An antigen is any molecule that is recognized by antibodies. Usually, an antigen is a protein or a polysaccharide, but it can be any type of molecule, even small molecules if coupled to a large carrier (haptens).
There are several kinds of antigens:
Antigens are presented by a cell to its environment via a histocompatibility molecule. Depending on the antigen presented and the histocompatibility molecule used, several types of immune cells can leap into action.
We can also classify antigens according to where they come from:
Exogenous antigens are antigens that have entered the body, e.g., by inhalation, ingestion, or injection. These antigens are taken up by endocytosis or phagocytosis into the antigen presenting cells (APCs), and degraded into fragments. The fragments are then presented on the surface of APCs by class II histocompatibility molecules to T helper cells (CD4+). Those that are specific for them, get activated, so they start to secrete cytokines. The cytokines then activate cytotoxic lymphocytes T (CTL), antibodies secreting lymphocytes B , macrophages and other cells.
Endogenous antigens are antigens that have been generated within the cell, as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. The fragments are then presented on the cell surface in the complex with class I histocompatibility molecules. If cytotoxic CD8+ T cells recognize them, they begin to secrete different toxins that cause the lysis or apoptosis of the infected cell. In order to keep the cytotoxic cells from killing cells just for presenting normal proteins, they run through a test cycle (negative control in the thymus) just after their production. Only those CTL that do not react to normal body protein fragments are allowed to enter the bloodstream.