What are the types of antigens?

One type of antigen is exogenous antigens. Exogenous antigens enter the body from the outside. For example, they can enter through inhalation, injection, or cuts on skin. These types of antigens include pollen, viruses, bacteria and food allergens, and are the most common type of antigens. Another type are autoantigens, which are proteins or nucleic acids that get attacked by their own immune system resulting in autoimmune diseases. The immune system can typically differentiate between foreign antigens and autoantigens. However, in some autoimmune diseases, like type I diabetes, the immune system is unable to differentiate between them. This leads to the production of autoantibodies against autoantigens. Endogenous antigens produced inside of the body because of bacterial or viral infections. These antigens signal to the immune system that they are either unharmful or harmful and presented to T cells for possible neutralization. When CD8+ T cells bind to the MHC-antigen complex, they become cytotoxic T cells if the antigen is recognized as a threat to the body. Tumor antigens are antigenic substances which are present on the surface of tumor cells that induce an immune response in the host (e.g. MHC I and MHC-II). Lastly, an antigen that has not been processed yet by an antigen-presenting cell is known as a native antigen.