How the Immune System Fights Disease
September 18, 2008
The immune system is a complex network of cells and chemicals. Its mission is to protect us against foreign organisms and substances. The cells in the immune system have the ability to recognize something as either "self" or "invader," and they try to get rid of anything that is an invader. Many different kinds of cells, and hundreds of different chemicals, must be coordinated for the immune system to function smoothly.
The immune system can mount a variety of responses to attack specific invader organisms. One of these responses is coordinated by T-helper cells (also known as T cells, T4 cells, or CD4 cells), which act as a kind of orchestra conductor. The T-helper cells tell other cells what to do when this response is triggered. We are interested in this immune response because it is the one that is most disrupted by HIV infection. As HIV succeeds in destroying more and more of these important cells, the ability to fight off other infections gradually declines. If the "coordinator" of the process, the T-helper cell, is no longer functioning, other cells in the immune system cannot perform their functions, leaving the body open to attack by opportunistic infections.
Let's look first at how the immune response coordinated by the T cells is supposed to work. Please keep in mind that we will be explaining only one of the body's immune responses.
Any infectious agent (Figure 1) that enters your body will eventually be taken up in your lymphatic system.
After displaying the agent's antigens, the macrophage will send out a message to a T-helper cell to read and recognize the antigens (Figure 4).
With HIV infection, this procedure does not work adequately. Initially, macrophages recognize the HIV, T-helper cells initiate the response, and B cells produce antibodies. However, although effective at first, the antibodies do not eliminate the infection. Although some HIV might get killed, many more viruses will actively infect T-helper cells -- the very same cells that are supposed to coordinate the defense against the virus. Infected T cells become virus factories which, if activated, will produce more copies of the virus instead of triggering the production of more antibodies against HIV.
Besides T cells, HIV is capable of infecting other cells (e.g., macrophages, B cells, and monocytes) and can cross the brain-blood barrier, infecting nervous system cells. Most immune cells cannot cross that barrier, which surrounds the brain and spinal cord, so HIV can retreat where the immune system cannot follow.
The immune system is very complex and many of its processes are still not understood. This brief explanation of the immune response coordinated by the T-helper cells will help you understand issues surrounding immune monitoring and treatment for HIV disease. Some of the tests that are used to monitor the health of HIV positive people show how well the immune system is working (e.g., T cell or CD4 cell counts), while others show the number of copies of the virus in the body (viral load). Monitoring and early treatment can be crucial in determining the course of HIV disease and making informed choices about treatment.
This article was provided by San Francisco AIDS Foundation. It is a part of the publication AIDS 101. Visit San Francisco AIDS Foundation's Web site to find out more about their activities, publications and services.