Although the role of CD28 in T cell costimulation is firmly established , the mechanisms by which it exerts its costimulatory actions are less clear .
although the role of cd28 in t cell costimulation is firmly established , the mechanisms by which it exerts its costimulatory actions are less clear .
O O O O B-protein_molecule O B-other_name I-other_name I-other_name O O O O O O O O O O O O O O O O O