B cells produce antibodies, which are secreted into the bloodstream.
These antigens can be protein, DNA, carbohydrate, or just about any other kind of molecule. Although the TCR also can bind to antigens, these antigens must be proteins. The MHC molecules are what doctors use to determine tissue types for transplants and grafts. There are two different types of T cells, identified by the cell-surface proteins CD4 and CD8, and they serve different functions. They are also involved in fine tuning the immune response between antibody production and CTL activation, since different pathogens have different susceptibilities to these two types of attack.