The immune system is a system of biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue. In many species, the immune system can be classified into subsystems, such as the innate immune system versus the adaptive immune system, or humoral immunity versus cell-mediated immunity.
Pathogens can rapidly evolve and adapt, and thereby avoid detection and neutralization by the immune system; however, multiple defense mechanisms have also evolved to recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess a rudimentary immune system, in the form of enzymes that protect against bacteriophage infections. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and insects. These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system. Jawed vertebrates, including humans, have even more sophisticated defense mechanisms, including the ability to adapt over time to recognize specific pathogens more efficiently. Adaptive (or acquired) immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination.
Disorders of the immune system can result in autoimmune diseases, inflammatory diseases and cancer. Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections. In humans, immunodeficiency can either be the result of a genetic disease such as severe combined immunodeficiency, acquired conditions such as HIV/AIDS, or the use of immunosuppressive medication. In contrast, autoimmunity results from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus. Immunology covers the study of all aspects of the immune system.
The immune system is a network of cells, tissues, and organs that work together to protect the body from infection. The human body provides an ideal environment for many microbes, such as viruses, bacteria, fungi, and parasites, and the immune system prevents and limits their entry and growth to maintain optimal health.
Although scientists have learned much about the immune system, they continue to study how the body targets invading microbes, infected cells, and tumors while ignoring healthy tissues. New technologies for identifying individual immune cells help scientists determine which cells trigger an immune response under various circumstances. Improvements in microscopy also allow for observations of living immune cells as they interact within lymph nodes and other body tissues.
In addition, scientists are rapidly unraveling the genetic blueprints that direct the human immune response, as well as those that dictate the biology of viruses, bacteria, fungi, and parasites. The combination of new technology and expanded genetic information promises to reveal more about how the body protects itself from disease. In turn, scientists can use this information to develop new strategies for the prevention and treatment of infectious and immune-mediated diseases.
What can you do?
On the whole, your immune system does a remarkable job of defending you against disease-causing microorganisms. But sometimes it fails: A germ invades successfully and makes you sick. Is it possible to intervene in this process and make your immune system stronger? What if you improve your diet? Take certain vitamins or herbal preparations? Make other lifestyle changes in the hope of producing a near-perfect immune response?
The idea of boosting your immunity is enticing, but the ability to do so has proved elusive for several reasons. The immune system is precisely that — a system, not a single entity. To function well, it requires balance and harmony. There is still much that researchers don’t know about the intricacies and interconnectedness of the immune response. For now, there are no scientifically proven direct links between lifestyle and enhanced immune function.
But that doesn’t mean the effects of lifestyle on the immune system aren’t intriguing and shouldn’t be studied. Quite a number of researchers are exploring the effects of diet, exercise, age, psychological stress, herbal supplements, and other factors on the immune response, both in animals and in humans. Although interesting results are emerging, thus far they can only be considered preliminary. That’s because researchers are still trying to understand how the immune system works and how to interpret measurements of immune function. The following sections summarize some of the most active areas of research into these topics. In the meantime, general healthy-living strategies are a good way to start giving your immune system the upper hand.
Immunity in action. A healthy immune system can defeat invading pathogens as shown above, where two bacteria that cause gonorrhea are no match for the large phagocyte, called a neutrophil, that engulfs and kills them (see arrows).
Photos courtesy of Michael N. Starnbach, Ph.D., Harvard Medical School
Adopt healthy-living strategies
Your first line of defense is to choose a healthy lifestyle. Following general good-health guidelines is the single best step you can take toward keeping your immune system strong and healthy. Every part of your body, including your immune system, functions better when protected from environmental assaults and bolstered by healthy-living strategies such as these:
- Don’t smoke.
- Eat a diet high in fruits, vegetables, and whole grains, and low in saturated fat.
- Exercise regularly.
- Maintain a healthy weight.
- Control your blood pressure.
- If you drink alcohol, drink only in moderation.
- Get adequate sleep.
- Take steps to avoid infection, such as washing your hands frequently and cooking meats thoroughly.
- Get regular medical screening tests for people in your age group and risk category.
Many products on store shelves claim to boost or support immunity. But the concept of boosting immunity actually makes little sense scientifically. In fact, boosting the number of cells in your body — immune cells or others — is not necessarily a good thing. For example, athletes who engage in “blood doping” — pumping blood into their systems to boost their number of blood cells and enhance their performance — run the risk of strokes.
Attempting to boost the cells of the immune system is especially complicated because there are so many different kinds of cells in the immune system that respond to so many different microbes in so many ways. Which cells should you boost, and to what number? So far, scientists do not know the answer. What is known is that the body is continually generating immune cells. Certainly it produces many more lymphocytes than it can possibly use. The extra cells remove themselves through a natural process of cell death called apoptosis — some before they see any action, some after the battle is won. No one knows how many cells or what kinds of cells the immune system needs to function at its optimum level.
Scientists do know more about the low end of the scale. When the number of T cells in an HIV/AIDS patient drops below a certain level, the patient gets sick because the immune system doesn’t have enough T cells to fight off infection. So there is a bottom number below which the immune system can’t do its job. But how many T cells is comfortably enough, and beyond that point, is more better? We don’t know.
Many researchers are trying to explore the effects of a variety of factors — from foods and herbal supplements to exercise and stress — on immunity. Some take measures of certain blood components like lymphocytes or cytokines. But thus far, no one really knows what these measurements mean in terms of your body’s ability to fight disease. They provide a way of detecting whether something is going on, but science isn’t yet sufficiently advanced to understand how this translates into success in warding off disease.
A different scientific approach looks at the effect of certain lifestyle modifications on the incidence of disease. If a study shows significantly less disease, researchers consider whether the immune system is being strengthened in some way. Based on these studies, there is now evidence that even though we may not be able to prove a direct link between a certain lifestyle and an improved immune response, we can at least show that some links are likely.
Inside your body there is an amazing protection mechanism called the immune system. It is designed to defend you against millions of bacteria, microbes, viruses, toxins and parasites that would love to invade your body. To understand the power of the immune system, all that you have to do is look at what happens to anything once it dies. That sounds gross, but it does show you something very important about your immune system.
When something dies, its immune system (along with everything else) shuts down. In a matter of hours, the body is invaded by all sorts of bacteria, microbes, parasites... None of these things are able to get in when your immune system is working, but the moment your immune system stops the door is wide open. Once you die it only takes a few weeks for these organisms to completely dismantle your body and carry it away, until all that's left is a skeleton. Obviously your immune system is doing something amazing to keep all of that dismantling from happening when you are alive.
The immune system is complex, intricate and interesting. And there are at least two good reasons for you to know more about it. First, it is just plain fascinating to understand where things like fevers, hives, inflammation, etc., come from when they happen inside your own body. You also hear a lot about the immune system in the news as new parts of it are understood and new drugs come on the market -- knowing about the immune system makes these news stories understandable. In this article, we will take a look at how your immune system works so that you can understand what it is doing for you each day, as well as what it is not.
Basics of the Immune System
Let's start at the beginning. What does it mean when someone says "I feel sick today?" What is a disease? By understanding the different kinds of diseases it is possible to see what types of disease the immune system helps you handle.
When you "get sick", your body is not able to work properly or at its full potential. There are many different ways for you to get sick -- here are some of them:
- Mechanical damage - If you break a bone or tear a ligament you will be "sick" (your body will not be able to perform at its full potential). The cause of the problem is something that is easy to understand and visible.
- Vitamin or mineral deficiency - If you do not get enough vitamin D your body is not able to metabolize calcium properly and you get a disease known as rickets. People with rickets have weak bones (they break easily) and deformities because the bones do not grow properly. If you do not get enough vitamin C you get scurvy, which causes swollen and bleeding gums, swollen joints and bruising. If you do not get enough iron you get anemia, and so on.
- Organ degradation - In some cases an organ is damaged or weakened. For example, one form of "heart disease" is caused by obstructions in the blood vessels leading to the heart muscle, so that the heart does not get enough blood. One form of "liver disease", known as Cirrhosis, is caused by damage to liver cells (drinking too much alcohol is one cause).
- Genetic disease - A genetic disease is caused by a coding error in the DNA. The coding error causes too much or too little of certain proteins to be made, and that causes problems at the cellular level. For example, albinism is caused by a lack of an enzyme called tyrosinase. That missing enzyme means that the body cannot manufacture melanin, the natural pigment that causes hair color, eye color and tanning. Because of the lack of melanin, people with this genetic problem are extremely sensitive to the UV rays in sunlight.
- Cancer - Occasionally a cell will change in a way that causes it to reproduce uncontrollably. For example, when cells in the skin called melanocytes are damaged by ultraviolet radiation in sunlight they change in a characteristic way into a cancerous form of cell. The visible cancer that appears as a tumor on the skin is called melanoma.
Components of the Immune System
One of the funny things about the immune system is that it has been working inside your body your entire life but you probably know almost nothing about it. For example, you are probably aware that inside your chest you have an organ called a "heart". Who doesn't know that they have a heart? You have probably also heard about the fact that you have lungs and a liver and kidneys. But have you even heard about your thymus? There's a good chance you don't even know that you have a thymus, yet its there in your chest right next to your heart. There are many other parts of the immune system that are just as obscure, so let's start by learning about all of the parts.
The most obvious part of the immune system is what you can see. For example, skin is an important part of the immune system. It acts as a primary boundary between germs and your body. Part of your skin's job is to act as a barrier in much the same way we use plastic wrap to protect food. Skin is tough and generally impermeable to bacteria and viruses. The epidermis contains special cells called Langerhans cells (mixed in with the melanocytes in the basal layer) that are an important early-warning component in the immune system. The skin also secretes antibacterial substances. These substances explain why you don't wake up in the morning with a layer of mold growing on your skin -- most bacteria and spores that land on the skin die quickly.
Your nose, mouth and eyes are also obvious entry points for germs. Tears and mucus contain an enzyme (lysozyme) that breaks down the cell wall of many bacteria. Saliva is also anti-bacterial. Since the nasal passage and lungs are coated in mucus, many germs not killed immediately are trapped in the mucus and soon swallowed. Mast cells also line the nasal passages, throat, lungs and skin. Any bacteria or virus that wants to gain entry to your body must first make it past these defenses.
Once inside the body, a germ deals with the immune system at a different level. The major components of the immune system are:
- Lymph system
- Bone marrow
- White blood cells
- Complement system