Antigen
Also, an antigen is a molecule that binds to Ag-specific receptors, but cannot necessarily induce an immune response in the body by itself. Antigens are usually proteins, peptides (amino acid chains) and polysaccharides (chains of monosaccharides/simple sugars) but lipids and nucleic acids become antigens only when combined with proteins and polysaccharides. In general, saccharides and lipids (as opposed to peptides) qualify as antigens but not as immunogens since they cannot elicit an immune response on their own. Furthermore, for a peptide to induce an immune response (activation of T-cells by antigen-presenting cells) it must be a large enough size, since peptides too small will also not elicit an immune response.
Anti-nuclear antibody
The human body has many defense mechanisms against pathogens, one of which is humoral immunity. This defence mechanism produces antibodies (large glycoproteins) in response to an immune stimulus. Many cells of the immune system are required for this process, including lymphocytes (T-cells and B-cells) and antigen presenting cells. These cells coordinate an immune response upon the detection of foreign proteins (antigens), producing antibodies that bind to these antigens. In normal physiology, lymphocytes that recognise human proteins (autoantigens) either undergo programmed cell death (apoptosis) or become non-functional. This self-tolerance means that lymphocytes should not incite an immune response against human cellular antigens. Sometimes, however, this process malfunctions and antibodies are produced against human antigens, which may lead to autoimmune disease.