Bleb: A bladder-like structure more than 5 mm in diameter with thin walls that may be full of fluid. Also called a bulla. From Healthy Resources 10 Rheumatoid Arthritis Symptoms How IL Inhibitors Work to Treat PsoriasisModerate to Severe UC: Targeted Solutions Featured Centers What Are the Best PsA Treatments for You?Understanding Biologics 10 Things People With Depression Wish You Knew