Lung alveolar histiocytes engaged in antibody production.

After introduction of antigen (sheep red blood cells) into rabbit lung alveoli by intrapulmonary or intratracheal injection antibody forming and releasing cells are found in subsequently evoked alveolar exudates. Intratracheal immunization results in slightly delayed antibody formation with a remarkably low involvement of mediastinal lymph nodes. At peak appearance of plaque-forming cells (PFC) in alveolar exudates 20–26 per cent of PFC are histiocytes or monocytes by microscopic and ultrastructural criteria. Plaque-forming histiocytes are less differentiated forms, different from typical macrophages. Their antibody production is sensitive to puromycin and to a lesser degree to actinomycin-D. In contrast to lymphoid PFC, some of these histiocytes are sensitive to a phagocyte-destructive agent-silica. Among the PFC, plasma cells and (early after immunization) activated lymphocytes and blasts are conspicuous in comparison with the overall composition of the cell populations of the alveolar exudate and mediastinal lymph node. It is calculated that 1 in 100–500 plasma cells and 1 in 20,000–100,000 histiocytes give demonstrable haemolysin formation during the peak of cellular response. This is suggested as the difference between the specialized and occasional or `primitive' antibody forming cell.