Immunology is the branch of medical science dealing with the body's specific responses to foreign substances. The function of the immune system is to recognize foreign, and therefore potentially harmful, microorganisms and their products and to assist their removal from the body. Immunology involves the study of the specialized cells of the immune system such as the lymphocytes, as well as antibodies, antigens, and the organs in which they are found, which include the lymph nodes, spleen, thymus, and bone marrow. Immunology also includes vaccinationÑthe use of weakened forms of pathogenic microbes or their products to produce immunity against infectious disease. Immunological responses are generally protective and therefore beneficial to the individual; however, occasionally the immune response can itself be harmful, such as in allergies and in autoimmune diseases, where the immune system attacks the cells of the body as if they were foreign invaders.