Computer-assisted morphometry of the intracapillary leukocyte pool in the rabbit lung

Computer-assisted morphometry was performed to evaluate the number and cell characteristics of capillary and alveolar leukocytes in rabbit lungs. An image-processing system and a programmable spread-sheet program were used, which allowed morphometric analysis of a large reference area. Neutrophils represented the largest intracapillary leukocyte population (2.2 x 10(7)/ml parenchyma, which corresponds to an approximately 104-fold microvascular enrichment of this cell type related to cell counts calculated for the capillary blood volume). In addition, large numbers of intracapillary lymphocytes (1.7 x 10(7)/ml parenchyma; 47-fold enrichment) and monocytes (0.3 x 10(7)/ml parenchyma; 86-fold enrichment) were detected. The total count of pulmonary leukocytes thus approximated the total number of pulmonary endothelial cells; and the total circulating pools of the different leukocytes were surpassed by the corresponding lung capillary pools, 3.2-fold for neutrophils, 1.2-fold for lymphocytes and 4.8-fold for monocytes. In contrast, alveolar cell numbers ranged from 1-2% of the capillary counts for all types of leukocytes. We conclude that the rabbit lung microvasculature harbours large pools of immunocompetent cells, which may contribute to host-defense mechanisms at the gas-exchange area.