A family of receptors that modulate the activation of t-lymphocytes by the T-CELL ANTIGEN RECEPTOR. The receptors are responsive to one or more b7 antigens found on antigen-presenting cells and, depending upon the specific ligand-receptor combination, modulate a variety of T-cell functions such as the rate of clonal expansion, cell survival and cytokine production. Although commonly referred to as costimulatory receptors, some of the receptors have inhibitory effects such as inducing peripheral tolerance.