<Tr> <Td> Natural killer cell </Td> <Td> CD16 +, CD56 +, CD3 -, CD31, CD30, CD38 </Td> </Tr> <P> Two commonly used CD molecules are CD4 and CD8, which are, in general, used as markers for helper and cytotoxic T cells, respectively . These molecules are defined in combination with CD3+, as some other leukocytes also express these CD molecules (some macrophages express low levels of CD4; dendritic cells express high levels of CD8). Human immunodeficiency virus (HIV) binds CD4 and a chemokine receptor on the surface of a T helper cell to gain entry . The number of CD4 and CD8 T cells in blood is often used to monitor the progression of HIV infection . </P> <P> While CD molecules are very useful in defining leukocytes, they are not merely markers on the cell surface . While only a fraction of known CD molecules have been thoroughly characterised, most of them have an important function . In the example of CD4 & CD8, these molecules are critical in antigen recognition . Others (e.g., CD135) act as cell surface receptors for growth factors . </P>

Clusters of differentiation (cd) molecules are important because they
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