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.
Regular and irregular antibodies
"Regular antibodies" usually refer to the isohemagglutinins, directed against antigens of the ABO system. They appear in the first years of life. They are of the IgM type. It is not certain whether they are natural antibodies (being produced without any previous infection, vaccination, other foreign antigen exposure or passive immunization) or are caused by infection in early life.