Final answer:
CD in CD4 T-cells stands for Cluster of Differentiation. CD4 T-cells are characterized by the expression of CD4 on their surface, which is associated with helper functions in the immune system. CD8 T-cells, on the other hand, are characterized by the expression of CD8 and are associated with cytotoxicity.
Step-by-step explanation:
CD in CD4 T-cells stands for Cluster of Differentiation. CD molecules are cell surface glycoproteins that are used to identify and distinguish between different types of white blood cells. CD4 T-cells are characterized by the expression of CD4 on their surface, which is associated with helper functions in the immune system. CD8 T-cells, on the other hand, are characterized by the expression of CD8 and are associated with cytotoxicity.