antigenul

An antigen is a molecule or a part of a molecule that can be recognized by the immune system and, in many cases, can provoke an adaptive immune response. Antigens are typically foreign to the host and can include proteins, polysaccharides, lipids, or nucleic acids from pathogens, allergens, or environmental substances. They may also be autoantigens, which can trigger immune responses in autoimmune conditions.

Antigenicity refers to the ability of a substance to bind specifically to antibodies or T cell receptors, whereas immunogenicity is the capacity to induce a broader immune response that leads to the activation of lymphocytes and production of antibodies. Not all antigens are immunogenic in every context; small molecules called haptens are antigenic when bound to a larger carrier protein, but are not immunogenic on their own.

Epitopes, or antigenic determinants, are the specific parts of an antigen that are recognized by immune receptors. An antigen can have multiple epitopes, enabling different antibodies or T cell receptors to recognize distinct sites on the same molecule. Epitopes can be linear or conformational, depending on the antigen’s structure.

Antigen processing and presentation are central to how the immune system detects antigens. Antigen-presenting cells, such as dendritic cells, macrophages, and B cells, process exogenous antigens and present peptide fragments on MHC class II molecules to CD4+ helper T cells. Endogenous antigens are presented on MHC class I molecules to CD8+ cytotoxic T cells. B cells can recognize intact antigens via their B cell receptors and differentiate into antibody-secreting plasma cells.

Antigens are foundational to vaccines, serology, and transplant biology. Understanding antigen structure and epitope mapping informs diagnostics, immunotherapies, and strategies to modulate immune responses.