Understanding the Immune System

The immune system is made up of cells and organs that protect your body from outside invaders such as bacteria, viruses, fungi, and parasites (germs) that can cause infection, disease, and even death. The immune system also gets rid of abnormal cancerous cells that are growing out of control. When functioning properly, it fights off infection and keeps you healthy. But when it malfunctions, germs that enter the body can more easily cause disease or death. Some important components of the immune system are:

• Dendritic cells and macrophages
• – T cells
• – Killer T cells
• – Natural killer cells
• – B cells
• – Neutrophils

Dendritic Cells and Macrophages

Dendritic cells and macrophages are the immune system’s first line of defense. Dendritic cells are found mostly in the skin and mucous membranes that protect the openings of the body (e.g., the mouth and throat). These cells capture and carry invaders to the lymph nodes or spleen. Macrophages (their name comes from Latin and means â big eaters ) protect different organs, including the intestines, lungs, liver, and brain.

These two types of white blood cells are known as scavengers. They engulf (eat) foreign invaders, break them apart, and display pieces of the intruders known as antigens on their surfaces. They also produce chemical messengers (known as cytokines) that instruct other immune cells to go into action.

T Cells

Once antigens are processed and displayed on the surface of macrophages, they can be recognized by helper T cells (also known as CD4 cells). When CD4 cells â see  the antigens displayed, they get busy and put the word out to other immune system cells. In other words, these cells coordinate and direct the activity of other types of immune cells such as killer T cells, B cells, and macrophages calling them into action to fight the intruder. CD4 cells produce many different cytokines in order to communicate effectively with other immune system cells.

Killer T cells directly attack and destroy (kill) cells infected by viruses as well as abnormal cancerous cells. Yet another type of T cell, called suppressor T cells, calls off the immune system attack once the invader is conquered. (This is to make sure the killer cells don’t go overboard, and relax once their job is done.) Both killer T cells and suppressor T cells are also known as CD8 cells.

Natural Killer Cells

Natural killer, or NK, cells also engulf and destroy infected cells and abnormal cancerous cells.

B Cells and Antibodies

B cells are another type of immune cell that is activated by CD4 cells. When a B cell recognizes an antigen, it goes into action and produces antibodies (also called immunoglobulins). Antibodies are proteins that attach to antigens like a key fits a lock. Each antibody matches a specific antigen. When an antibody matches up with an antigen, it has in essence marked the intruders for destruction by scavenger immune cells. Antibodies also activate a complex chemical chain reaction, called the complement system. This system ™s purpose is to destroy bacteria, which it essentially does by punching holes in bacterial membranes (or thin walls), which kills them.

When you are exposed to a pathogen for the first time, it usually takes awhile (several weeks to a few months) for your body to produce antibodies to fight it. But if you were exposed to a germ in the past, you will usually still have some B cells (called memory cells) lingering in your body that recognize the repeat invader. This allows the immune system to go into action right away. This also is why people get some diseases, such as chickenpox or measles, only once. And, this is the basis for how vaccines work they cause your immune system to produce antibodies even though you have not actually had the disease. This is why the expression â vaccinated against  some disease (e.g., smallpox) is used.

Neutrophils

Neutrophils are another type of white blood cell. They are made in the bone marrow. When they are needed to fight infection, they leave the marrow and travel anywhere in the body to fight it. These cells are your main defense against bacteria. They eat bacteria and produce toxic chemicals that destroy them.

Key Organs of the Immune System

The immune system has special organs, called lymph organs, that serve as a home base for all of these white blood cells. They are spread throughout the body. Lymph organs include the bone marrow and the thymus, as well as the lymph nodes, spleen, tonsils and adenoids, the appendix, and clumps of tissue in the small intestine known as Peyer’s patches. (Some people would also consider the blood and vessels that carry these blood cells to and from the other structures to be lymph organs.)

Lymph nodes are located along the so-called lymphatic routes. There are nodes, or clusters, in the neck, armpits, abdomen, and groin. Each lymph node contains B cells, and T cells, and other cells, ready to fight invaders.

The spleen is a very important organ for a healthy immune system. It is about the size of a fist, and it is located at the upper left of the abdomen. One of its most important roles in the immune defense it to help the body weed out and discard worn-out white blood cells. It also houses various white blood cells, waiting for instruction to go out and fight infection.

The lymphatic vessels carry lymph, which is a clear fluid that â bathes  the body’s tissues, helping to clean out invaders or germs. The vessels carry the fluid to the lymph nodes, which can sort out the antigens in order to begin the fight against them.

HIV and the Immune System

In HIV+ people, the virus attacks the CD4 cells that coordinate the immune response. This causes the CD4 cells to lose the ability to communicate with the rest of the immune system. And without CD4 cells organizing the rest of the immune system, important immune cells don ™t know which invaders need to be removed from the body. When this coordination breaks down, people are at risk for opportunistic infections (OIs) and cancers that usually do not harm people with healthy immune systems.

HIV can also infect macrophages and other immune cells. Your immune system recognizes and produces antibodies to HIV, but antibodies alone are not enough to eliminate the virus. This is partly because HIV mutates so rapidly that it can change faster than the immune system can respond to it.

Other problems may result from suppression of the bone marrow, which can occur as a side effect of certain HIV drugs like AZT (Retrovir). The bone marrow is where immune cells are produced, so when it ™s suppressed, you may have lower numbers of immune cells available, which again may cause you to be a little more vulnerable to infections.

Effective combination HIV treatment can stop the virus from replicating (making copies of itself) and infecting more CD4 cells. Since CD4 cells are key to a healthy immune response, this can give your immune system a fighting chance to replenish its supply of CD4 cells and to defend itself (you!) against opportunistic infections. Researchers are also studying new HIV therapies that they hope will help repair the damaged immune system and restore lost immune function.

1. McMichael, Andrew. (1996). How HIV fools the immune system. Medical Research Council News, London. Retrieved July 2003 from http://www.tulane.edu/~dmsander/WWW/335/McMichael/McMichael.html.

2. Merck Manuel. Biology of the immune system: Retrieved July 2003 from http://www.merck.com/mmhe/index.html.

3. National Institutes of Health. (1993). The immune system how it works. National Institutes of Health Publication no. 96. 3229.

4. Project Inform. (2002). HIV vaccines and your immune system: Retrieved July 2003 from http://www.projinf.org/fs/vaccines.html.

5. Project Inform. (2000). Immune therapy: In brief: Retrieved July 2003 from http://www.projinf.org/fs/ibt.html.

6. Schindler, L. et. al. (2003). Understanding the immune system. National Cancer Institute ™s Science Behind the News.

7. Seattle Treatment Education Project. (1993). The immune system: An overview: Retrieved July 2003 from http://www.thebody.com/step/immune.html.