HIV and the immune system

We know that HIV attacks the immune system, but what is a virus and why is this particular virus so unique?

What is a virus?
A virus is a very small organism. Unlike other life forms (e.g. human cells) they can't replicate themselves or build new viruses within their own cores, because they don't have the “machinery” to do so.

They are “parasites” in the sense that they need to use the biochemical facilities of living cells (such as human cells) to reproduce. Outside a living cell, a virus is nothing but inactive, lifeless and harmless chemicals. Thus, viruses need to transport their own genetic material into living cells, and to then use the “machinery” of the cells to make more copies of themselves.

Most viruses contain DNA in their cores (like all other cells), but a few viruses, called retroviruses, have RNA instead of DNA in their nuclei – the only living organisms with RNA in their nuclei. (HIV is a retrovirus because it has RNA in its core.)

A virus consists mainly of three parts:

1. A core where the genetic material or genes are housed (DNA or RNA). (The genetic material is necessary for the replication of the virus).
2. A shell or capsule protecting the core and acting as a vehicle to transfer the virus from cell to cell and from person to person. This shell is composed of protein projections (or glycoprotein projections) looking like spikes, and the virus uses these spikes to attach itself onto the specific receptors (or binding sites) of a host cell that it will infect. We can compare the interaction between the outermost proteins on the shell of the virus and the proteins of the cell’s receptor sites to lock and key. The key on the virus (projection) will fit only into a specific lock - (the receptor).
3. Some viruses (such as HIV) also have a loose envelope, covering the capsule.

What makes HIV so different from other viruses?
If all viruses use human cells to reproduce, what makes HIV so different and dangerous?

The HI virus does something that no other virus known to humankind has ever done: It directly attacks and hijacks the most important immune cells in our bodies, namely the CD4 cells (or the T helper cells) and use them to reproduce or to make more viruses.

Why are the CD4 cells so important?
The CD4 cells are important because they are the managers of our immune systems.

Or to use a war metaphor: If we picture the immune system as the body’s army, the CD4 cells are the generals of this army. Certain types of white blood cells, namely phagocytes act as “spies” and they patrol the body to look for any invading, foreign substances or organisms. When they spot an enemy - such as a virus - they warn the generals (CD4 cells) to mobilise the body’s army. The generals (CD4 cells) then order the rest of the “soldiers” of the immune system (e.g. killer T-cells, B-cells and antibodies) to take up their weapons and to fight.