Most cases of conjunctivitis may be categorized as either papillary or follicular, according to the macroscopic and microscopic appearance of the conjunctiva (Fig 5-3). Neither type is pathognomonic for a particular disease entity. Papillary conjunctivitis shows a cobblestone arrangement of flattened nodules with central vascular cores (Fig 5-4). It is most commonly associated with an allergic immune response, as in vernal and atopic keratoconjunctivitis, or it is a response to a foreign body such as a contact lens or ocular prosthesis. Papillae coat the tarsal surface of the upper eyelid and may reach large size (giant papillary conjunctivitis). Limbal papillae may occur in vernal keratoconjunctivitis (Horner-Trantas dots). The histologic appearance of papillary conjunctivitis is identical, regardless of the cause: closely packed, flat-topped projections, with numerous eosinophils, lymphocytes, plasma cells, and mast cells in the stroma surrounding a central vascular channel.
Follicular conjunctivitis (Fig 5-5) is seen in a variety of conditions, including inflammation caused by pathogens such as viruses; atypical bacteria; and toxins, including topical medications (glaucoma medications, especially brimonidine, or over-the-counter ophthalmic decongestants). In contrast to papillae, follicles are small, dome-shaped nodules without a prominent central vessel. Accordingly, whereas a papilla clinically appears more red on its surface and more pale at its base, a follicle appears more pale on its surface and more red at its base. Histologically, a lymphoid follicle is situated in the subepithelial region and consists of a germinal center, containing immature, proliferating lymphocytes; and surrounding corona, containing mature lymphocytes and plasma cells. The follicles in follicular conjunctivitis are typically most prominent in the inferior palpebral and forniceal conjunctiva.