The Immune System

The immune system is our defense system. It is a complex set of organs, white blood cells, other specialized molecules and a separate circulatory system, called the lymph system (Read about "The Lymph System") that works with the cardiovascular system (Read about "The Heart & Cardiovascular System") to move these defenders around the body to where they are needed. There are sentries in place to look for invaders (which are called antigens) and call in other resources. (Read about "Microorganisms") There are front line troops to do the fighting and there are clean-up crews to clear out the casualties and leave the battlefield, our bodies, clean. Left behind after any attack are even more sentries that keep a watch out for the bad guys from the previous attack. (Read about "Immune System Glossary")

Immune system components

Any discussion of the immune system should start with the lymph system. Lymphatic vessels circulate the lymph, which is almost colorless and carries many of the immune system fighters around the body. Lymph is familiar to most people from the term "swollen lymph nodes." The lymph nodes are spread around the body as part of the lymphatic vessels. They serve as gathering places for the fighters of the immune system and become enlarged during an illness because they are swollen with the casualties of the battle. Other parts of the immune system, according to the National Institute of Allergies and Infectious Diseases (NIAID) include:

• bone marrow
• thymus
• spleen (Read about "The Spleen")
• white blood cells also called leukocytes
• antibodies
• complement
• tonsils (Read about "Tonsils & Adenoids")
• adenoids

Tonsils and adenoids play important roles early in life but become less important as we grow older. Filtering organs such as the liver (Read about "The Liver") and the kidneys (Read about "Kidney Disease") also play a role with the immune system of clearing our bodies of infection.

The bone marrow is where those defenders, the white blood cells or leukocytes are manufactured. There are three types of leukocytes:

• lymphocytes
• monocytes
• granulocytes

Granulocytes are the foot soldiers and the most common. They attack on a broad basis wherever they are told. Granulocytes are further subdivided into neutrophils, eosinophils and basophils. Monocytes mature into what are called macrophages. These are the largest of the defenders. They can surround and destroy antigens as well as clean up after other defenders. Macrophages are usually stationed around the body in various tissues and organs, ready to spring into action.

Lymphocytes turn into very specialized cells. Some migrate to the thymus were they mature. They are called T cells. T cells themselves can be of various types such as helper T cells, killer T cells and even suppressor T cells, which have the job of helping call off the attack of the immune system.

Natural killer (NK) cells are another lymphocyte that can act on their own to destroy a diseased cell. They appear to be part of a first line of defense against tumors and virus infected cells, according to NIAID.

The last type of lymphocytes is called B cells. B cells produce antibodies. Antibodies attach themselves to antigens and mark them for attack. There is also a type of B cell called a memory B cell. Its job is to remember exactly what an attacker looked like so the next time it shows up it can be surrounded and destroyed quickly. Immunizations (Read about "Immunizations") use this process to prime our immune system for certain diseases such as the flu, measles etc. (Read about "Influenza" and about measles & other "Childhood Illnesses")

There are millions of antibodies in our system. According to the Centers for Disease Control and Prevention (CDC), each of them is primed to fight a specific invader. Antibodies are separated into five classes. They are:

• Immunoglobulin A (IgA) concentrates in body fluids - tears, saliva, the secretions of the respiratory tract and the digestive tract - guarding the entrances to the body. (Read about "Respiratory System" "Digestive System")
• Immunoglobulin D (IgD) remains attached to B cells and plays a key role in initiating early B-cell response.
• Immunoglobulin E (IgE) whose natural job is suspected to be protecting against parasitic infections, is the suspected villain responsible for the symptoms of allergy. (Read about "Allergies")
• Immunoglobulin G (IgG) works efficiently to coat microbes, speeding their uptake by other cells in the immune system.
• Immunoglobulin M (IgM) is very effective at killing bacteria

Each class of antibody stands ready to fight specific types of attackers in specific places. NIAID says for example, IgA is usually found in the body secretions and the mucous membranes lining the airways and digestive tract. Likewise, IgG is the main type of antibody in the blood and the tissues. Antibodies also attract free floating proteins called complement that combine with the antibodies to help destroy invaders and remove them from the body.

The complement system is made up of about 25 proteins that work together to "complement" the action of antibodies in destroying bacteria. Complement also helps to rid the body of antibody-coated antigens (antigen-antibody complexes). Complement proteins, which cause blood vessels to become dilated and then leaky, contribute to the redness, warmth, swelling, pain, and loss of function that characterize an inflammatory response.

Components of the immune system communicate with one another by exchanging chemical messengers called cytokines. These proteins are secreted by cells and act on other cells to coordinate an appropriate immune response. Cytokines include a diverse assortment of interleukins, interferons and growth factors. Some cytokines are chemical switches that turn certain immune cell types on and off. Other cytokines chemically attract specific cell types. These so-called chemokines are released by cells at a site of injury or infection and call other immune cells to the region to help repair the damage or fight off the invader.

How it works

Though not usually considered part of the immune system, our skin (Read about "Skin") is our first line of defense. Until the skin is breached in some manner, invaders are kept outside. A cut is one way that bacteria can invade our bodies. But there are also natural breaches in the skin such as our eyes (Read about "The Eye"), mouth and nose that provide entry points for things like bacteria and viruses. Once they get inside, the resources of the immune system are called upon. The antigen or invader eventually runs into one of the white blood cells that act as sentries. Those include the macrophages, any of the granulcytes or the NK cells. Each of these has the ability to fight the invader on their own but they also aren't afraid to call in reinforcements. Chemical signals are sent out and soon others join the battle. The blood flow to the area increases and inflammation occurs as various leukocytes arrive to do battle. T cells and B cells arrive to check and see if this invader has been here before. If the antigen has, the B cells that fought last time (memory B cells), launch into action producing the needed antibodies. If it's a new invader, other B cells produce the required antibody to stop the invader in its tracks. Sometimes the antibodies attract scavenger cells, like macrophages, that engulf the invader and destroy them. Other times the antibodies can combine with complement to destroy the antigen or they can attach themselves to viruses in such a way that the viruses are no longer able to invade cells.

Immunity

Long ago, physicians realized that people who had recovered from the plague would never get it again - they had acquired immunity. This is because some of the activated T and B cells become memory cells. The next time an individual meets up with the same antigen, the immune system is set to demolish it. Immunity can be strong or weak, short-lived or long-lasting, depending on the type of antigen, the amount of antigen, and the route by which it enters the body.

CDC says immunity can also be influenced by inherited genes. When faced with the same antigen, some individuals will respond forcefully, others feebly, and some not at all. An immune response can be sparked not only by infection but also by immunization with vaccines. Vaccines contain microorganisms - or parts of microorganisms - that have been treated so they can provoke an immune response but not full-blown disease. Immunity can also be transferred from one individual to another by injections of serum rich in antibodies against a particular microbe (antiserum).

For example, immune serum is sometimes given to protect travelers to countries where hepatitis A (Read about "Hepatitis A") is widespread. Such passive immunity typically lasts only a few weeks or months. Infants are born with weak immune responses but are protected for the first few months of life by antibodies received from their mothers before birth. Babies who are nursed (Read about "Breastfeeding") can also receive some antibodies from breast milk that help to protect their digestive tracts.

Vaccines

Medical workers have long helped the body's immune system prepare for future attacks through vaccination. Vaccines consist of killed or modified microbes, components of microbes, or microbial DNA that trick the body into thinking an infection has occurred. An immunized person's immune system attacks the harmless vaccine and prepares for subsequent invasions. CDC says vaccines remain one of the best ways to prevent infectious diseases and have an excellent safety record. Previously devastating diseases such as smallpox, polio, and whooping cough (Read about smallpox in "Biological Health Threats" and about whooping cough in "Childhood Illnesses") have been greatly controlled or eliminated through worldwide vaccination programs.

Immune system disorders

Sometimes things go wrong and the immune system doesn't work the way it is supposed to. That can be the result of the immune system going after invaders that present us no danger, from the immune system attacking our own bodies and from the immune system missing certain components. The results are allergies, autoimmune diseases such as rheumatoid arthritis and conditions such as immunodeficiency diseases. (Read about "Allergies" "Rheumatoid Arthritis" "Primary Immunodeficiency")

• Allergic diseases - The most common types of allergic diseases occur when the immune system responds to a false alarm. In an allergic person, a normally harmless material such as grass pollen or house dust (Read about "Pollen Allergies" "Dust Allergies") is mistaken for a threat and attacked. Allergies such as pollen allergy are related to the antibody known as IgE. Like other antibodies, each IgE antibody is specific; one acts against oak pollen, another against ragweed. (Read about "Allergies")
• Autoimmune diseases - Sometimes the immune system's recognition apparatus breaks down, and the body begins to manufacture T cells and antibodies directed against its own cells and organs. Misguided T cells and autoantibodies, as they are known, contribute to many diseases. For instance, T cells that attack pancreas cells may contribute to diabetes (Read about "Diabetes"); while an autoantibody known as rheumatoid factor is common in people with rheumatoid arthritis. (Read about "Rheumatoid Arthritis") People with systemic lupus erythematosus (Read about "Lupus") have antibodies to many types of their own cells and cell components. No one knows exactly what causes most autoimmune diseases, but multiple factors are likely to be involved. These include elements in the environment, such as viruses, certain drugs and sunlight, all of which may damage or alter normal body cells. An example of one of these types of disease is rheumatic fever. (Read about "Rheumatic Fever") Rheumatic fever results from an inflammatory reaction to certain streptococcus A bacteria, the bacterium that causes strep throat. (Read about "Sore Throat & Strep Throat") The body produces antibodies to fight the bacteria, but instead the antibodies attack some of the body's own connective tissues. Hormones are also suspected of playing a role, since most autoimmune diseases are far more common in women than in men. Heredity, too, seems to be important. Many people with autoimmune diseases have characteristic types of self marker molecules.

• Immunodeficiency disorders - When the immune system is missing one or more of its components, the result is an immunodeficiency disorder. Immunodeficiency disorders can be inherited, acquired through infection or produced unintentionally by drugs such as those used to treat people with cancer or those who have received transplants. (Read about "Transplants") Temporary immune deficiencies can develop in the wake of common virus infections, including influenza, infectious mononucleosis, and measles. (Read about "Influenza" "Mononucleosis" and measles in "Childhood Illnesses") Immune responses can also be depressed by blood transfusions, surgery, malnutrition, smoking and stress. (Read about "Quit Smoking" "Stress") Some children are born with poorly functioning immune systems. Some have flaws in the B cell system and cannot produce antibodies. Other people, whose thymus is either missing or small and abnormal, lack T cells. Very rarely, infants are born lacking all of the major immune defenses. This condition is known as severe combined immunodeficiency disease or SCID. (Read about SCID in "Primary Immunodeficiency")

  AIDS (Read about "HIV / AIDS") is an immunodeficiency disorder caused by a virus (HIV) that infects immune cells. HIV can destroy or disable vital T cells, paving the way for a variety of immunologic shortcomings. HIV also can hide out for long periods in immune cells. As the immune defenses falter, a person with AIDS falls prey to unusual, often life-threatening infections and rare cancers. A contagious disease, AIDS is spread by intimate sexual contact, transfer of the virus from mother to infant during pregnancy, or direct blood contamination. (Read about "Blood Donation") There is no cure for AIDS, but newly developed antiviral drugs can slow the advance of the disease, at least for a time. Researchers also are testing HIV vaccines in clinical studies. (Read about "Clinical Studies")

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