Structure of the Conjunctiva
The conjunctiva of the eye is a membrane that lines the inside of the eyelid and extends over the white part of the eye (the sclera). The conjunctiva and eyelid normally function as a natural and immunologic barrier to a vast number of foreign invaders, including bacteria and viruses.
A crucial role in the normal defense against ocular invaders is played by many important cells, including mast cells, plasma cells, lymphocytes, and neutrophils—white blood cells and their derivatives. The mast cell, which is found throughout the eyelid and conjunctiva, is one of the most important players in allergic reactions. Plasma cells are actively involved in making antibodies (immunoglobulins) such as IgE. In allergic individuals, IgE is found in their tears and glands that make tears (lacrimal glands). Both immediate (IgE-mediated) and delayed hypersensitivity (lymphocyte-mediated) immune responses have been shown in experimental models of allergic conjunctivitis.
Although red blood cells (eosinophils) are not normally present in the conjunctiva, they may be found in conjunctival scrapings from people with allergic conjunctivitis. The presence of a single eosinophil (or its granules) usually points to allergic conjunctivitis; however, the frequency of finding eosinophils in conjunctival scrapings may vary with the duration of exposure.