CD4 protein. Computer model of a molecule of the CD4 protein that plays a vital role in the immune system. Protein structural units are the coloured balls, each colour for a different amino acid. The CD4 protein is found on the surface of white blood cells called T-helper cells. As a surface protein, CD4 binds to the surface proteins on other cells and pathogenic micro-organisms. This interaction helps the T-helper cells to recognise a threat and initiate an immune response. CD4 also binds to HIV (the human immunodeficiency virus) as part of the process by which HIV kills the T-helper cells and compromises the immune system. CD4 is a monomeric glycoprotein chain of around 450 amino acids.