The main components of immune reaction—antigen and antibodies—are discussed in detail. The various cells that participate in the reaction are described in length. The reaction allied to this which results in injury—the hypersensitivity—come next in this chapter. Their four types are important. The next important components of immune reaction are the complements and cytokines, an understanding of which is important. Allergic conjunctivitis is mentioned in detail—including the etiopathology and management.