Composite cellular defence stratagem in the avian respiratory system: functional morphology of the free (surface) macrophages and specialized pulmonary epithelia

Qualitative and quantitative attributes of the free respiratory macrophages (FRMs) of the lung – air sac systems of the domestic fowl ( Gallus gallus variant domesticu s) and the muscovy duck ( Cairina moschata ) were compared with those of the alveolar macrophages of the lung of the black rat ( Rattus rattus ). The birds had significantly fewer FRMs compared to the rat. In the birds, the FRMs were found both in the lungs and in the air sacs. Under similar experimental conditions, the most robust FRMs were those of the domestic fowl followed by those of the rat and the duck. Flux of macrophages onto the respiratory surface from the subepithelial compartment and probably also from the pulmonary vasculature was observed in the birds but not in the rat. In the duck and the domestic fowl, a phagocytic epithelium that constituted over 70% of the surface area of the blood–gas (tissue) barrier lines the atrial muscles, the atria and the infundibulae. The epithelial cells of the upper respiratory airways contain abundant lysosomes, suggesting a high lytic capacity. By inference, the various defence strategies in the avian lung may explain the dearth of FRMs on the respiratory surface. We counter‐propose that rather than arising directly from paucity of FRMs, an aspect that has been over‐stressed by most investigators, the purported high susceptibility of birds (particularly table birds) to respiratory ailments and afflictions may be explained by factors such as inadequate management and husbandry practices and severe genetic manipulation for fast growth and high productivity, manipulations that may have weakened cellular and immunological defences.