Any substance (molecule) capable of inducing a specific immune response (humoral or cellular) against that substance, or can be recognized by products of an immune response, such as antibodies or lymphocytes, is commonly called an antigen. Antigens exist in many forms: typically they can be simple molecules, toxins, chemicals, proteins, carbohydrates, lipids or nucleic acids derived from invading microorganisms, such as viruses, bacteria, protozoans, and fungi, or other foreign substances that are usually not found in the body (non-self or heteroantigens). In addition, the body's tissue and cells, including cancer cells, can also have antigens (self or autoantigens, or mutated neoantigens) in them and can cause an immune response (Zamvil et al., 1986; Khodadoust et al., 2017). These autoantigens can be used as biomarkers to identify those specific tissues or cells (Schumacher and Schreiber, 2015; Balachandran et al., 2017). Although antigens exist in numerous forms, they all share a common mechanism of triggering an immune response. The immune system in humans and other mammals can be divided into two general categories: the innate, which is a more general immune system that can recognize foreignness, but not specific antigens, and the adaptive, which is more a specialized immune system with receptors for specific antigens. These two systems work closely together and take on different tasks—while the innate immune response is immediate, the adaptive immune response is not. However, the effect of the adaptive immune response is long-lasting, highly specific, and manifests sustained long-term memory (Janssen et al., 2003; Polonsky et al., 2018).