Antigeeni

Antigeeni, a term used in several languages including Italian and Finnish, refers to a molecule or a part of a molecule that can be recognized by the immune system. In English, the equivalent term is antigen. Antigeeni are typically foreign to the host and can provoke an immune response, although some self-molecules can become targets in autoimmune conditions.

An antigen is recognized by specific receptors on B lymphocytes and T lymphocytes. The exact portion of an antigen that is recognized is called an epitope or antigenic determinant, which may be linear (a short sequence of amino acids or nucleotides) or conformational (dependent on the three-dimensional structure). A substance can be antigenic (able to bind to antibodies or T cell receptors) without being highly immunogenic, and some small molecules called haptens are antigenic but require a larger carrier to elicit a full immune response.

Antigen processing and presentation are central to how the immune system responds. Exogenous antigens, taken up by antigen-presenting cells, are processed and presented on MHC class II molecules to CD4+ helper T cells. Endogenous antigens, from within cells, are processed and presented on MHC class I molecules to CD8+ cytotoxic T cells. B cells can bind native antigens through their B cell receptors and, with help from T cells, can produce high-affinity antibodies.

Immunogenicity is the ability of an antigen to elicit an immune response, whereas antigenicity is the ability to bind specifically to antibodies or T cell receptors. Adjuvants are substances that enhance immunogenicity. Vaccines, diagnostic tests, and research tools rely on well-characterized antigeeni to achieve specificity and efficacy.