The term antigen refers to a substance recognized by the immune system. The term epitope refers to each specific portion of an antigen to which the immune system can respond. Antigenic epitopes exist on both native self tissues as well as foreign nonself tissues. A complex, 3-dimensional protein has multiple antigenic epitopes that the immune system can recognize as well as many other sites that remain invisible to the immune system.
The adaptive immune response, unlike the innate immune response discussed in Chapter 1, is a learned response to highly variable but specific antigenic epitopes rather than a response to conserved patterns. The purpose of the adaptive immune system is to continuously sample antigenic epitopes, determine if they are self or nonself, and then mount an immune response to eliminate foreign antigens. Several immunologic concepts, especially those of the immune response arc, the primary adaptive immune response, and the secondary adaptive immune response, are involved in this process. See Clinical Example 2-1.
Analogous to the neural reflex arc, the immune response arc—the interaction between antigen and the adaptive immune system—can be subdivided into 3 phases: afferent, processing, and effector (Fig 2-1).
Excerpted from BCSC 2020-2021 series: Section 9 - Uveitis and Ocular Inflammation. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.