In vitro synthesis of bovine IgA was detected by a radioimmunoelectrophoretic assay in 17 of 20 different tissues obtained from cows and 2 of 6 different tissues obtained from a 1-week-old calf. Conclusions drawn from these qualitative studies are supported by the results of preliminary quantitative data. Synthesis of IgA was most pronounced in the adult ileum, duodenum, colon, lungs, lower nasal mucosa, oral pharyngeal mucosa, parotid gland, lacrimal gland and thymus and the calf parotid and lacrimal glands. Correspondingly, IgA, not IgG1, is the principal immunoglobulin in bovine tears and saliva. In comparison to other secretory tissues of the cow and the mammary glands of the rabbit, human and monkey, the bovine mammary gland shows moderate deficiency in the synthesis of IgA. This deficiency is reflected in the fact that IgA does not become the predominant immunoglobulin of normal bovine milk, although an increase in the IgA to IgG1 ratio does occur after parturition. The adult bovine thymus is an immunologically active secretory tissue, but the ileum of the 1-week-old calf appears to lack an IgA-producing system. The concentration of serum IgA is lower than in the human and is greater before calving than after. Most of the IgG1 synthesized in the mammary gland is eluted from Sephadex G-200 before 7S IgG1.

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