HIV glycoprotein (gp120) complexed with human CD4 proteins. Gp120 is an important surface protein of Human Immunodeficiency Virus, or HIV, that allows the virus to recognize and infect white blood cells bearing the CD4 protein. Normally, the CD4 protein assists in the activation of these white blood cells in the presence of pathogens, but HIV turns this defensive protein into a liability. It does so through gp120 (reddish-brown molecule in image), which tightly grips CD4 proteins (violet), anchoring the HIV virus to the target cell and providing a toehold for fusion with the host cell and subsequent infection. Magnification: 41 million times at 5"x7" print size.