In meicine, bleb is a large blister (usually approximately hemispherical) filled with serous fluid. Blebs can form in a number of tissues due to different pathologies, including frostbitten tissues, and as a cause of spontaneous pneumothorax. In the lungs, a bleb is a collection of air within the layers of the visceral pleura. In ophthalmology, blebs may be formed intentionally in the treatment of glaucoma.