[ c luster d esignation] a system for classifying cell-surface markers expressed by lymphocytes based on a computer analysis of monoclonal antibodies against human leukocyte antigens , with antibodies having similar specificity characteristics being grouped together and assigned a number (CD1, CD2, CD3, etc.); these CD numbers are also applied to the specific antigens recognized by the various groups of monoclonal antibodies . See also CD antigen .