Conjunctiva‐associated lymphoid tissue (CALT) – the physiological protective MALT of the conjunctiva

Abstract

Abstract Purpose Conjunctiva‐associated lymphoid tissue (CALT) is a part of the mucosal immune system – the local MALT of the conjuncitva. The presence of lymphatic cells in the conjunctiva is known for about a century but had remained controversial for a long time. Methods Complete conjunctival sacs in a large number of rabbits and humans were investigated in combined studies of whole‐mount observation together with histology, scanning and transmission electron microscopy and immunohistochemistry. Results A diffuse lymphoid tissue composed of lymphocytes and plasma cells along with accessory cells of the immune system occurs in the sub‐epithelial lamina propria with intraepithelial lymphocytes. Lymphoid follicles are regularly interspersed. These are less frequent and flat in elderly humans, but prominent in the rabbit. They show a typical structure, are composed of B‐cells with parafollicular T‐cells and high endothelial venules, and are covered by a specialised follicle‐associated epithelium (FAE) devoid of goblet cells. This has specialised M‐cells that contain groups of lymphocytes for uptake of antigens. Follicles frequently have a bright germinal center which indicates that antigens were in fact taken up and lymphocyte proliferation and differentiation was induced. Conclusion The conjunctiva of the rabbit and human have typical components of a physiologically protective mucosal immune system in the form of diffuse lymphoid tissue and lymphoid follicles that form the efferent and the afferent limb, respectively, of an immune answer. CALT can hence detect antigens from the ocular surface, pre‐sent them to lymphoid cells and generate protective effector cells and hence repre‐sents a part of the mucosal immune system at the conjunctiva. (DFG KN 317/11)