How does HIV enter the body?
This article discusses the various ways the sneaky HIV virus dodges and avoids your natural defences to cause an infection. It is a war story. Your mucosal membranes are like the beaches of Omaha lined with fences, trenches, mines and guns. Big guns. But the HIV virus are many, and they are determine to breach the beach. This is how it plays out.
HIV is a virus. That means it is a tiny microscopic particle. Imagine it is a really small ball. Inside this ball, is an even tinier strand of genetic material called RNA. Like a thread inside the ball.
The objective of HIV is to get this tiny bit of RNA into your T-cells. Once it reaches the T-Cells, it will combine with the DNA of the T-Cell and from there, it is unstoppable and incurable.
The problem for HIV is the T-Cells are located deep inside the body. To get from the outside of the body all the way to these T-cells deep inside, there are lots of hurdles it must overcome. These are the body’s defences.
The first few layers of skin are made of tough layers of dead cells called keratin. They might not look like much to you but to the HIV virus it looks like the Great Wall of China and then some. In short, HIV cannot get through intact skin.
However, during intercourse, HIV does not only come into contact with skin. The inner lining of the vaginal wall, the inner lining of the mouth, the tube inside the penis and deep in the rectum are not covered by skin. They are covered by Mucosa. The mucosa itself is not defenseless, it has tight junctions that prevent the HIV virus from getting through.
This is the minefield of the body. They are basically proteins that sit around to kill any microbe that comes its way. Yes, even and especially viruses. HIV viruses.
These are the sentries of the body. They float around looking for any and all viruses. Once it spots a virus, it either goes in for the kill or sounds the alarm so that other immune cells respond to kill or eat up the viruses.
The skin is tough. We know that. We also know the HIV virus cannot penetrate skin. But wait. The HIV virus found in semen and vaginal fluid does not only come into contact with the skin during intercourse.
It also comes into contact with the area inside the vagina and inside the urinary tube of the penis. These wet areas are not skin. They are made of cells different from the skin and is called mucosa.
Mucosa unfortunately does not have the thick layer of dead cells to protect it like the skin.
The HIV virus can squeeze its way in between the mucosal cells into the sub-mucosal area. Imagine you have gaps in your wall defences and the enemy has squeezed its way between these gaps and now are in the compound of your fortress.
But fear not! You still have your minefields and sentries. Immediately, they start killing the HIV virus.
The virus knows that in order to survive, it has to find a place to hide. And what better place to hide than inside one of your own cells?
The problem is the HIV virus cannot go inside any cell. Each cell has a cell membrane that itself acts as a wall against HIV. But the sneaky HIV virus has one key (gp120) that can open one particular lock (CD4, CCR5) that is found only on a very few cells in the area underneath the mucosa.
Desperately the HIV virus looks for such a cell. More often than not, they are all killed before they manage to hide inside one of these cells.
But every so often, the HIV virus gets lucky and finds a cell with the correct lock. It fits its key into the lock and hides inside the cell.
The Sneak Attacks
Like every good enemy, the HIV virus knows that a full frontal assault like that will not very likely succeed and probably cause lots of casualties. So the HIV virus has come up with some tactics to catch the body unawares.
Dendritic cells are the guardians of the mucosa. They have long arms and provide defence for the body. The problem is, they are also one of the cells that have the special lock (CD4, CCR5) that the HIV virus has the key (gp120)to. The other problem is sometimes these Dendritic cells stretch out their arms too far until they are on the mucosal surface i.e. outside the protective wall. This provides a portal for HIV to enter the cell even without engaging the rest of the defences.
Imagine the HIV virus scanning the wall of your fortress. Suddenly, it realizes you have parked one of your tanks outside the wall and coincidentally the HIV virus has the keys to your tank! They open the door and sneak inside. Then in the morning you drive the tank back into your fortress carrying the HIV virus along with it and helping it avoid your minefields and sentries.
Trojan Horse Leukocytes
Yes they are literally called Trojan Horse white blood cells.
The HIV virus is not just found in it’s naked form in the semen and vaginal fluid. The semen and vaginal fluid also contain white blood cells. HIV has found a way to hide inside these cells.
Your body does not recognize these white blood cells as dangerous. So will happily let them in and pass all your defences. Once inside, HIV is free to infect T-cells and cause an infection. See why it is called a Trojan Horse cell?
The end of the Battle
There can only be one of 2 outcomes. Either all the HIV viruses get killed by your body’s defences or it manages to reach the T-cells. Once the HIV viruses reaches a T-cell, it enters the cell and combines with the cell’s DNA. From there, is can force the cell to make billions more HIV viruses which will go on to infect many other T-cells. This forms what is known as the infection reservoir. Our current treatment has no way of killing HIV viruses hiding in this reservoir. So the infection remains incurable.
Fortunately, more often then not, all the HIV viruses get killed before they manage to reach the T-cells. This also explains why the risk of getting infected with HIV is small even if you had a direct sexual exposure to a HIV +ve person.
NB: This is a massive over-simplification of a very complex subject with great and detailed basic science studies done by many renown scientist. This article is in no way intended to simplify their work but more to put it in a way that can be more easily appreciated by the layman.
HIV Signs and Symptoms
About Dr. Tan
Dr. Tan graduated from the National University of Singapore in 2001. His residency was in the two largest public hospitals in Singapore; Tan Tock Seng Hospital and Singapore General Hospital.
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