The Immune System Part II - HIV/AIDS

How many of you have heard of AIDS? Can I get a show of hands? Ok good. How about HIV? Ok. So what's the difference? I mean aren't they the same? Well, actually they are not, though you cannot have one without the other.

HIV stands for Human Immunodeficiency Virus. Therefore it is a virus. AIDS stands for Acquired Immune Deficiency Syndrome. Therefore it is a syndrome or a combination of symptoms resulting from disease or infection. HIV is the virus that causes AIDS. Thus, as you probably figured out, if one can prevent getting HIV then this will stop the spread of AIDS.

The key to understanding HIV and AIDS is knowing what an immune deficiency is and what it results in. If you recall from last week, our immune system is what protects us from foreign substances that damage our body's normal function. Normally, our immune system is fighting off microscopic bacteria, fungi, single celled organisms called protozoa, and, more rarely, viruses (collectively referred to as "germs"). These germs are unavoidable contaminations from other people, door handles, hookahs, and anything else that is contacted by many different individuals (Jordan Raney's nest of hair for example was a highly susceptible environment for these germs, though no longer since his haircut - thanks Jordan). It does this through a variety of mechanisms that include those wonderful antibody producing B cells and T cells. Last week I covered B cells (see part I) so this week I will introduce to you T cells. Then, I will discuss what happens when they malfunction or disappear and how that contributes to HIV/AIDS.

T cells, like B cells, are first formed in the bone marrow and then, like B cells, go through a maturing and selection process so that our body doesn't produce immune cells that will attack itself. Unlike B cells, much of the T cell maturation process occurs in an organ of the body right on top of our heart called the Thymus (hence "T" cells).

Also like B cells, the mature T cell is transformed during its maturation into different forms that differ in function. There are essentially two forms of mature T cells, helper or CD4 T cells and cytotoxic or CD8 T cells. Helper CD4 T cells do precisely what their name implies, they help other cells. Namely, they help B cells to produce antibodies and another cell, called a macrophage ("big eater"), to consume and degrade foreign organisms. Cytotoxic or "killer" CD8 T cells release chemicals that kill infected cells in the body. Both CD4 and CD8 T cells work in concert to control infection so the body can return to its normal function.

When a person gets infected with HIV the virus actually infects CD4 T cells. But this takes time since T cells and B cells are not activated until several days later after an infection. This is why it usually takes over a week or two to get over a flu virus, if not longer. During this time lag the immune system is constantly producing new T cells and other immune cells to fight off the infection. Also, after HIV infects an individual, it is frequently mutating into different forms. This is part of the reason why it takes such a long time before symptoms of HIV actually develop in someone who's been infected, anywhere from 2-15 years. The number of CD4 T cells infected and killed increases significantly during this lag period as the amount of virus or the viral load increases. Remember that T cells play an important role in the immune response so as their number decreases so does the body's ability to fight infection. It's when the CD4 T cell level drops below a certain number, 200, that someone can be then diagnosed with having AIDS.

It is precisely the lack of ability to fight infection that makes AIDS an immunodeficiency. And as you can imagine the lack of immune ability means there is a greater opportunity for the body to be infected and damaged. Remember when I said that your body is continually, daily fighting micoscopic organisms; well when your immune system becomes deficient your body can no longer prevent these organism from infecting you. These infections are termed "opportunistic" since they only infect when they have the opportunity. Once infected, an immunodeficient person cannot stop the infection and it will progress to present various symptoms that fall under the "syndrome" part of AIDS. Eventually the infection or group of infections will progress to a point when an individual cannot repair damaged cells and will die.

So the take home lesson is DON'T GET HIV! And if you have it, don't spread it. Obvious no? Well why does this horrible virus still persist if preventing it's spread was so easy? Why is in fact growing at a rapid rate in some areas of the world like India and Southern Africa? Why is it spreading more and more among the heterosexual population here in the US? Why am I not the leader of the free world? Hmmm . . . sounds like a future blog to me. I hope this was a little more simpler than my last blog. Let me know if it wasn't.

This marks the end of my immunology course in two weeks at medical school. The next two weeks we'll be venturing into that marvelous, dynamic machinery known as the musculoskeletal system. Being as buff as I am, I think that I would be an expert in that system already. I guess we'll find out . . .