The proteins

(1) The virus uses the proteins (binds with them) so that the virus can enter a cell and infect the cell?
(2) The function of the antibodies is to slip in the proteins and then block the proteins?

Context:

that prevent antibodies from slipping in and blocking the proteins the virus uses to latch onto a cell and infect it.

More context:

Most of the antibodies that the body produces to fight HIV, however, are ineffective. The surface of the virus is cloaked with sugar molecules that prevent antibodies from slipping in and blocking the proteins the virus uses to latch onto a cell and infect it. To make matters more complicated, HIV is constantly mutating, so there are multiple HIV strains that antibodies elicited in any vaccine must be able to sense and destroy.

http://www.sciencedaily.com/releases/2010/06/100601171846.htm