|MadSci Network: Virology|
Thank you very much for your interesting question.
Let's start at the beginning,
No, viruses cannot sense color and smell...
And most viruses do not attack T-cells.
But! Some do! For example, HIV, the virus that causes AIDS, infects T- cells that are responsible for the control of the immune system. When the T-cells die, the immune system becomes unable to protect the body from normally unharmful bacteria, viruses and fungi.
How HIV can recognizes a T-cell is a very well known matter, but a little bit complicated. Let's start by saying that proteins on the surface of the HIV viruses recognize a specific protein on the surface of T-cell lymphocytes. The protein on the surface of the HIV virus is called gp120 and the molecule on the T-cell surface is called CD4 or T4. This is why some people say thay HIV infects CD4 cells or T4 cells.
But how does the gp120 molecule recognize the CD4 molecule?
Proteins are made from amino acids. The amino acids are like small "lego" blocks. As with "lego" blocks, they can build many things with just a limited set of "lego" blocks.
Every protein in every life form on Earth (yes, even you!) is built using only 20 different "legos" that each have different characteristics. Some proteins are 30 blocks long... other are 250 blocks long... and other are even larger (normally we should say "amino acids" instead of "blocks").
The properties of these "legos" allows them to interact with one another (in some case with one and not another), and to form very complex structures.
These structures could be compared to a "key hole", that requires a "key" that could easily fit in it. This "key" can be another protein and in that case, the "key" will "lock" in the "keyhole" and the two proteins will bind to one another.
The gp120 on the surface of the HIV is the "key" that fits into the "keyhole" provided by the CD4. When HIV encounters a T-cell that expresses CD4, it can bind strongly to it, and it will give it time to infect that cell. If it encounters a cell that does not express CD4 (for example a skin cell), it will not be able to infect it.
Most viruses use this "technique" to infect their target cells but they always use different "keys" and "keyholes".
You can go to http://www.critpath.org/aric/library/img005.htm to see a nice picture of a HIV virus binding to a T-cell.
If you are interested, go to http://www.critpath.org/aric/library/img.htm and look at some of the images on this very interesting site. If you go to their home page ( http://www.critpath.org/aric/library.htm), you can even have access to some important published articles on HIV and lots of information on this nasty virus.
If you are interested on other viruses, the definitive site to visit is All the Virology on the World-Wide-Web at Tulane University.
I hope this answer has not raised too many questions but I do hope that it has raised some in your mind...
If it did, don't hesitate to ask!
Try the links in the MadSci Library for more information on Virology.