T cell receptor and CD4 proteins. Illustration of a cell membrane with embedded proteins and other molecules. At centre is a T-cell receptor (dark blue), with two CD4 molecules (light blue) at right and left. Also shown are glycolipids (orange). The T cell receptor activates the immune response to antigens in T cells (a form of lymphocyte). The CD4 protein (cluster of differentiation 4) 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.