Editing Antigen (section)

===Exogenous antigens===
Exogenous antigens are antigens that have entered the body from the outside, for example, by [[inhalation]], [[ingestion]] or [[Injection (medicine)|injection]]. The immune system's response to exogenous antigens is often subclinical. By [[endocytosis]] or [[phagocytosis]], exogenous antigens are taken into the [[antigen-presenting cell]]s (APCs) and processed into fragments. APCs then present the fragments to [[T helper cells]] ([[CD4]]<sup>+</sup>) by the use of [[MHC class II|class II histocompatibility]] molecules on their surface. Some T cells are specific for the peptide:MHC complex. They become activated and start to secrete cytokines, substances that activate [[cytotoxic T lymphocytes]] (CTL), antibody-secreting [[B cell]]s, [[macrophages]] and other particles.

Some antigens start out as exogenous and later become endogenous (for example, intracellular viruses). Intracellular antigens can be returned to circulation upon the destruction of the infected cell.