ABSTRACT
The ocular immune system has both innate and acquired components. Physical factors, including the blink response, third eyelid and tear film, are a significant means of defence from microbes, chemicals or aeroallergens. The tear film comprises an inner mucin layer produced primarily by the conjunctival goblet cells, and also transmembrane mucin derived from epithelial cells. The middle aqueous layer is secreted by the lacrimal and accessory lacrimal glands as well as the nictitans glands, and there is also an outer oily layer produced mainly by the meibomian glands of the eyelids. Tears contain Ig produced by plasma cells within the lacrimal tissue. In the dog the dominant Ig in tears is IgA (1.0 ± 0.7 mg/ml), with less IgG (0.6 ± 0.6 mg/ml) and very small quantities of IgM (0.02 ± 0.00 mg/ml). Aggregates of lymphocytes and plasma cells are a normal feature of the subepithelial connective tissue of the bulbar conjunctiva and the inner aspect of the third eyelid, and interepithelial lymphocytes are also recognized. This conjunctivaassociated lymphoid tissue (CALT) is poorly
characterized in the dog and cat. The normal structure of the eye is reviewed in (508).
