Final answer:
CD (Cluster Designation) is a term used to refer to specific molecules on the surface of cells, which are used to identify different types of cells in the immune system.
Step-by-step explanation:
The term CD (Cluster Designation) is a shorthand way to refer to specific molecules on the surface of cells. These molecules, also known as cluster of differentiation markers, are used to identify different types of cells in the immune system. For example, CD4 is a marker found on helper T cells, while CD8 is a marker found on cytotoxic T cells.