Tour the Immune System

An Organ-by-Organ Introduction to the Immune System

The organs of the immune system protect your body. Let's look at each organ and learn what it does.

The immune system is a collection of organs, cells and specialized tissue that work together to defend your body against harmful bacteria and viruses (pathogens). One of the most fascinating abilities of a properly functioning immune system is that, while defending against dangerous pathogens, it can determine between what is supposed to be in the body and what is foreign and should be attacked. The picture above illustrates some of the major parts, but actually some are not shown. These other important components are your tonsils, appendix, heart and stomach which all function to make this defense system work exceptionally well. As we begin our tour of the system, let's start in a place you might not expect: the blood.

It's Crowded in Your Blood

Blood vessel with blood cells
Blood vessel with blood cells. Getty Images/KATERYNA KON/SCIENCE PHOTO LIBRARY

Everyone knows that blood goes with the heart. What's blood got to do with the immune system?

Blood is usually associated with the heart and blood vessels which are part of the circulatory system. As it circulates throughout the body, red blood cells carry oxygen to all parts of your body and they remove carbon dioxide. However, did you know that the blood has more than just one type of cell?

There are many components in the blood and each has a different function. The most abundant are red blood cells that carry oxygen. There are also little cell-like pieces called platelets that are essential in blood clotting. One of the most fascinating components of blood, and the reason why blood is featured on a tour of the immune system, are the white blood cells. These can be further divided into five types of cells which defend the body against bacteria, viruses and parasites. The blood cells are suspended in plasma, which consists mostly of water with clotting factors, cell nutrients, sugar, and hormones.

Make No Bones About It, Marrow Is Important

Internal Anatomy of Bone
Internal anatomy of bone and marrow. Getty Images/Science Picture Co

Marrow is a yellow-white tissue in the center of many bones. It's the location of a type of stem cell (specifically, the pluripotential hemopoietic stem cell), from which come the many types of blood cells. It's a marvelous biological feat that all the cells in our blood -- red, white and the platelets -- come from one type of cell that develops into such a variety of results.

A Closer Look at Blood Cells

red and white blood cells.
red and white blood cells. Getty Images/KATERYNA KON/SCIENCE PHOTO LIBRARY

Is the only thing you know about blood is that it's red? Your blood is actually made up of seven types of cells, and all of them are important to your health.

Here are drawings of the cells in your blood. Let's look at them more closely, but especially focus on the white blood cells which are tremendously important to your immune system.

The familiar red blood cell, pictured in the lower right, is formally called the erythrocyte. It was once a living cell, but by the time it gets to your bloodstream, it's only a "biological box" that can carry large amounts of oxygen.

On the lower, left are the platelets. When you have a cut, the platelets bind together and form a blood clot that stops the bleeding.

The white blood cells are a collection of five types of cells that patrol the blood stream and tissues of the body looking for bacteria, viruses and parasites. Together they are known as white blood cells or leukocytes. The most common type of leukocyte is the neutrophil and is the body's front line of defense against harmful bacteria. You have probably seen evidence of neutrophils if you've ever had a cut that got infected. Surrounding the infection is usually a fluid called "pus" which mostly contains the remains of dead neutrophils.

Monocytes will patrol the bloodstream for a little while, but soon develop into macrophages that can actually "eat" bacteria in the body that isn't supposed to be there. Because of this, macrophages are large and able to engulf the invader.

are especially valuable as fighters of parasites. Because of this association, doctors may suspect a parasitic infection if you have a blood test that shows a higher number of eosinophils in the blood than normal.

are the least common white blood cell. In addition to fighting bacteria, they are involved in releasing histamine, a biochemical that ultimately leads to an increase in swelling. While histamine can have some unpleasant side effects, it's a necessary part of the immune response.

Not pictured are , which are the second most common type of leukocytes. Lymphocytes are commonly found in the blood, but also in the lymphatic system. They develop into either B-lymphocytes or T-lymphocytes and have many functions in fighting bacteria and viral infections.

Edited by Richard N. Fogoros, MD

Macrophages: Nature's Garbage Disposal

Macrophage white blood cell,
Macrophage white blood cell. Getty Images/ROGER HARRIS/SCIENCE PHOTO LIBRARY

Wouldn't it be great to have cells that just travel around in your blood and eat harmful viruses and bacteria?

Remember the white blood cell called the monocyte? At some point in its development, it becomes a macrophage, which is Greek for "Large Eater." It's kind of like a powerful garbage disposal that is useful to our immune system because it can actually eat (the process of phagocytosis) foreign invaders, whether microbial or not. Here is a drawing of a phagocyte (macrophage) preparing to engulf an entire bacterium. Once the phagocyte surrounds the bacterium, it will destroy it by "digesting" it. Macrophages patrol the body looking for anything it can "eat." However, macrophages are particularly on the lookout for anything marked with a "special sign." Other white blood cells, especially the lymphocytes, attach antibodies to foreign microbes which effectively identifies the germ as something bad that needs to be destroyed.

Lymph: Kind of Like Blood Except It's Not

Your body has two circulatory systems. One is for blood and the other one is for lymph.

You probably know about the body's extensive network of arteries and veins. Maybe less familiar is the distribution of another network of vessels that are similar to veins, but not as extensive. Instead of transporting blood, they carry a clear fluid called lymph (pronounced "limf") that is similar to plasma (the liquid part of blood). As nutrients seep from the blood into the tissues, the lymphatic system collects this fluid (which is now called lymph) along with any associated wastes and returns it to the blood. Lymph is a great place to fight microbes and it's filled with lymphocytes and other white blood cells. Before the lymph gets recycled into the bloodstream, lymphocytes work to identify any harmful microbes so they can be destroyed.

Lymph Nodes: Filter Stations for Lymph

Along the lymphatic system are collections of specialized tissue called lymph nodes. These are places where large amounts of lymphocytes stay, which can attack any microbes found in the lymph as it filters through the lymph node.

What's the Spleen? Here's an Ex-Spleen-ation

Colored CT scan of human spleen, stomach & liver
Colored CT scan of human spleen (in pink), stomach & liver. Getty Images/ ALFRED PASIEKA/SCIENCE PHOTO LIBRARY

Ask someone to locate their spleen, and you'll probably get a blank stare in return. That's because most people don't think much about their spleen, and it's rarely, if ever, something they talk about. The spleen is an oval-shaped organ located in the upper-left side of your belly, between the stomach and diaphragm. It's where old, worn-out blood cells go to be recycled. However, since it's also where bacteria are filtered out of the blood, it's the largest single organ of the immune system. Oddly enough, the spleen is a non-essential organ. You can actually live without it, but you'll be more likely to get certain types of bacterial infections.

Thymus, Where Art Thou?

Male lymphatic system,
Male lymphatic system (thymus in yellow). Getty Images/PIXOLOGICSTUDIO/SCIENCE PHOTO LIBRARY

The thymus, located between your lungs and behind your sternum, is where T-lymphocytes develop. Though these white blood cells begin from stem cells in bone marrow, they further specialize into T-lymphocytes here. The "T" actually stands for "thymus" to reflect this origin. The thymus is an interesting organ: Although it's active in young people and teenagers, it shrinks and becomes much less active in adults.

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