Antigen

An antigen is a molecule that stimulates the production of antibodies. Usually, it is a protein or a polysaccharide, but can be any type of molecule, including small molecules (haptens) coupled to a protein (carrier).

• Immunogen - Any substance that provokes the immune response when introduced into the body. An immunogen is always a macromolecule (protein, polysaccharide). Its ability to provoke the immune response depends on its foreignness to the host, molecular size, chemical composition and heterogeneity (e.g. different amino acids in a protein).
• Tolerogen - An antigen that invokes a specific immune non-responsiveness due to its molecular form. If its molecular form is changed, a tolerogen can become an immunogen.
• Allergen - An allergen is a substance that causes the allergic reaction. It can be ingested, inhaled, injected or comes into contact with skin.

Cells present their antigens to the environment via a histocompatibility molecule. Depending on the antigen presented and the type of the histocompatibility molecule, several types of immune cells can become activated.

We can also classify antigens according to their origin.

Exogenous antigens

Exogenous antigens are antigens that have entered the body form the outside, for example by inhalation, ingestion, or injection. By endocytosis or phagocytosis, these antigens are taken into the antigen-presenting cells (APCs) and processed into fragments. APCs then present the fragments to T helper cells (CD4+) by the use of 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. Cytokines are substances that can activate cytotoxic T lymphocytes (CTL), antibody-secreting B cells, macrophages and other cells.

Endogenous antigens

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 an activated cytotoxic CD8+ T cells recognize them, the T cells 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 self-proteins, self-reactive T cells are deleted from the repertoire as a result of central tolerance (also known as negative selection which occurs in the thymus). Only those CTL that do not react to self-peptides that are presented in the thymus in the context of MHC class I molecules are allowed to enter the bloodstream.

There is an exception to the exogenous/endogenous antigen paradigm, called cross-presentation.