Pulmonary alveolar macrophages (AM) of rabbits inbred for resistance and susceptibility to tuberculosis were compared with respect to their ability to ingest tubercle bacilli in vitro. AM from the resistant T family ingested about twice as many bacilli as those from the susceptible C and FCCa families. This was due to differences in the number of phagocytes ingesting bacilli rather than differences in the number of bacilli ingested by each phagocyte.
These results suggest that the AM of certain susceptible inbred rabbit families trap fewer bacilli from the alveolar air. This could explain why they were resistant to attack by tuberculosis via the respiratory route, even though they were highly susceptible to its progress.