CD4 is called CD4 because it is the fourth cluster of differentiation antigen identified on leukocytes (white blood cells).
Explanation
The "CD" in CD4 stands for "cluster of differentiation." These CD markers are surface proteins used to identify and differentiate between different types of cells, particularly immune cells. The numbering system (1, 2, 3, 4, etc.) reflects the chronological order in which these markers were discovered and grouped into distinct clusters based on their reactivity with specific monoclonal antibodies. So, CD4 was the fourth cluster to be defined.
Essentially, when scientists were identifying and classifying immune cells, they found a particular protein on the surface of certain cells that consistently reacted with a specific antibody. They designated this protein as belonging to the "cluster of differentiation" and, because it was the fourth such cluster they identified, it became known as CD4. This system helps researchers and clinicians identify and study various immune cell populations, such as CD4+ T helper cells, which are critical for immune function.
