A symbol for glycoprotein expressed on the surface of some lymphocytes, including Helper T cells. Human CD4 is the receptor that serves as a docking site for HIV viruses on certain lymphocyte cells. Binding of the viral glycoprotein gp120 to CD4 is the first step in viral entry, leading to the fusion of viral and cell membrane. In a healthy adult, helper T-cells account for between 32% and 68% of the total number of lymphocytes (which includes B-cells and other types of T-cells). The CD4 percentage is sometimes a more reliable measurement than the CD4 count because it tends to vary less between measurements. For example, one person’s CD4 count may vary between 160 and 240 over a period of several months while their CD4 percentage remains constant at, say, 15%. The reason for this is that the CD4 count isn’t actually a direct count of CD4 cells, but rather a calculation based on the results of three other tests (the CD4 percentage, the lymphocyte percentage, and the WBC count), each of which can vary slightly each time it’s measured. Occasionally the CD4 count may be relatively high while the CD4 percentage is low (less than 21%). In this situation, many healthcare providers would consider the immune system to be significantly impaired based on the CD4 percentage.