1. When infection occurs, cellular infiltration occurs around the invading organism, leading to the formation of subepithelial small pin point grayish follicles with a center of mononuclear cells surrounded by lymphocytes and giant multinucleated cells (Leber's cells). These follicles are not raised above the surface, non expressible and are called immature follicles Inclusion bodies can be detected in conjunctival scrapings by Giemsa stain
2. The follicles enlarge and become pinkish, raised above the surface and are expressible (TIla)
3. As infiltration increases around a core of blood vessels, papillae are formed (Tut) where the papillae are soft, vascular and pinkish in color.
4. Healing occurs by cicatrization (Till), which may take the form of lines, patches, or a dense white line at the sulcus subtarsalis called ArIt's line . Post trachomatous degenerations (PTDs) may occur in the conjunctival crypts between adjacent papillae or in the deeper parts of the papillae. Calcification may occur in these degenerate areas leading to sandy white calcified spots called post trachomatous concretions (PTCs). As recurrent infection is common, the conjunctiva may show different stages of the disease (follicles, papillae and scarring).
5. Complete healing (TIV) is the end point where cicatrisation is complete with no follicles or papillae and absence of inclusion bodies in conjunctival scraping.