Dendritic cells- the first responders. They present germs to the immune system.
What happens when the body sees a germ?
The lymphatic system includes the spleen, the thymus, and a network of lymph veins that carry white blood cell fluid through the body and into the bloodstream. White blood cell fluid leaks out of blood veins around our glands and organs, and lymph nodes can be found under the jaw, under the arms, in the gut and the groin.
When macrophages or dendritic cells see a germ, they bring it to the lymph nodes -- often, when we are fighting off an infection, or coming down with something, they will be swollen and sore. The lymph nodes are a major hub of immune system activity. The germ is trapped in the lymph node and held so that passing T4 cells can look it over and see if they recognize it.
The T4 cell sends signals to the rest of the immune system and coordinates the response to germs — you could think of a T4 cell like a record producer or orchestra conductor.
Each T4 cell is made to recognize a particular germ. When the T4 cell "sees" that germ, it starts to communicate with other immune system cells by sending chemical messages called chemokines and cytokines.
It also starts rapidly copying itself so that it can keep helping the immune system make its music.
A special group of T8 cells, called "killer" cells" or cytotoxic lymphocytes, will respond to messages from the T4 "helper" cells by killing infected cells.
Both T4 and T8 cells can "remember" certain infections, and will be prepared to fight off infections that they have seen before.
The T4 and T8 cells make up what is called the cellular immune response. While the cellular immune response is happening, the B cells of the immune system are also responding by producing antibodies.
Antibodies are small molecules shaped like the letter Y. After the body has responded to a germ by making antibodies to that germ, antibodies seek out germs and stick to them. The sticky antibody will either disable the germ or mark it for other immune system cells to destroy. This antibody response is called humoral immunity.
Both types of immune response have "memory". If a person becomes infected with hepatitis A (or vaccinated against it) they develop antibodies to hepatitis A. These antibodies protect a person from getting hepatitis A again.
A 'flu shot will help your body develop antibodies to a particular type, or strain, of 'flu. That one 'flu shot won't be able to generate antibodies to all of the different types of 'flu, so it won't protect a person against getting sick from the 'flu if they get exposed to a different strain. Each year people who track diseases make a guess-ti-mate about which strain of 'flu will be going around. Usually, they are right.
What happens with HIV?
When a person first gets infected with HIV, they usually feel sick — swollen glands, fever, sore throat, aches and pains, stomach pain --- this is because they have a large amount of virus in their blood. Usually, this is when people have the most HIV in their blood. During these first few weeks of acute HIV infection, the immune system is trying to get rid of as much HIV as it can - - macrophages and dendritic cells bring as much HIV as they can into the lymph nodes. Once HIV gets into the lymph nodes, it begins to infect T4 cells.
Before we knew how the immune system worked, we thought HIV had a latent period — where it was just resting in a person's body. This turned out to be wrong — HIV was busy in the lymphatic system. Samples of tissue from the lymph nodes of people living with HIV were full of virus, which was busy making copies of itself.
Over time, people with HIV begin to lose T4 cells. This doesn't happen overnight‹each day, a person's body makes about 1 billion new T4 cells. But at the same time that a person's body is making more T4 cells, HIV is using those T4 cells to make more copies of itself, and many of the T4 cells die or don't work properly. As a person starts losing T4 cells, the amount of HIV in their body usually goes up.
While all of this is going on, the body continues to fight against HIV.
How does the body fight against HIV?
Each of our bodies has both a cellular and a humoral response to HIV. We all produce antibodies to HIV, although they aren't successful at getting rid of HIV. We also have special squads of T4 cells that recognize and respond to HIV, but HIV enters the very T4 cells that come to fend HIV off and warn the rest of the immune system. After entering T4 cells, HIV hijacks the T4 cells and uses them to make more HIV. Over time, the infected T4 cells either stop working or die.
Over time, people with HIV begin to lose T4 cells. This doesn't happen overnight‹each day, a person's body makes about 1 billion new T4 cells. But at the same time that a person's body is making more T4 cells, HIV is using those T4 cells to make more copies of itself.
Source: Prepared in 2002 for the International Treatment Preparedness materials working group by Tracy Swan, US AIDS treatment advocate (tracyswan9@aol.com)
