Understanding the Immune System

Table of Contents

The immune system is made up of cells and organs that protect your body from outside invaders such as bacteria, viruses, fungi, and parasites (types of germs) that can cause infection and disease. The immune system also gets rid of abnormal pre-cancerous cells and cancerous cells that are growing out of control. When it works correctly, it fights off infection and keeps you healthy. However, when it does not work correctly, germs and other abnormal cells in the body can more easily cause disease.

Key Organs of the Immune System

The first line of defense against germs is your skin, the single largest organ of the body. It provides a physical barrier that keeps bacteria and viruses from entering the body. Viruses such as HIV cannot get through normal, healthy, unbroken skin. HIV can, however, get into the body through unbroken mucous membranes, which are the moist membranes of the vagina (birth canal), rectum, and urethra ('pee hole').  

Germs that get inside the body are taken care of by the internal parts of the immune system. The white blood cells that defend the body from invaders and get rid of possibly dangerous abnormal cells begin their lives in the bone marrow. Once they leave the bone marrow, they travel to the lymph organs, which serve as a home base or a staging area for mature white blood cells. There, the white blood cells await instruction to go out and fight infection.

Lymph organs are spread throughout the body and include the lymph nodes, thymus, spleen, appendix, tonsils and adenoids, and clumps of tissue in the small intestine known as Peyer's patches. Lymph nodes are located in the neck, armpits, abdomen, and groin. Each lymph node contains cells ready to fight invaders. The lymphatic vessels connect the lymph nodes and carry lymph, which is a clear fluid that "bathes" the body's tissues and helps to clean out invaders or germs.

The spleen is an 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 key roles is to filter blood and to identify and get rid of worn-out white blood cells.

Key Cells of the Immune System

Some key cells of the immune system are:

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


Neutrophils are one of the "first-responders" in the body's response to infection. Normally, you have more neutrophils than any other type of white blood cell. They circulate in the bloodstream and are your main defense against bacteria. They eat bacteria and produce toxic chemicals that destroy them. These are the cells that give pus its whitish/yellowish color.

Dendritic Cells and Macrophages

Dendritic cells are found mostly in the skin and mucous membranes that protect the openings of the body (e.g., nose, mouth, and throat). These cells capture and carry invaders to the lymph nodes or spleen. Macrophages (their name comes from Latin and means "big eaters") protect different organs, including the intestines, lungs, liver, and brain. Like dendritic cells, macrophages capture and carry invaders to the lymph organs.

These two types of white blood cells are known as scavengers. They engulf (eat) foreign invaders, break them apart, and display pieces of the germs—known as antigens (from antibody-generating) —on their surfaces. The body can then make antibodies to that specific germ, which helps the body get rid of that invader faster and remember it in the future. These cells 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 "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 cells infected by viruses as well as abnormal cells that may become cancerous. Suppressor T cells call off the immune system attack once the invader is conquered. This is to make sure the killer T cells stop killing 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 destroy infected or abnormal cells. NK cells are an important part of the early response to viral infections because they kill cells infected by virus while the body is recruiting killer T cells into action.

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). An antibody is a protein that attaches to an antigen like a key fits a lock. Each antibody matches a specific antigen. When an antibody has matched up with an antigen, it has marked the intruders for destruction by scavenger immune cells. Antibodies also activate a complex chemical reaction that destroys bacteria by punching holes in their outside surface (the bacterial membrane), which kills them.

When you are exposed to a germ for the first time, it usually takes a while (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) in your body that recognize or 'remember' the repeat invader. This allows the immune system to go into action right away. This is why people get some diseases, such as chickenpox or measles, only once. They may get chickenpox or measles germs in their bodies more than once, but their bodies remember the germs and react so quickly that they do not get sick.

This is also how vaccines work— they introduce your body to an inactive form of a particular germ and trigger your immune system to produce antibodies to that germ. The inactive form prevents you from getting sick while still allowing your body to make important memory B cells. This is why the expression "vaccinated against" some disease (e.g., smallpox) is used.

HIV and the Immune System

In people living with HIV (HIV+), 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. Without CD4 cells organizing the rest of the immune system, important immune cells do not know which invaders need to be removed from the body. When the immune response is not properly activated and organized, 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 changes or mutates faster than the immune system can respond to it.

Other problems may result from limiting the activity of the bone marrow, which can occur as a side effect of certain HIV drugs like Retrovir (zidovudine). Since the bone marrow is where immune cells are produced, if its activity is reduced you may have lower numbers of immune cells available, which again may cause you to be more vulnerable to infections.

Effective combination HIV treatment can stop the virus from multiplying (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.

Additional Resources

Select the links below for additional material related to the immune system.

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