Immunology and the Immune Response

Immunology, a branch of biomedical sciences pioneered by Edward Jenner in 1796, investigates the ability of an organism to respond to antigenic challenge and differentiate between “self” and “nonself.” Building on the work by Jenner and other influential immunologists, Emil von Behring and Kitasato Shibasaburo developed the theory of how antibodies function in the late 1890s; at the same time, Elie Metchnikoff developed the theory of phagocytosis. The work of these scientists initiated studies of the adaptive and innate immune responses, respectively. Invading infectious agents first encounter the host innate immune system, present in some capacity in all multicellular organisms. Innate immunity does not recognize pathogen-specific antigens and consists of anatomical barriers (skin, cilia, mucus, peristalsis, etc.), commensal microbes (nonpathogenic bacteria producing antimicrobial substances such as bacteriocidins, cationic proteins, and lactoferrin), and antimicrobial peptides (b-defensins, phospholipase A, and lysozyme). When anatomical barriers become compromised, phagocytes (macrophages and neutrophils) immediately recognize, ingest, and destroy invading pathogens. Recognition occurs through pattern recognition receptors (PRRs) on the surface of the phagocytic cells, evolved to respond to evolutionarily conserved characteristics of infectious microbes. PRRs activate signaling cascades to communicate with the adaptive immune system and produce proteins (cytokines, chemokines, and cell-adhesion molecules) to attract effector cells to the site of an infection. As the invading pathogen is contained, the innate immune system signals to the adaptive immune system to help eradicate the remaining pathogen and program an immune response for future invasions by the samemicroorganism; host survival depends on these critical actions.