Distribution of secretory component and immunoglobulins in the developing lung.

The distribution of secretory component and immunoglobulins, IgA, IgG, and IgM in developing human lungs, ranging in age from 12 gestational weeks to 8 yr, was studied using the indirect peroxidase-labeled antibody method. Secretory component appeared first in the serous bronchial epithelium near the duct openings at the sixteenth gestational week, in goblet cells at the twentieth gestational week, and in bronchiolar epithelium at the twenty-second gestational week. The number of secretory component-positive cells (Y) in bronchial epithelium increased with age in weeks (X) according to the formula Y = 15.328 + 0.094 log X (p less than 0.001). Secretory component was almost absent in bronchiolar epithelium in atelectatic lungs with or without hyaline membrane. Secretory component-positive cells were never observed below the levels of respiratory bronchioles. Immunocompetent cells appeared in the bronchial walls after birth in normal lungs, but they appeared at the thirty-ninth gestational week in cases of pulmonary infection. The IgA- and IgM-containing cells were present around the bronchial glands in normal lungs, whereas IgG-containing cells were not associated with the glands. In normal lungs, IgA-containing cells were most prominent, followed by IgM-containing cells and IgG-containing cells. The apical portions of serous epithelial cells of bronchi and bronchial glands were positive for IgA and IgM. In bacterial and viral pulmonary infections, IgA-containing cells increased in number in the bronchial glands. The appearance of secretory component-positive cells long before immunoglobulin-containing cells in fetal lungs suggests that secretory component may play some role in the formation of the mucociliary blanket, instead of selective transport of IgA or IgM.