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.
Isoantibodies
Isoantibodies (formerly called alloantibody) are antibodies produced by an individual against isoantigens produced by members of the same species. In the case of the species "Homo sapiens", for example, there are a significant number of antigens which are different in every individual. When antigens from another individual are introduced into another's body, these isoantibodies immediately bind to and destroy them.