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In mammals, surface respiratory macrophages (SM) are known to play a foremost role in protecting the respiratory system by providing first line of defense through engulfing pathogens and particulate matter respired with air. It has been reported that the pulmonary cellular defense system of domestic birds is inadequate. In particular, low number of SM and even lack of the cells in a healthy avian respiratory system have been associated with susceptibility of domestic birds to respiratory diseases. In an endeavor to resolve the existing controversy, the quantitative and qualitative attributes of the surface respiratory macrophages of the domestic duck and of the domestic rabbit were compared under similar experimental conditions. Quantitatively, the rabbit SM were on average approximately fourteen times more than the duck SM. The SM were found to have comparable diameters measuring about 12 m in the duck and 13 m in the rabbit. Similarly, the duck and the rabbit SM were structurally similar. Typically, they were round granular cells possessing filopodial extensions and variable electron dense bodies in the cytoplasm. The phagocytic capacity measured using polystyrene particles revealed that the duck SM had a higher phagocytic capacity than the rabbit SM. The volume density of the engulfed polystyrene particles, i.e. the volume of the particles per unit volume of the cell was estimated at 20 % in the duck and 9 % in the rabbit. These results suggest that the comparatively low numbers of SM in domestic birds may contribute to susceptibility of the birds to diseases. However, given the high phagocytic capacity of the avian SM, susceptibility of the domestic birds may not be due to dearth of the SM alone but some other factor (s) such as persistent exposure of the birds to particulate matter which is known to reduce robustness of the SM may be involved.
Mutua et al. (Wed,) studied this question.