Cluster of Differentiation, also known as CD, is a system for categorizing cell surface molecules that are expressed on white blood cells. These molecules are used to identify the different types of immune cells and their functions. However, the term "Cluster of Differentiation" has many synonyms. Some of the common synonyms include cell surface marker, cell surface antigen, leukocyte antigen, and lymphocyte antigen. Other synonyms for CD include cell activation antigen, differentiation antigen, and antigenic determinant. While these terms all refer to the same system of categorizing immune cells, they may be used interchangeably in different scientific fields. Understanding these synonyms is essential for anyone working in immunology and related fields.