An antigen is a substance specifically recognized by an antibody or an antigen receptor of the immune system. Antigens are the molecular targets of adaptive immunity, which distinguishes particular structures rather than merely broad categories of foreign material. They may originate from microorganisms, environmental substances, or the body’s own tissues. Recognition does not necessarily produce an immune response: a substance capable of inducing such a response is more precisely called an immunogen. (ncbi.nlm.nih.gov)
Antigenicity and immunogenicity
Antigenicity is the capacity for specific recognition by antibodies or lymphocyte receptors; immunogenicity is the capacity to induce an adaptive immune response. Although the terms antigen and immunogen are sometimes used interchangeably, the distinction explains why a molecule can bind an antibody without independently stimulating antibody production. Antigen recognition and immune activation are therefore separate biological events. (ncbi.nlm.nih.gov)
A hapten illustrates this distinction. Haptens are small molecules that can be recognized by antibodies but generally require attachment to a larger carrier, commonly a protein, to elicit an antibody response. The combined hapten–carrier structure can act as an immunogen. Antibodies generated against it may subsequently bind the free hapten, even though the free molecule alone did not initiate their production. (ncbi.nlm.nih.gov)
The outcome also depends on the biological context. For naive T cells, recognition normally must be accompanied by appropriate costimulatory signals. Recognition without adequate additional signals can instead produce functional inactivation or deletion. Thus, antigenicity does not imply harmfulness, foreign origin, or an inevitable inflammatory response. (ncbi.nlm.nih.gov)
Molecular structure and epitopes
Many important antigens are proteins or complex carbohydrates, including bacterial surface polysaccharides. Antibody recognition is not restricted to proteins: antibodies can also recognize small chemical compounds and other molecular structures. An antigen may be soluble, part of a microbial surface, or associated with a host cell. (cdc.gov)
The particular region recognized by an antibody or antigen receptor is an epitope, also called an antigenic determinant. A large antigen can contain numerous epitopes, allowing different antibodies to recognize different parts of the same molecule. An antibody’s corresponding binding surface is its paratope. Consequently, recognition is defined by the interaction between a particular binding site and a particular molecular region, rather than by the antigen as an indivisible whole. (ncbi.nlm.nih.gov)
Protein epitopes may be linear, involving a continuous sequence of amino acids, or conformational, involving residues brought together by protein folding. Altering a protein’s three-dimensional structure can therefore change antibody recognition. Specificity is also not absolute exclusivity: structurally similar epitopes on different antigens can produce cross-reactivity, in which one antibody recognizes more than one target. (ncbi.nlm.nih.gov)
Recognition by B cells and T cells
B cells recognize antigens through membrane-bound immunoglobulins, which serve as B-cell receptors. These receptors and secreted antibodies can bind accessible structures on intact antigens. By contrast, most conventional T cells recognize antigen-derived peptide fragments displayed together with major histocompatibility complex molecules, or MHC molecules. Their target is the peptide–MHC complex, not simply a free antigen molecule. (ncbi.nlm.nih.gov)
This distinction requires antigen processing and presentation. Proteins are broken down into peptides, selected peptides bind MHC molecules, and the resulting complexes appear at the cell surface. MHC class I generally presents peptides derived from intracellular proteins to CD8 T cells. MHC class II generally presents peptides derived from material taken up from outside the cell to CD4 T cells. These pathways allow immune cells to inspect different sources of protein. (immunology.org)
Professional antigen-presenting cells, including dendritic cells, participate in initiating T-cell responses. The intracellular–extracellular division is not absolute: cross-presentation allows extracellular antigens to enter the MHC class I pathway, while intracellular material can also reach MHC class II pathways. Different processing routes can therefore expose different epitopes from the same original antigen. (immunology.org)
Self-antigens and immune tolerance
Antigens are not necessarily foreign. Self-antigens originate in the host’s own tissues, whereas non-self antigens originate elsewhere. Self-derived peptides are routinely presented by MHC molecules. Whether recognition leads to an immune response depends partly on immune tolerance, which normally limits responses against the body’s own constituents. (cancer.gov)
Tolerance mechanisms include removal or inactivation of self-reactive lymphocytes, changes in B-cell receptors, and suppression by regulatory T cells. Failure of these controls can permit autoimmunity. The existence of a self-antigen, however, is not itself evidence of disease: normal tissues contain many potential immune targets without undergoing immune attack. (ncbi.nlm.nih.gov)
Cells involved in cancer can also carry recognizable antigens. Some tumor-associated antigens are absent from normal cells or occur there at lower levels. Such targets are studied in cancer immunotherapy, including cancer treatment vaccines designed to stimulate responses against antigen-bearing tumor cells. (cancer.gov)
Vaccination and laboratory detection
A vaccine supplies antigens, or a means of producing them, in a form intended to induce protective immunity and immunological memory. Different vaccines use different antigenic materials. A subunit vaccine contains selected microbial components; a toxoid vaccine uses an inactivated bacterial toxin. A conjugate vaccine links a polysaccharide antigen to a protein, enabling a more durable response than the polysaccharide alone may produce. (cdc.gov)
Antigens also serve as laboratory targets and cellular markers. In infectious-disease testing, antigen assays detect pathogen components, such as viral proteins. Antibody tests instead detect the host’s immune response, while molecular tests detect genetic material. These approaches measure different biological entities: detecting an antigen is not the same as detecting an antibody directed against it. (cancer.gov)