Quantification and origin of the immunoglobulins in porcine respiratory tract secretions.

The immunoglobulin content of porcine nasal, tracheal and bronchio-alveolar secretions was determined. IgA was the predominant immunoglobulin in nasal and tracheal secretions, there being no significant difference (P greater than 0.01) between their IgA:IgG ratios of 2.7:1 and 2.4:1, respectively. In contrast, IgG was the predominant immunoglobulin in bronchioalveolar secretions, the IgA:IgG ratio being 0.7:1. The ability of tracheal and lung tissue to synthesize IgA and IgG was determined in vitro. Tracheal cultures synthesized more IgA than IgG whereas in the lungs the reverse occurred, the ratio of IgA:IgG synthesis being 1.4:1 and 0.4:1, respectively. The proportion of serum-derived IgA and IgG in respiratory tract secretions was determined by measuring the transfer of 125I- and 131I-labelled immunoglobulins from serum. It appeared that more than 97% of the IgA was produced locally. More significantly a proportion of the IgG was also synthesized locally. This proportion was smallest in nasal secretions (21.8%) and greatest in bronchio-alveolar secretions (62.6%). The results suggest that IgG, a large proportion of which is locally synthesized, is important in the homeostatic mechanisms of the lower respiratory tract.