How does AIDS effect the immune system?

Have you ever wondered what the immune system actually is? This explains the main features of the immune system and what AIDS does to destroy it.

AIDS gets a lot of press. Most of us know that it affects the immune system, and that the name is short for Acquired Immune Deficiency Syndrome. Most of us probably also know that the immune system helps protect us from infection. But how many of us know what the immune system actually is, or how it works?

The fact is, the immune system is no one thing. You can't pinpoint any specific part of the body and say "That's the immune system." It is so complex, that no body of science yet fully understands exactly how it all works. So let's go through the key points of what we do know, in order to gain a better understanding of this strange condition called AIDS.

The biggest organ of your immune system is your skin. Does that surprise you? Remember that the microbes that cause infection, whether they are bacteria, viruses or fungi, are all invisible to the human eye. So although we are surrounded by millions of them every day, we can't see them. Your skin helps prevent all those microbes from gaining access to your body. Think of it as fortress walls with electric fencing – that's the immune function of your skin. Your mucous membranes also form part of this fortress that keeps the microbes out.

Of course, no fortress is impenetrable. You may have a cut on your skin, or a wound. And of course, there are doorways into your body through your nose and mouth. Fortunately, these have little burglar alarms that alert the immune system when an intruder tries to make an uninvited entry.

The immune system, of course, has to be very smart. It has to know which cells belong to the body, and which don't. It does this by looking for little markers called antigens. Think of antigens as being little uniforms. All the cells in your body wear the same uniform, even though they may have different functions. Microbes coming into the body have their own uniforms, which are usually very different. Maybe they're even dressed in civvies, who knows.

Guard cells patrol the body all the time. There are small, quick-footed, lightly armed guards called microphages ("little eaters") which patrol the bloodstream. There are also bigger, slower, better-armed guards called macrophages ("big eaters") which hang around the spleen and lymph nodes, waiting to be called. They also patrol the body tissues.

Microphages are cells with powerful digestive enzymes and antibacterial substances. When a microphage sees a foreign uniform, it rushes over with a baton, bangs the microbe over the head and then gobbles it up to get rid of it. They are always first at the scene of the crime, and call for reinforcements from more powerful immune cells. It's quite possible that the microbes may be more powerful than the microphages, in which case it's the microphage that gets shot before it can wield the baton. The macrophages arrive afterwards, to collect the evidence and destroy any microbes that may have escaped the microphage guards.

Both microphages and macrophages come from the bone marrow. But the bone marrow also makes stem cells, which move to different parts of the body. These include other important immune organs like the thymus gland, spleen, lymph nodes, tonsils, appendix and Peyer's patches in the intestinal wall. Once settled into the organ, the stem cells produce white blood cells called lymphocytes.

Cells produce more cells by dividing into two cells. The daughter cells divide again, and so the process continues. The stem cells that go into the thymus gland divide very quickly, to produce a large number of daughter cells, most of which don't survive. Those that do, then leave the thymus gland and travel between the other immune organs at will. But they never return to the thymus.

These particular cells, formed in the thymus gland, are called T-lymphocytes or T-cells. And they learn special skills in the thymus gland which are very useful in fighting off foreign microbes.

When a T-cell reaches the scene of a crime, three things can happen. The foreign microbe can either inactivate or kill the T-cell. If it kills all the T-cells, the body can't see the foreign uniform any more and the microbe can go where it likes.

The T-cells can also start dividing. This means there are more cells that recognise the foreign uniform, which in turn means that if any more of these uniforms try and get into the body, the body can respond quicker and better.

The third thing the T-cell can do is release cytokines. This is a bit like casting a magic spell which makes the macrophages in the area stronger and more powerful, so that they can hit the microbes faster and have more chance of devouring them.

T-cells make up about 70% of the lymphocytes in the body. The rest are B-lymphocytes, which never go to the thymus gland. They don't travel as easily as T-cells, and they have different skills, including the ability to make antibodies which inactivate foreign particles.

HIV targets mainly the T-cells, specifically those which have a special receptor called a CD4 receptor. The virus attaches onto that receptor so that it can take over the inside of the cell. So we have a virus which looks – and acts like – a CD4 T-cell.

During the first stage of infection with HIV, the virus will kill a number of T-cells as it infects them. The immune system will react to the infection, and the body will display all the symptoms of an immune response such as fever, headache, tender lymph nodes, and generally feeling unwell.

However, once the B-cells have formed antibodies, the spread of infection stabilises. The symptoms disappear, and nothing seems to happen for a few months to several years.

But think what happens in the meantime. The virus has not been destroyed, just stabilised. There are antibodies, but not enough. The virus can continue to spread, although at a much slower rate.

Now think what happens when a microbe breaches the skin and enters the bloodstream. The immune systems sends the T-cells to fight the new microbe. But some of these T-cells are infected with HIV. They've lost their magic spells, and can either die from a microbe attack, or start dividing to make more T-cells. Except that they start making more virus-infected T-cells, which will eventually succumb to the virus. As more and more T-cells die, the body is less able to recognise foreign uniforms.

As time passes, the number of CD4 T-cells slowly drops, and the amount of virus slowly increases until a critical level is reached. Then the number of CD4 T-cells plummets, and the amount of virus in the bloodstream shoots up. Without the T-cells, the immune system becomes virtually useless. The foreign uniforms can march in unchallenged. Even an army of 98 lb. weaklings can come in and create havoc. These diseases – caused by everyday, normally harmless microbes – are the ones that usually define the beginning of AIDS.

Scientists have now developed tests to measure the amount of CD4 T-cells and the amount of virus in the body. When the T-cells get too low, or the virus gets too high, they can adjust the HIV treatment to restore a measure of law and order in the body. When pharmaceutical companies test their new anti-HIV drugs, they measure the T-cells and amount of virus in order to measure exactly how effective their drug is.

As a result of understanding the immune system better, people infected with HIV are able to live longer, healthier, productive lives. New research is leading scientists to consider the possibilities of cell transplants. Perhaps one day, our understanding will grow to the point that we know how to rebuild a defective immune system completely.