CD4 protein. Computer model of a molecule of the CD4 protein that plays a vital role in the immune system. This is the amino acid backbone which has alpha helices (red) and beta sheets (pink). 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.