Phlycten is due to-
Phlyctenular keratoconjunctivitis is a characteristic nodular affection occurring as an allergic response of the conjunctival and corneal epithelium to some endogenous allergens to which they have become sensitized. Phlyctenular conjunctivitis is of worldwide distribution. However, its incidence is higher in developing countries. ref: A K KHURANA OPHTALMOLOGY,E4, Page-77
Exogenous allergy
Endogeneous allergy
Viral Keratitis
Fubgal keratitis
Conjunctiva