Medicines against HIV work by gumming up some part of the infection process. If the virus can't get into cells, or can't make new virus, then the infection would be stopped.
Can blocking CCR5 stop HIV? Scientists are working hard to find out. Drugs are being developed that barricade the CCR5 receptor so HIV can't use it. This thwarts the virus as it attempts to enter a cell.
However, there are many things that could go wrong. One danger is that blocking one of our own proteins to stop HIV infection might weaken our immune system.
A drug to block CCR5 may have bad side effects and make people sicker. Fortunately, this is unlikely since people lacking CCR5 seem perfectly healthy.
But blocking CCR5 is still unlikely to be a miracle cure. Why not? The reason for this lies in the special properties of HIV's genes.
Most living things carry the instructions for making themselves in DNA. HIV (and a few other viruses) uses a related molecule, RNA, instead.
To make new virus, this RNA has to first be copied into DNA. The HIV virus carries its own unique protein for this purpose, called reverse transcriptase. (It carries out the reverse of the usual process of copying DNA to make RNA.)
The reverse transcriptase of HIV is very sloppy when it copies the genetic instructions. It makes lots of mistakes so that random changes frequently appear in its genes. The new viral offspring are now slightly different from the original.
Some of these viruses might not work at all. But some might work better, or infect cells in a different way.
So a form of evolution occurs -- Darwin's 'survival of the fittest'. Some of these changed viruses can evade the immune system and the drugs given for treatment.
This is how the virus becomes resistant to drugs--most viral particles are stopped by a drug but some have a mutation that lets them survive and reproduce.
How can the virus enter cells if CCR5 is blocked? A mutant virus could use a different co-receptor to get in. In fact this happens during the normal course of infection.
CCR5 is used during early stages of infection when the HIV virus is spreading slowly. Later on the virus evolves to use a different co-receptor called CXCR4 and that's when people get more sick.
But, the virus might not be able to easily mutate around a blocked CCR5. If the virus can't get into cells, then very few new viral particles would be produced. This greatly decreases the chance of a mutant form being created that doesn't use CCR5.
It's like throwing darts while blindfolded. Most darts would miss the bull's eye unless you got very lucky. However, if you throw a whole lot of darts, there's a greater possibility of hitting the target just by chance.