Immune System and Allergies

Immune System and Allergies


The immune system protects the body from infections, diseases and foreign substances. It involves a number of organs, tissues and cells. Whenever possible, these components work together to keep foreign invaders (known as antigens) from entering the body and bloodstream. However, when antigens do penetrate into the blood, the immune system is capable of immediately identifying and attacking the threat.

The immune system has a number of ways to combat antigens. The eyes, nose and mouth are all capable of keeping most types of antigens from penetrating the body through mucus and cilia (tiny hair-like projections).

Leukocytes (white blood cells) also play a very large role in the immune system. They identify and/or attack antigens that make it to the bloodstream. Some leukocytes aggressively bind themselves to antigens and release chemicals to kill them (such as killer T-cells). Others trigger the release of antibodies that are specifically designed to combat specific invaders.

The symptoms associated with an immune system response are often uncomfortable to the individual, but are actually quite necessary. Symptoms such as sneezing, runny nose, fever, coughing, itching, nausea, diarrhea or shortness of breath are all produced as the immune system tries to fight foreign invaders.

At times, the immune system mistakenly attacks a generally harmless substance. An allergic reaction is the most common example of such an attack. Other types of improper immune system reactions are responsible for type 1 diabetes, rheumatoid arthritis and other conditions. These are called autoimmune disorders.

The immune system, although functioning properly, may present problems for people undergoing organ transplants. The body perceives transplanted organs or body tissue as attacking foreign invaders and rejects them. This can have serious consequences for the donor recipient. Therefore, transplant patients are required to take medications to suppress this immune system response (immunosuppressants).

About the immune system

The immune system is a complicated system of specialized cells and organs throughout the human body designed to recognize foreign substances (antigens) and react to them. Antigens commonly include toxins, viruses, bacteria, fungi, parasites and cancer cells, although they can also include transplanted cells or tissues. Immune system reactions usually result in the antigen being turned into a less dangerous substance or being removed from the body entirely.  

Sometimes the immune system will mistake a harmless substance (such as pollen or a protein in peanuts) as a threat, triggering an allergic reaction. Allergies and allergy symptoms are a result of the immune system’s response to allergens (see Response to allergens).

The immune system is spread throughout the human body, which allows it enough access to effectively protect against invaders in almost every part of the anatomy. With a variety of components all acting together, the immune system is capable of rapidly overcoming most invaders. Many of these components operate at the cellular level. These include:

  • White blood cells. While there are many different types of white blood cells, all of them work together to destroy bacteria, fungi, viruses and other harmful substances in the bloodstream.  All white blood cells are called leukocytes, and each one is capable of moving and acting independently to defend the body against invaders. White blood cells are divided into three different classes:

    • Granulocytes. Made up of basophils, eosinophils, and neutrophils.

    • Lymphocytes. Made up of T-cells and B-cells:

      • T-cells. This type of cell comes in two forms: Killer T-cells are capable of distinguishing between foreign and friendly cells on their own, directly attacking invaders when they are detected. Helper T-cells are used to stimulate B-cells.

      • B-cells. These types of cells need to be stimulated by helper T-cells before they can respond. However, once they are stimulated, they are capable of tuning in to a specific germ or antigen. They then begin cloning themselves and produce millions of antibodies designed to attack that specific antigen.

    • Monocytes. Evolve into macrophages (scavenger cells that remove foreign bodies from the blood).

  • Antibodies. Produced by white blood cells, these proteins are able to defend against dangerous substances by responding to specific antigens (e.g. bacteria, viruses). Antibodies are shaped like the letter Y, with the two branches sensitized to bind together with an antigen if it is encountered. This binding action usually disrupts the chemical action of the invader.  The five classes of antibodies are:

    • Immunoglobulin A (IgA)
    • Immunoglobulin D (IgD)
    • Immunoglobulin E (IgE)
    • Immunoglobulin G (IgG)
    • Immunoglobulin M (IgM)

  • Hormones. The hormones generated by the immune system are called lymphokines. Some of these hormones are used by the body to either suppress or encourage other parts of the immune system.

  • Complement system. This series of proteins floats freely in the blood and works together with antibodies to either cause cellular bursting or signal that a certain cell needs to be removed from the body.

A number of organs and tissues are involved in the immune response by manufacturing or regulating the types of cells present in the bloodstream. These include:

  • Bone marrow. Capable of producing new red and white blood cells, bone marrow is vitally important to the immune system. Marrow is located in the cavities of the bones and produces new blood cells from stem cells, which are capable of becoming many different types of cells.

  • Thymus. Located in the front of the chest at the base of the neck, this organ is most important to infants, who depend on it for the maturation of T–cells. Adults can usually make enough T–cells elsewhere in the body that the thymus is not an essential organ.

  • Spleen. The spleen is an organ located under the rib cage on the left side of the abdomen. It contains macrophages (scavenger cells that remove foreign bodies from the blood).

  • Lymphatic system. A network of nodes that extends throughout the body. Lymph (blood plasma that bathes cells in water and nutrients) is distributed through the system to provide cells with the food, water and oxygen they need to survive.

  • Adenoids and tonsils. These masses of lymphoid tissue are located in the back of the throat. They store white blood cells that produce antibodies against foreign materials that are breathed in or swallowed.

About the immune response

The process by which the immune system deals with antigens is complicated. The body is capable of defending itself in many different ways and in many different places. Just getting into the bloodstream is difficult for antigens. Some of the very first processes through which the immune system defends the body include:

  • External protection. The first and most basic layer of the immune system is the skin. While not a true immune system component, skin is able to function as a first line of defense against antigens, keeping many antigens out of the body entirely and providing an early warning system for those that manage to penetrate it. Skin prevents bacteria and viruses from passing, and actually secretes antibacterial substances to prevent bacteria and spores from growing on the skin.

  • Entrances to the body. While the nose, mouth and eyes are gateways to the inner body, they are also part of the first line of defense against antigens. All three use mucus and enzymes to combat and keep out many types of invaders. Antigens frequently become trapped in mucus, and are removed by the body through actions such as coughing and sneezing. The nose and airways also contains cilia (hair-like projections) which sweep mucus up and away from the lungs.

  • Boundary tissues. Before an antigen can enter the bloodstream, it needs to make it past the bodies various boundary tissues. Macrophage cells live in specific areas, such as the lining of the lungs. When they come into contact with a foreign particle, they help to neutralize it by moving it out of the bloodstream. A form of macrophages (Langerhans cells) live in the skin, where they fight against any invaders that could otherwise be absorbed through the skin and into the bloodstream.

Those antigens that succeed in making it into the bloodstream quickly find themselves identified as dangerous. Several types of cells known as antigen presenting cells (APCs) are located throughout the body, constantly checking their surrounding environment for any foreign antigens. When an antigen is encountered by the APCs, the cells actually break down the molecules of the antigen and display its components. This action makes any antigen easily identifiable to those parts of the immune system tasked with attacking foreign substances.

With the help of the APCs, neutrophils (the most common type of white blood cells) are able to quickly recognize dangerous invaders and release enzymes to kill them. Antibodies can also identify many types of antigens, and will attach themselves to invaders to disrupt them. Antibodies and some types of leukocytes (white blood cells) will trigger chemical responses, such as the release of histamines or leukotrienes, which make the body less hospitable for the invading antigen.

Most of the attacking cells that make up the immune system are able to distinguish between friendly local cells and foreign cells through the major histocompatibility complex (MHC). This process actually marks, through genes in protein molecules, every individual cell normally found in the body as friendly. When the immune system encounters a cell that is not identified as part of the MHC, it reacts against it.

When dealing with an antigen invader, the immune system will often produce symptoms that people find unpleasant. Runny nose, itching, swelling and coughing are all common immune system responses to antigens. While the onset of these symptoms might make it seem like the immune system is malfunctioning, it is actually producing these symptoms for very good reasons.

For example, the immune system favors inflammation and swelling because they dilate capillary walls (through which blood flows) and increase blood flow to an infected area. This allows more immune system cells to get to the point of infection faster.

Response to allergens

In some cases, an individual’s immune system becomes sensitized to a substance that is actually not a threat. In allergies, the immune system has incorrectly identified a substance (called an allergen) as dangerous. As a result, the immune system attacks the allergen and triggers an allergic cascade.

Common allergens include pollen, dander, mold, dust, foods, and other substances. An allergic reaction to these substances produces symptoms similar to those felt when the immune system attacks an actual threat, such as a bacteria or virus. These symptoms may include sneezing, runny nose, coughing, itching, nausea, diarrhea or shortness of breath.

The first time a person ever encounters an allergen, they become sensitized and do not actually have an allergic reaction. Sensitization is the process through which the immune system is able to classify an allergen as potentially dangerous, and remember it in case of future contact. Once a person has been sensitized to an allergen, they will have an allergic response the next time (and any other time) that allergen is encountered.

With an allergic response, the immune system reacts in much the same way it would if a dangerous antigen were present. The chain of events that the immune system follows includes:

  1. The allergen to which a person is sensitized enters the body. It may be inhaled through nasal passages, come in direct contact with the skin or be ingested. The allergen flows through the bloodstream and encounters immunoglobin E (IgE) antibodies.

  2. The IgE antibodies recognize the invader and bind themselves to the allergen molecule. They also trigger the mast cells and basophils to which they are attached to self–destruct.

  3. Powerful chemicals from inside the mast cells and basophils are released into the bloodstream. These include histamines, leukotrienes and other allergy stimulators.

  4. Allergy symptoms begin to appear. These may be localized (only in the area where these chemicals were first released) or systemic (throughout the entire body). These chemicals mainly affect the blood vessels, mucous glands and bronchial tubes.

The allergens are therefore the match that lights the fuse (IgE) that triggers the bomb (mast cells and basophils) to explode.

Sometimes one substance is similar enough to another that the immune system will mistake it for a known allergen and trigger the allergic cascade by mistake. For example, a protein in latex is similar to a protein found in fresh fruits, vegetables and nuts. People with an allergy to latex often have allergic reactions to these foods. This phenomenon is known as cross-reactivity.

A number of medications are used in the treatment of allergies. Some of these work to prevent or relieve symptoms by interrupting the immune system’s response to allergens. For instance, antihistamines and mast cell stabilizers prevent the release of, or block the effects of, histamines. Leukotriene modifiers are another type of allergy drug which work by blocking the effects of leukotrienes.

Immunotherapy is another form of allergy treatment. Also known as allergy shots, this method involves injecting a patient with increasing, controlled doses of an allergen over a period of time. This allows the immune system to slowly build immunity to the allergen, and as a result, it no longer reacts as strongly when the body comes in contact with the substance.

Related conditions

In some individuals, the immune system can become a problem, either by not working adequately or working too aggressively. Conditions of this nature are called either immune-mediated (the immune system allows damage to take place without intervening) or autoimmune (the immune system mistakenly attacks the body’s own tissues). Some conditions that result from the immune system behaving incorrectly include:

  • Allergies. Reaction caused when the immune system “mistakes” a non–threatening substance as dangerous. The body reacts to remove the threat, causing symptoms such as sneezing, 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 the airway passages in the lungs to become inflamed, resulting in symptoms such as coughing, wheezing and shortness of breath.

  • Type 1 diabetes. Condition where insulin–producing cells in the pancreas are mistaken as dangerous by the immune system, making it difficult for the body to convert food into energy.

  • Rheumatoid arthritis. Condition in which the immune system incorrectly causes an inflammation of the membrane lining the joints of the hands and feet, causing pain and stiffness in the joints.

  • Multiple sclerosis. Condition in which the immune system damages the nerve tissues in the central nervous system.

  • Inflammatory bowel diseases. Condition in which the immune system attacks the intestines. This includes Crohn’s disease and ulcerative colitis.

  • Lupus erythematosus. Condition in which the immune system may attack and damage 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. Condition in which the immune system causes 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.

  • Sjogren’s syndrome. Condition in which immune system reacts against some of the bodies own tissues, damaging the glands that produce tears and saliva.

  • HIV (human immunodeficiency virus). Condition whereby the immune system becomes depressed and is unable to fight infections.

Even a properly functioning immune system may present problems for some people – such as those undergoing organ or tissue transplants. Trying to protect the body, the immune system attacks transplanted organs or body tissue as foreign invaders. This can have serious consequences for the donor recipient.  For this reason, transplant patients are required to take medications (immunosuppressants) to hold back the immune system from attacking.

Questions for your doctor

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. 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. Does having allergies mean that my immune system is not functioning properly?
  3. What role does the immune system play in my allergies?
  4. Do my symptoms suggest an immune system response?
  5. What allergen may be triggering my immune system to react?
  6. Are there drugs I can take to prevent my immune system from responding to allergens?
  7. How can I keep my immune system healthy?
  8. Am I at risk for any immune system-related conditions?
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