A large part of normal middle ear cavity and Eustachian tube is provided with a mucociliary transportation system which is considered the first line of defense of the middle ear. Secretory cells of the normal mucosa of the middle ear and the tube can be classified into three main types: mucous, mixed and dark granulated (or serous). This may mean that the secreta of the mucosa is biochemically diverse and/or in a dynamic state. The presence of immunoglobulin producing plasma cells (IgA, IgG and IgM) and macrophages in the Eustachian tube and the middle ear mucosa supports the concept that the middle ear is protected by an immunologic defense system. Normal mucosal epithelial cells of the middle ear and the tube possess the ability to transport macromolecules by intact epithelial cells toward connective tissue layers where the macromolecules are either phagocytized by histiocytes or enter into the capillary system. The macrophages of regional lymph nodes also picked up the macromolecules. Demonstration of lamellar substances similar to phospholipids in the secreta of the tube and middle ear mucosa of the guinea pig suggests the presence of auditory surface active agent(s) (surfactant). Close association between secretory cells of the mucosa and acid phosphatase is demonstrated by cytochemical methods. It is suggested that secretory enzymes such as lysozyme and acid phosphatase may be either produced or transported by secretory cells of the middle ear mucosa. These enzyme secretions are considered a part of the enzymatic defense system of the middle ear.
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