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.
Antigen-antibody interaction
Normally antibodies can detect and differentiate molecules from outside (foreign) of the body and those produced inside the body (self) as a result of cellular activities. Self molecules as ignored by the immune system. However, in certain conditions, the antibodies recognise self molecules as antigens and triggers unexpected immune responses. This results in different autoimmune diseases depending on the type of antigens and antibodies involved. Such conditions are always harmful and sometimes deadly. The exact nature of antibody-antigen interaction in autoimmune disease is not yet understood..