As far as viruses go, HIV is a pretty diabolical virus. It infects mainly cells of our immune systems, specifically cells called “T Helper cells” (known to immunologists as CD4 T cells). Unfortunately, it’s these helper cells that tell the rest of the immune system (and the rest of the body) about viral infections; if you kill the messenger, the alarm can never be raised. It floats around the body, and when it finds one of these T cells, it uses proteins on its surface that let it enter the cell. (Interestingly, there’s a mutation in the population that actually prevents HIV from entering the cell. It’s in a protein called CCR5.) All viruses uses the cell’s machinery to make new viruses, but in most cases, cells have multiple mechanisms to let the immune system know that they’ve been infected:
* They can put a protein on their surface that lets the body know. “Hey! I’ve got a virus inside me! Help!”
* They can release chemicals that turn on the immune system, make it harder for other cells to get infected, and make it more likely to put the alarm protein on the surface.
* If they’re not displaying an alarm signal or a normal signal, the body kills the cell with NK cells, assuming that the virus is probably preventing the signal from getting to the surface.

But HIV bypasses all these methods. It prevents the cell from giving an alert signal, but it specifically allows only the normal signal to be displayed, so the body assumes everything’s fine. (And even if the NK cell kills the HIV-infected cell, HIV can slow down the cell’s death process so that more virus can be created.)

Now, why can’t we just make a vaccine to protect ourselves? It’s a great question.

HIV is a retrovirus. That basically means that instead of using DNA like our bodies, it uses a less-stable genetic molecule called RNA. Once it gets inside the cell, the virus uses a protein that it itself makes (called reverse transcriptase) to turn the RNA into DNA, and then the body turns the DNA into protein (I’m simplifying here, but it’s the general idea we’re going for). It turns out that the HIV version of the reverse transcriptase is a really crappy copier. It makes a ton of mistakes and errors. But it’s the error-prone nature that makes HIV so difficult to make a vaccine against.

Vaccines are basically a way to trick our bodies into thinking we’ve been infected with a virus or bacteria, and causing the immune system to react, so that the next time we’re actually infected with the real virus or bacteria, we’ll be able to respond quicker and stronger. But in order for the vaccine to work, the immune system has to recognize a certain shape. Sometimes it’s a protein on a bacteria, sometimes it’s a protein from a virus, or sometimes it’s a toxin (like tetanus toxin). But in every case, the immune system only responds to shapes it can see on the outside of cells; it can’t respond to anything that’s hiding away inside another cell.

HIV sits inside a cellular forcefield, called an envelope, when it’s not inside a cell. Like everything else, the envelope shape is coded by the virus. Other viruses have these envelopes, too, and the body can make an immune response against them, recognize them, and get rid of them. But because the HIV is such a crappy copier, the shape of the HIV envelope is constantly changing. So even if your body could respond to one HIV virus shape, there’d be 5,000 other shapes that it hasn’t responded to yet. Some immunologists like to think of HIV as not just one virus, but thousands of different viruses that are all attacking the same targets. So even if we could vaccinate someone against one envelope shape of HIV, there’s nothing to say that the body could fight off a different envelope shape.

The drugs that slow down HIV (called an “HIV cocktail”) try to stop the virus in lots of directions, and the body even creates more CD4 cells as it detects that others are being killed. But ultimately, the virus wins, and maims the person’s immune system. This leaves the person at risk for all types of infections from bacteria and viruses that normally the immune system clears without our even knowing it.

The good news is that HIV is preventable. Blood transfusions in the US are safe, and compared to other viruses, HIV isn’t all that contagious. If you practice safe sex, take precautions when you’re around blood, and don’t share needles, you’ll greatly reduce any risks. And research is showing that if pregnant women take certain HIV drugs, they can greatly reduce the odds that the fetus will have HIV, too.

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