Introduction 
Figure 19- Stereoview of a region on the macromolecule CD4, the T cell receptor for HIV. CD4 
contains four tandem immunoglobulin-like domains, D1-D4. The view represents Dl's major 
binding region for the virus coat glycoprotein gpl20. Atoms of residues 41-59 are shown in red. 
They are enveloped by the surface in contact with a water molecule probe. This figure was produced 
with the display program QUANTA. 
Reprinted by permission from Ryu, S.-E., Kwong, P.D., Truneh, A., Porter, T.G., Arthos,/., Rosen- 
berg, M., Dai, X., Xuong, N.-h., Axel, R., Sweet, R.W., and Hendrickson, W.A. Nature 348:419-426. 
Copyright ® 1990 Macmillan Magazines Ltd. 
Biologic 
Activation Regulation Effects 
Ag clearance 
Tumor killing 
Cellular activation 
Hemolysis 
fragments 
Figure 20. The complement system. Ag, antigen; Ab, antibodies. 
Courtesy of V. Michael Holers. 
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