Immune System

Immune System

Also called: Immune Response, Immune Reaction

Reviewed By:
David Slotnick, M.D.


The immune system protects the body from infections, diseases and foreign substances. It involves a number of organs, tissues and cells. These components work together to keep foreign invaders called antigens from entering the body and bloodstream. When antigens do enter the body, the immune system identifies, weakens and eliminates them.

There are a number of ways antigens are prevented from entering the body. The skin and mucous membranes (e.g., lining of the eyes, nose and mouth) are all capable of keeping most types of antigens from entering the body by blocking them or trapping them in mucus. The mucous membranes of the nose are further protected by cilia (tiny hair-like projections), which also help to trap the antigens.

When antigens make it into the body, the immune system begins to respond. White blood cells play a major role in immune system functioning. These cells identify and/or attack antigens that make it to the bloodstream. Some white blood cells aggressively bind themselves to antigens and release chemicals to kill them. Other white blood cells trigger the release of antibodies that are designed to combat specific invaders.

The symptoms associated with an immune system response are often uncomfortable for a person, but are actually quite necessary. Symptoms such as sneezing, a runny nose, fever, coughing, itching, nausea, diarrhea or shortness of breath are a result of the immune system fighting foreign invaders. Inflammation is a common immune system reaction and may appear as red or swollen areas of the body.

At times, the immune system makes a mistake and attacks the body’s own tissues. These autoimmune disorders can cause a wide range of diseases, including type 1 diabetes, rheumatoid arthritis and other conditions.

Modern medications have provided ways to produce immunity when the body cannot. Vaccinations provide immunity by forcing an immune system response and creating antibodies to a particular disease.

About the immune system

The immune system is a complex collection of specialized organs, tissues and cells spread throughout the human body that protects the body from outside cells or threats. This allows a rapid response by almost every part of the body. With a variety of body components acting together, the immune system is capable of quickly overcoming most foreign invaders.

The immune system’s flexibility is a result of its complexity. Most organ systems have defined parts and functions within the body, such as the heart and blood vessels of the circulatory system or the stomach, intestines and other organs that make up the digestive system. In addition to their primary role in the body, many of these systems and bodily tissues also help to fight foreign invaders. The immune system must be capable of fighting threats from any source outside the body and even from cells inside the body.

The body is regularly threatened by pathogens (viruses, bacteria, fungi, parasites) and toxins that can be found in the environment. When these materials penetrate the body’s barriers (e.g., skin, mucous membranes), the immune system works to prevent or limit the damage they can cause. Disease occurs when the immune system does not adequately function.

Before foreign invaders can cause infection or tissue damage, they must pass through barriers. Some body systems provide these barriers, in addition to their main function in the body. These body systems include:

  • Skin. The body’s largest organ blocks toxic substances and pathogens in the environment from reaching internal tissues and organs.
  • Mucous membranes. The moist lining of body cavities and passageways, including the inside of the mouth, nose and eyes, form a barrier to trap foreign substances and organisms. The upper respiratory tract is also protected by internal cilia (tiny hair-like structures) that further help to trap these antigens. The foreign invaders are then removed from the body. Materials caught in the mucous membranes of the mouth and nose may be expelled through coughs and sneezes.
  • Digestive system. Pathogens may be consumed with food. They are usually killed in the stomach by acids that are part of the digestive process.

If foreign invaders get past these barriers, the main organs and cells of the immune system may become engaged.

The immune system produces many reactions throughout the body to help it fight infection. In general, it requires inflammation of tissues to be effective, which causes redness, heat and swelling. The redness results from the dilation, or widening, of blood vessels in the area of infection. This allows more white blood cells, which are highly involved in fighting disease, to reach the area.

Inflammation also raises the temperature of the infected area or much of the body. This helps chemicals the body makes to kill foreign organisms to be more effective. It also allows the cells that fight infection to reproduce more rapidly. The resulting fever may be uncomfortable for the patient, but is important for the immune system to work.

Swelling related to inflammation is often associated with the collection of cells and fluids in the infected area. This may include immune system cells, bacteria that have been killed and chemicals used in the body’s response. Gradually they are removed from the tissues by the circulatory and lymphatic system.

Other signs that the immune system is functioning include sneezing, coughing and a runny nose. These are all mechanisms used to eliminate pathogens and foreign materials from the body.

Antigens and the immune system

Any organism or substance that causes an immune system reaction is considered an antigen. These commonly include pathogens (viruses, bacteria, fungi and parasites), toxins, and body cells that become cancerous. Transplanted cells or tissues are also foreign to the immune system and may cause a response. Immune system reactions are aimed at turning the antigen into a less dangerous substance or removing it from the body entirely.

When functioning properly, the immune system is able to differentiate between the body’s own cells and foreign substances. It recognizes certain protein molecules on the surface of every healthy cell in the human body. These molecules are part of the major histocompatibility complex (MHC) that occurs in specific patterns and differs from person to person, with the exception of identical twins. These surface molecules are either absent or unrecognizable in foreign substances.

Thus, organisms or substances that penetrate body tissues and do not have the proper MHC are identified as foreign by the immune system, provoking an immune system response. During an immune system response, foreign invaders are typically surrounded and bound by proteins called antibodies that are produced by white blood cells in the body. This prevents the invading substances from reproducing or being transported throughout the body. These foreign substances are then removed from the body or killed.

Some pathogens, such as viruses and some bacteria, invade the body’s cells to reproduce and spread. When this occurs, body cells will often change their MHC surface proteins. This signals to the immune system that the cell is infected and needs to be removed. Cells of the immune system may also use part of the antigen to signal other immune cells that an infection is occurring, thus triggering a larger scale response.

Cancer cells are mutated body cells that begin to uncontrollably reproduce. This frequently causes a change in the MHC surface proteins, similar to what happens when a pathogen infects a cell. The immune system response typically removes the cancer cells to prevent further spread. The immune system can become weakened or overwhelmed and allow cancer cells to continue to reproduce. This is one route that can lead to disease.

Toxins and other foreign molecules are often far smaller than cells. After they are recognized by the immune system as potentially harmful substances, they are engulfed and removed from tissues.

Transplanted cells and organs also have external molecules that differ from those the immune system recognizes as part of its body. The immune system reacts to these transplanted tissues much like it does to pathogens and will attempt to reject the transplanted organ. Immunosuppressant medications are often used to reduce the body’s efforts to kill and remove these tissues.

Organs and tissues of the immune system

The organs and tissues involved in immune system activity are spread throughout the body. Some are solely dedicated to protecting the body from foreign organisms and toxins. Others fulfill multiple functions, with their involvement in immune system activity only part of their overall role in the body.

Most major organs and tissues involved in immune system response may be considered part of the lymphatic system. This system consists of organs and tissues that are responsible for draining fluid (lymph) from spaces in tissues and returning it to the bloodstream. Lymph is composed of plasma, white blood cells and other substances. It continuously flows out of the thin walls of the capillaries (tiny blood vessels that carry oxygen and nutrients to individual body cells) into surrounding body tissues where its proteins, minerals and nutrients provide nourishment.

Although most lymph is reabsorbed into the capillaries, some remains in the spaces surrounding the cells. The lymph vessels are a network of tubes that branch off into tissues throughout the body. They collect excess lymph and also remove and transport damaged cells, bacteria, and foreign material that may have entered body tissue.

A major component of the immune system, the lymphatic system produces and stores some of the cells needed to fight infections and disease. The system includes the lymph nodes (also known as lymph glands), small, bean-shaped organs that are clustered in various areas of the body, including the neck, armpit, chest and groin. The lymph nodes are located along the lymph vessels and their functions include producing immune system cells and filtering foreign materials from lymph. When the lymph nodes recognize antigens in the lymph fluid, they respond by enlarging and producing additional cells to help combat infection. Thus, enlarged lymph nodes, or swollen glands, are a common result of immune system activity – and a symptom of many different diseases.

In addition to the lymph fluid, lymph vessels and lymph nodes, the lymphatic system also involves a number of organs and tissue masses, including:

  • Spleen. An organ located under the rib cage on the left side of the abdomen. It produces cells to fight infection, stores healthy blood cells and filters out damaged or worn-out blood cells, platelets and cellular waste.

  • Thymus. An organ located in the front of the chest at the base of the neck that is most active before birth and in infancy. Its main function is the creation of certain types of white blood cells (T cells).

  • Bone marrow. The soft, inner section of most human bones are composed of blood-forming cells, fat cells and tissues that support the growth of blood cells, including the white blood cells that respond to antigens within the body.

  • Adenoid and tonsils. These masses of lymphoid tissue are located at the back of the throat. They store white blood cells that produce antibodies against foreign materials that are breathed in or swallowed. The tonsils and adenoid often become swollen when they are fighting off an infection in the throat. Physicians may recommend that they be surgically removed if they become chronically enlarged (e.g., in cases of recurring tonsillitis).

Cells and chemicals of the immune system

In addition to the major body organs, a complex set of cells are involved in immune system activity. These cells are produced throughout the body and have been classified into multiple layers.

Most cells involved in immune system activity are types of white blood cells (also called leukocytes). While there are many different varieties of white blood cells, all of them work together against pathogens and harmful substances. In addition, many types are also capable of moving and acting independently to defend the body against foreign invaders.

Lymphocytes are one class of white blood cells. There are several types of cells considered lymphocytes that respond to infection in different ways, including:

  • T cells. This type of cell exists in two forms: killer T cells and helper T cells. Killer T cells detect and directly attack antigens. Helper T cells recognize the presence of foreign organisms and stimulate B cells into action.

  • B cells. Once signaled by helper T cells, they reproduce by cloning into plasma cells. The plasma cells produce chemical defenses (antibodies) against specific antigens. The body produces millions of types of B cells because each type responds to only one type of antigen.

  • Memory cells. Following an infection, some B and T cells become memory cells. These cells remain in the system in case the patient is infected with the same antigen again. These memory cells are one factor that helps the body develop immunity to some diseases.

Phagocytes are another class of white blood cells. These cells engulf and digest foreign particles and organisms. Phagocytes called monocytes travel through the bloodstream and remove foreign bodies. When monocytes move into tissues, they undergo changes and become macrophages. After they become macrophages, they can remove antigens from body tissue.

Macrophages also serve as antigen presenting cells (APCs). These cells are located throughout the body, checking tissues for foreign antigens. When an APC encounters an antigen, it breaks the antigen down. The APC then moves molecules from the antigen to its surface so other parts of the immune system will recognize the antigen.

Neutrophils are another type of phagocyte and are one of the most common white blood cells. They engulf pathogens and then inject them with granules of chemicals to kill the organism. Because they use these granules, they are also classified as granulocytes. Other granulocytes include basophils and eosinophils.

Mast cells are also involved in immune system activity but are not blood cells. They are found in many tissues (e.g., lungs, skin, tongue) and release chemicals called histamines in response to infections, including the presence of certain allergens (e.g., pollen, dander, mold). Many chemicals other than blood cells and mast cells also participate in immune system activity. These chemicals work to weaken or kill pathogens and neutralize potential toxins.

Antibodies are the major response cell of the immune system. They are proteins produced by the plasma cells created from cloned B cells. There are millions of types of B cells and each can produce antibodies to respond to one specific antigen. Antibodies work in many ways to disrupt infecting organisms.

Antibodies are also known as immunoglobulins (Ig) and are divided into five classes: IgA, IgD, IgE, IgG and IgM. Each group may perform different functions to respond to antigens or what is perceived to be an antigen. For example, IgE is involved in allergic reactions and signals the production of histamines from mast cells. It is also believed to be active in the immune system response to parasitic infections. Other antibodies have other functions, both known and unknown.

Chemicals such as hormones are messenger chemicals also used in immune system activity. They are often released by one tissue to initiate or end activity by another tissue. The hormones generated by the immune system are called lymphokines. Some of these hormones are used by the body to either suppress or encourage immune system activity.

The complement system of proteins floats freely in the blood. When they encounter an antigen, they work with antibodies to cause the cells of the pathogen to burst. They also may signal that a cell is infected and needs to be removed from the body.

Histamines and leukotrienes are chemicals released by granulocytes and mast cells. They may be injected into a pathogen to kill it. They also cause the blood vessels to dilate, or widen, so more agents of the immune system can reach the site of infection.


Immunity is the body’s ability to resist infection by certain pathogens. Some immunity in the body is natural (or innate) and is provided by genetics and through the mother’s immune system during gestation. It provides some protection from disease as the child develops its own acquired immunity.

Most immunity is acquired, which means the body produces it as a result of environmental exposure to antigens. As a patient is repeatedly exposed to antigens, acquired immunity is strengthened. Each time the immune system encounters an antigen, memory cells (made from B cells and T cells) are stored. The next time the same antigen is encountered, the body is able to launch a faster immune system response to kill the pathogen.

Immunization is the process of providing acquired immunity to a patient. It is administered through a vaccine, or an inactivated amount of a particular disease-causing agent. Vaccines can be attenuated (weakened) live viruses, killed pathogens, toxoid (made from toxins produced by bacteria) or biosynthetic (a combination of natural and synthetic substances). Through vaccines, an antigen is introduced into the body so the immune system can develop protective antibodies without risk of exposure to the actual pathogen that causes disease. Some diseases for which vaccines have been developed include smallpox, measles, whooping cough, and tetanus.

Vaccines are usually injected, although some can be inhaled through a nasal spray or ingested through the mouth. Many vaccines provide a lifetime of protection against disease. Others need periodic updating (boosters). The Centers for Disease Control and Prevention(CDC) has schedules of the vaccines it recommends for infants (beginning at 2 months of age), children, adolescents and adults.

Another example of deliberate exposure to an antigen is the use of antiserum (immune serum). Given by injection, this provides antibodies to a patient for temporary immunity to a particular microorganism. Some serums are used prior to potential disease exposure, such as those administered to travelers before they visit places where tropical diseases may occur. Other serums are given following exposure to a disease or toxin, such as rabies or snake bite venom.

The other main source for acquired immunity is breastfeeding. Breast milk contains antibodies and can be an infant’s primary source of acquired immunity.

Conditions related to the immune system

In some individuals, the immune system may not work adequately or may work against its own body tissue. Such conditions are called either immune-mediated (the immune system allows damage to take place without intervening) or autoimmune (the immune system mistakenly attacks its own tissues).

Some immune-mediated conditions include:

  • Allergic reaction. Caused when the immune system reacts to a non-threatening substance (an allergen) as if it were dangerous. The body attempts to remove the threat, causing symptoms such as sneezing, a runny nose, coughing, itching, nausea, diarrhea or shortness of breath.

  • Allergic asthma. Condition where allergens (e.g., pollen, dander, mold) trigger an asthma attack. When inhaled, the allergens cause inflammation of the airway passages in the lungs, resulting in coughing, wheezing and shortness of breath.

  • AIDS (acquired immune deficiency syndrome). Condition caused by HIV (human immunodeficiency virus) where the immune system becomes suppressed and is unable to fight infections. Bacteria and viruses that are normally repelled by a healthy immune system cause infections that may be life-threatening.

Some of the major autoimmune conditions include:

  • Type 1 diabetes. Occurs when insulin-producing cells in the pancreas are mistaken as antigens by the immune system, making it difficult for the body to convert food into energy.

  • Rheumatoid arthritis. Condition in which the immune system causes an inflammation of the membrane lining the joints. It often results in pain and stiffness in the joints.

  • Multiple sclerosis. Disease that results when the immune system damages the nerve tissues in the central nervous system.

  • Inflammatory bowel diseases. Multiple conditions may result from the immune system working against the intestines. These include Crohn disease and ulcerative colitis.

  • Lupus erythematosus. Occurs when the immune system attacks and damages the kidney, brain or lungs.

  • Psoriasis. Condition in which the immune system sends signals that cause skin growth to speed up, resulting in the itchy, flaky buildup of skin.

  • Scleroderma. This disease results from the immune system causing the skin and blood vessels to thicken, resulting in a loss of movement and shortness of breath.

  • Autoimmune thyroid diseases. Conditions in which the immune system damages or stimulates the tissues of the thyroid. Conditions of this type include Hashimoto’s thyroiditis and Grave’s disease.

  • Sjögren syndrome. Occurs when the immune system reacts against some of the body’s own tissues, damaging the glands that produce tears and saliva and causing dry mouth and dry eyes.

Immune system activity plays a major role in organ and tissue transplantation. The immune system functions as it should in such cases because it reacts to the new tissue as a foreign invader. However, for transplantation, such a response must be circumvented so the body will accept the new organ or tissue.  For this reason, transplant patients are often required to take medications (immunosuppressants) to reduce the ability of the immune system to work.

Questions for your doctor

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their condition. Patients may wish to ask their doctor the following questions related to the immune system:

  1. How can I tell if my immune system is functioning properly?
  2. How does my immune system keep me from getting sick?
  3. What is an antigen?
  4. How can I keep my immune system healthy?
  5. Am I at risk for any immune system-related conditions?
  6. What are autoimmune disorders?
  7. If I have one autoimmune condition am I likely to develop another?
  8. What factors can lead to immunosuppression?
  9. How are allergies and my immune system related?
  10. What are vaccines?
  11. How do I know when I need a vaccination?
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