HIV (AIDS)

HIV, or human immunodeficiency virus, is a virus that targets cells in the immune system.

Over time, the immune system begins to fail which is called immunodeficiency, and this increases the risk of infections and tumors that a healthy immune system would usually be able to fend off.

These complications are referred to as AIDS, or acquired immunodeficiency syndrome.

Now there are two distinct types of HIV—HIV-1 and HIV-2.

HIV-1 is the more commonly associated with AIDS in the US and worldwide, HIV-2 is more rare, and typically restricted to areas in western Africa and southern Asia.

HIV-2 is so uncommon that “HIV” almost always refers to HIV-1.

Alright HIV targets CD4+ cells, meaning cells that have this specific molecule called CD4 on their membrane. Macrophages, T-helper cells, and dendritic cells are all involved in the immune response and all have CD4 molecules; therefore they can be targeted by HIV.

The CD4 molecule helps these cells attach to and communicate with other immune cells, which is particularly important when the cells are launching attacks against foreign pathogens.

So this little molecule is pretty important for our immune system, but it’s also extremely important for HIV. HIV targets and attaches to the CD4 molecule via a protein called gp120 found on its envelope.

HIV then again uses gp120 to attach to another receptor, called a co-receptor.

HIV needs to bind to both the CD4 molecule and a coreceptor to get inside the cell.

The most common co-receptors that HIV uses are the CXCR4 co-receptor, which is found mainly on T-cells, or the CCR5 co-receptor which is found on T-cells, macrophages, monocytes, and dendritic cells.

These coreceptors are so important that some people with homogeneous genetic mutations in their CCR5 actually have resistance or immunity to HIV, since HIV can’t attach and get into the cell.

In fact, even heterozygous mutations which lead to fewer co-receptors on the cells, can make it harder for the virus to spread, and results in a slower disease progression.

For those without this mutation though, once HIV binds to CD4 and either CCR5 or CXCR4, it gains access to the cell.