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 cannot 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.
T independent antigens are divided into 2 classes by the mechanism of activating B cells.