiated 140 days after primary immunization, in contrast, was characterized by the simultaneous appearance of 19S and 7S antibody-forming cells after an observed latent period of 2–3 days.

The cellular dynamics of the recruitment phase of the respective immunoglobulins in the primary and secondary response was interpreted as evidence for the derivation of the two classes of immunoglobulins from separate progenitors. The 19S antibody-forming cells were derived predominantly by a process of transformation and maturation and 7S antibody formers by a process of cellular division with a doubling time of about 12 hr.

The draining lymph node exhibited maximal immunological reactivity due to its capacity to retain the particulate antigen. This capacity was considerably enhanced in the sensitized draining lymph node. Minimal cellular activity was also noted in distal lymphoid tissues which included the thymus.

Focal cellular activity was observed in the draining lymph node for 60 days after immunization. Subsequently, very low level plaque-forming cellular activity was observed in association with persistence of maximal antibody activity. The appearance at 120 days of a generalized peak of cellular activity in lymphoid tissues throughout the host was considered an explanation for this discrepancy.

The change in distribution of cellular antibody-forming activity, from a local to a generalized lymphatic response during the late phase of the immune response, implied a fundamental alteration in homeostatic mechanisms associated with maintenance of immune reactivity. Further manifestations of such an alteration were indicated by the appearance of 2-ME-sensitive 7S antibody nearly 3 months after primary intradermal immunization, which in the ensuing 5 months was associated with, and inversely related to, two major fluctuations in 2-ME-resistant 7S antibody.

Evidence for the existence of immunological memory in the 19S system was not established in the present work. 19S anamnesis, for which evidence was derived from measurements of circulating antibody levels, was interpreted from cellular studies as the result of the substantial activity of previously uncommitted 19S lymphoid cells in distal lymphoid tissue associated with previously committed 19S cells contained in the draining lymph node.

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