The immune system is a network of cells, tissues, and organs that work together to defend the body against attacks by foreign invaders. These are primarily microbes—tiny organisms such as bacteria, parasites, and fungi that can cause infections. There are three types of lymphocytes: natural killer cells, thymus-derived lymphocytes (T cells) and bone marrow-derived lymphocytes (B cells), each with different functions. Immunity is defined as the ability to destroy pathogens or other foreign material and to prevent further cases of infectious disease. The chapter mainly focuses on immune system, lymphocytes, immunity, allergic rhinitis, atopic dermatitis (eczema), anaphylaxis, food allergy, graft versus host disease, adverse drug reaction, and latex allergy.