Basic components of the immune system part 12

Most antigens are either thymus-dependent or thymus-independent antigens. Thymus-dependent antigens require T-cell participation. Most proteins and foreign red cells are examples of these molecules. Thymus independent  antigens do not require T-cell participation for antibody production. Instead, they directly stimulate specific B lymphocytes by cross linking antigen receptors on the surface of B cells. These molecules  produce primarily IgM and IgG2 antibodies and do not stimulate long-lasting memory cells. Most bacterial polysaccharides (found in bacterial cell walls) fall into this category. Certain polysaccharides,  such as LPS (lipopolysaccharide), not only induce specific B-cell activation but also can act as polyclonal B-cell stimulant.