IgA and secretory component (SC) in the third eyelid of domestic animals: a comparative study.
The third eyelid of domestic animals is important for the production and distribution of tears, in removing ocular debris and in protection of the globe, and has significant immunologic functions. Although it is known that tears contain antibodies of the immunoglobulin A (IgA) isotype which are produced mainly by plasma cells of the lacrimal gland, very little is known about the antibody repertoires in the third eyelid of domestic animals. To assess whether IgA is derived from local synthesis, we analyzed the location of IgA-producing cells and the cellular distribution of secretory component (SC) in the third eyelid of domestic animals in a comparative study. A total of 83 third eyelids of dogs, cats, pigs, cows, sheep, goats and horses were investigated in the course of this study. Third eyelids were obtained immediately after death, cut length-wise, fixed overnight and processed for immunohistochemical detection of IgA and SC by the ABC technique. The results show that IgA-producing plasma cells are densely populated in subepithelial spaces of the surface epithelium as well as in the nictitating gland in a species-specific manner. In contrast, the SC could be demonstrated exclusively in glandular acinar and ductal epithelial cells and in different cell types of the surface epithelium, preferentially located on the bulbar side of the nictitating membrane. It is suggested that most of the SC is locally produced by resident plasma cells and subsequently transferred through the surface epithelium and glandular duct cells by transcytosis. This indicates that the third eyelid is an important member of the secretory immune system in domestic animals.