B - and T-lymphocyte attenuator (BTLA), also known as CD272, is a transmembrane glycoprotein with a molecular weight of 35kD and is a member of the CD28 family of T cell co stimulators. B - and T-lymphocyte attenuator (BTLA), also known as CD272, is a transmembrane glycoprotein with a molecular weight of 35kD and is a member of the CD28 family of T cell co stimulators. BTLA contains 127aa of extracellular domain, 21aa of transmembrane domain, and 111aa of cytoplasmic domain. Unlike other members of the CD28 family, the Ig domain in the extracellular domain of BTLA is type I rather than type V, and it does not form homologous dimers. In addition, BTLA interacts with HVEM, a member of the TNF superfamily, rather than with the B7 family ligand. The interaction between BTLA and HVEM transmits inhibitory signals and downregulates the immune response of lymphocytes.