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.
T independent antigen (TI)
For most protein antigens, the production of antibodies by B lymphocytes is dependent on stimulation of helper T cells. However bacterial polysaccharides and lipopolysaccharides, and some polymeric proteins, can stimulate B lymphocytes without involvement of helper T cells. The non-protein microbial antigens can not stimulate classical T cell response by themselves, but they are able to elicit the production of antibodies, so that is why we call them T cell or thymus independent antigens.