Allergic sensitization

We are all exposed to a number of substances that could potentially cause allergy, yet only some of us become allergic. There are alternative paths that the immune system can take when encountering a new molecule.

When an allergen first enters the body, an antigen-presenting cell digests it, presents it on its surface and introduces it to a T cell. What happens next determines whether the person will develop an allergy or not.

In a normal reaction the T cell differentiates into a Th1 cell, which in turn interacts with a B cell, releasing cytokines that promote the B cell to differentiate into a plasma cell, which will start producing IgG antibodies. The IgG antibodies have a protective function. They help to immobilize and destroy the allergens without causing an allergic response.

However, if the T cell differentiates into a Th2 cell, a different path is activated. The Th2 cell also interacts with the B cell and promotes it to differentiate into a plasma cell, but instead of IgG the plasma cell will produce IgE antibodies. The IgE’s bind to the surface of mast cells, priming them to recognize the allergen the next time it enters the body. This process is called sensitization.

It is not clear why some people become sensitized to certain allergens and some don't, but after sensitization each new encounter with the allergen induces an allergic reaction.

GLOSSARY OF ALLERGY TERMS:

« Allergen

An antigen (i.e. allergen) that induces an allergic or hypersensitive response.
« Allergenic

A substance (such as a food or pollen), that has the capacity to induce allergy is defined as allergenic.
« Allergen immunotherapy

A process of administering allergenic extracts to patients who suffer from allergic rhinoconjunctivitis and allergic asthma to decrease the degree of hypersensitivity and symptoms by reducing immunologic responses to environmental allergens like pollen, dust, animal dander, and molds; also highly effective for preventing anaphylaxis in patients allergic to bee venom.
« Allergic asthma

An inflammatory disease of the lungs characterized by (in most cases) reversible airway obstruction. Originally, a term used to mean “difficult breathing”; now used to denote bronchial asthma.
« Allergic reaction

An adverse reaction involving the immune system, caused by inhaling, swallowing, or touching a substance to which a person is sensitive. It can also follow injections of medicines, insect stings, or insect bites.
« Allergic rhinitis

An allergic reaction affecting the lining of the nose and the eyes caused by inhaling airborne allergens such as dust mite, pollens or animal dander and also by eating certain foods. Symptoms may include itching, sneezing, blocked nose, runny nose, and itchy/watery eyes; may also occur concurrently with allergic conjunctivitis.
« Allergy

A chronic disease characterized by an overreaction of the immune system to protien substances - either inhaled, ingested, touched or injected - that normally do not cause an overreaction in non-allergic people. Hypersensitivity is typically caused by exposure to a particular antigen (allergen) resulting in a marked increase in reactivity to that antigen on subsequent exposure, sometimes resulting in harmful immunologic consequences. See also: allergic reaction.
« Anaphylaxis

The most severe type of allergic reaction involving many organs in the body. Onset of symptoms are immediate often affecting the breathing, and may be fatal if not treated immediately.
« Animal dander

Tiny scales from the skin, hair or feathers of an animal. Dander is a normal effluvium, but can induce an allergic reaction in people sensitized to it.
« Antibody

Antibody (also called immunoglobulin) is a molecule produced by B-lymphoid cells. It recognizes a unique molecule, which is called an antigen. Antibodies are present in the blood and body fluids. After binding an antigen, some antibodies can bind to surface receptors on immune cells, and in some cases they may neutralize pathogens. There are different classes of antibodies. The antibodies produced in an allergic reaction are of the IgE class.
« Antigen

An antigen is any substance that causes the immune system to produce antibodies against it. Antigens may be foreign substances from the environment, such as chemicals, bacteria, viruses, or pollen, or they may be formed inside the body, as with bacterial toxins or tissue cells.
« Antihistamines

Antihistamines are drugs commonly used for treating the symptoms of allergic reactions. They inhibit the action of histamine in the body by blocking the H1 histamine receptors in smooth muscles, vascular endothelial cells and central nervous system.
« Anti-inflammatory

Anti-inflammatory refers to the property of a substance or treatment that reduces inflammation or swelling. About half of analgesics (painkillers) have anti-inflammatory properties.
« Atopic dermatitis

Atopic dermatitis (AD, a type of eczema) is an inflammatory, relapsing, non-contagious and pruritic (itchy) skin disorder. The cause of AD is unclear, but factors like genetics, dysfunction of the immune system, defective skin barrier and environmental exposures have been suggested. There is no cure to AD. Various treatments are used to manage the condition, the most common ones being moisturizers and topical corticosteroids.
« Atopy

Genetic tendency to develop certain allergies. Atopic individuals usually suffer from one or more of the following: eczema (atopic dermatitis), allergic rhinitis (hay fever), or allergic asthma. These maladies may also be accompanied by food allergies, allergic conjunctivitis and other allergic symptoms.
« Basophil

Basophils (basophil granulocytes) are a type of white blood cells that are involved in inflammatory reactions, particularly those related to allergies and asthma. When stimulated, basophils release histamine and other enzymes that can lead to inflammation, bronchoconstriction, and asthma symptoms. Basophils and mast cells are quite similar in appearance and function, but basophils are found in the blood, whereas mast cells reside in tissues.
« Beta blockers

Beta blockers are a class of drugs that target the beta receptors on the surface of cells that are part of the sympathetic nervous system (heart muscle, smooth muscles, airways, arteries, kidneys, etc.). Beta receptors lead to stress responses, especially when stimulated by epinephrine (adrenaline). Beta blockers interfere with the binding of epinephrine and other stress hormones to the receptor, and weaken the effects of stress hormones.
« Conjunctivitis

Conjunctivitis (aka pink eye) is an inflammation of the conjunctiva. The conjunctiva lines the inside of the eyelids and covers the white part of the eye (sclera). Conjunctivitis can be due to a viral or bacterial infection or an allergic reaction. Redness and swelling of the conjunctiva, as well as watering and itching of the eye are typical symptoms of all types of conjunctivitis. Bacterial conjunctivitis also involves mucopurulent discharge.
« Dermatitis

Dermatitis, or eczema, is a general term describing a non-contagious inflammation of the skin. It can be due to an inherent skin defect, direct contact with an irritating substance, or an allergic reaction. Symptoms of dermatitis can include redness, dryness, itching, swelling, crusting, flaking and even cracking of the skin, which can cause oozing and bleeding.
« Desensitization

The reduction or abolition of allergic sensitivity or reactions to the specific antigen (allergen). In allergen immunotherapy, desensitization is accomplished by administrating increasing doses of the antigen, starting from very small doses that induce little or no symptoms, and increasing the dose over time until the immune system develops a tolerance to the substance.
« Drug Allergy

Sensitivity (hypersensitivity) to a drug or other chemical. Allergic symptoms may occur regardless of the form the drug is taken in (pill, injection, liquid), and vary in severity: itching, swelling, rash, wheezing, abdominal pain, vomiting or even anaphylaxis. The most common drug allergy is penicillin.
« Dust Mite

House dust mites are small mites invisible to the naked eye. They feed on organic matter shed by humans, such as flakes of skin, and need at least 50% humidity to survive. House dust mites thrive in the stable environment provided by houses, and are widely distributed in homes. House dust mite is a common cause of asthma & allergic rhinitis.
« Eczema

Generic term for inflammatory conditions of the skin. Eczema is characterized by one or more of the following symptoms: redness, swelling, itching and burning, crusting, flaking, blistering, cracking, oozing and bleeding. In a later stage lichenification and scaling can follow. Eczema is often hereditary and associated with allergic rhinitis and asthma.
« Eosinophils

Eosinophils are a type of white blood cells whose natural role is to defend the body against parasites. The number of eosinophils in the blood increases during parasite infections, but also during allergic reactions. At the site of the allergic reaction eosinophils release chemicals that cause damage to the surrounding tissues.
« Food allergy

An adverse IgE-mediated immune response to specific foods such as peanuts, tree nuts, shellfish, egg, and milk. There are also non-allergic food hypersensitivities that can be caused by a lack of digestive enzymes, inability to absorb certain nutrients or non-IgE-mediated immune responses, for example. These should not be confused with a true food allergy.
« Hay fever

Hay fever (allergic rhinitis) is an acute irritative inflammation of the mucous membranes of the eyes and upper respiratory passages induced by seasonal allergens such as pollen from trees, grasses, weeds or flowers. The inflammation is accompanied by itching and profuse, watery nasal secretion. Despite the name, hay fever usually doesn’t involve temperature elevation. Hay fever episodes recur annually approximately at the same time of the year, depending on the flowering season of the inducing plant.
« Histamine

A vasodepressor amine found in all body tissues. Histamine causes capillary dilation, contraction of smooth muscle cells, stimulation of gastric acid secretion and acceleration of the heart rate. It is involved in local immune responses, but also has gastrointestinal regulatory functions and acts as a neurotransmitter. In the immediate hypersensitivity reaction (allergic reaction) histamine is released by mast cells and basophils. It is responsible for the symptoms of allergic rhinitis.
« Hives

Urticaria is commonly referred to as hives. It looks like nettle rash, and consists of itchy, raised lumps surrounded by red area. Hives are often caused by an allergic reaction, but other causes are also possible.
« Immunoglobulins

Immunoglobulins, also known as antibodies, are proteins found in blood and in tissue fluids. Immunoglobulins are produced by cells of the immune system called B-lymphocytes. Their function is to bind to substances in the body that are recognized as foreign antigens (often proteins on the surface of bacteria and viruses). This binding is a crucial event in the destruction of the microorganisms that bear the antigens. Immunoglobulins also play a central role in allergies when they bind to antigens that are not necessarily a threat to health and provoke an inflammatory reaction.
« IgE

Immunoglobulin E, a class of antibody associated with the allergic response. IgE normally plays a role in defending the body against parasites, but in an allergic response it triggers the release of histamine from the mast cells by binding to the high-affinity FcεRI receptors on the mast cell surface.
« IgG

Immunoglobulin G, the most common immunoglobulin (antibody) in blood circulation and extracellular fluid. IgG protects the body from various types of infections. It can bind viruses, bacteria, fungi and toxins, and neutralize them by immobilizing them, tagging them for phagocytic cells or activating the complement system, a part of the innate immune system that helps in clearing pathogens. Allergen immunotherapy aims to reduce the IgE-mediated response to allergens, allowing the IgG-mediated response to neutralize the allergens.
« Immune system

An intricate complex of interrelated cellular, molecular, and genetic components that provides a defense, the immune response, against foreign organisms or substances and aberrant native cells.
« Immunotherapy

Treating a disease by inducing, enhancing or suppressing the immune response. Originally, immunotherapy was therapeutic administration of serum or immune globulin containing preformed antibodies produced by another individual. Currently, immunotherapy includes nonspecific systemic stimulation, adjuvant, active specific immunotherapy, and adoptive immunotherapy. New forms of immunotherapy include the use of monoclonal antibodies. Activation immunotherapy stimulates the immune system, and is used in cancer treatment, for example, to destroy tumors. Suppression immunotherapy, by contrast, dampens the immune response and is used for treating autoimmune diseases and preventing the rejection of transplanted organs. Allergen immunotherapy is also suppression immunotherapy as it decreases the hypersensitivity reaction of the immune system.
« Latex

Natural latex is an emulsion or suspension produced by some seed plants. Latex contains suspended microscopic globules of natural rubber. There are similar synthetic materials as well: polystyrene, polyvinyl chloride, etc. The word latex is also used for natural latex rubber, which is used in latex gloves, condoms, etc. Many people are allergic to rubber latex.
« Lymphocyte

Lymphocytes are white blood cells that can be found throughout the body in lymphatic tissue. They are divided in two main groups, B cells and T cells. When activated by antigens, B cells can differentiate into antibody-secreting plasma cells, whereas T cells can divide rapidly and produce new T cells sensitized to the antigen. Lymphocytes are an essential part of the adaptive immune system, which mounts a tailor-made defense when dangerous invading organisms penetrate the body's general defenses.
« Mast cell

Mast cells are leukocytes that are found in tissues throughout the body, particularly in tissues where they can participate in recognizing foreign substances. Typical examples of such tissues are skin, airways and the intestinal tract. Mast cells are involved in the immediate hypersensitivity reaction by releasing histamines, leukotrienes and other chemical mediators.
« Mold

A filamentous fungus, generally appearing as a circular colony that may be cottony, wooly, or glabrous, but with filaments that are not organized into large fruiting bodies, such as mushrooms. Molds release spores that spread through the air. These spores are the cause of symptoms in people allergic to mold.
« Pollen

Pollen consists of microspores of seed plants carried by wind or insects before fertilization. Pollen allergy is a major cause of allergic rhinitis. Many wind-pollinated grasses, trees and weeds produce a large amount of light-weight pollen that induces allergic symptoms in sensitized individuals. The prevalence of specific pollen allergies varies greatly depending on the geographic location.
« Respiratory Allergies

Allergy caused by inhaled substances such as pollen, hours dust mite, moulds and pet dander. Respiratory allergy usually doesn’t cause severe, anaphylactic reactions, but the symptoms can still have a significant impact on the quality of life. The irritation of airways, eyes and skin can lead to allergic rhinitis, asthma, eczema and urticaria.
« Rhinitis

Irritation and inflammation of the mucous membrane inside the nose, often due to an allergy to pollen, dust or other airborne substances. Seasonal allergic rhinitis also is known as "hay fever," a disorder which causes sneezing, itching, a runny nose and nasal congestion.
« Sinusitis

Inflammation of the mucous membrane of any sinus, especially the paranasal. See also: rhinosinusitis
« Skin Prick Test

A test to identify reactions to allergens. A common method to perform skin prick test is to apply separate droplets of allergens on the skin and prick the skin under each droplet with a tiny lancet that only penetrates the surface of the skin. This will carry the allergen into the skin. A positive test result is a reaction where a raised itchy lump (wheal) develops within 15-20 minutes.
« Urticaria

An eruption of swollen, itching wheals, colloquially called hives. Acute urticaria, lasting less than 6-8 weeks, is often due to a state of hypersensitivity to foods or drugs, but can also be triggered by infection, for example. Chronic urticaria can be due to many things, and often the cause will remain unclear. In addition to the hypersensitivity and infection, possible underlying causes are physical agents (heat, cold, light, friction), autoimmune reactions or psychic stimuli.