Allergy – Mechanism, Symptoms, Risk factors, Diagnosis, Treatment and Prevention, Animation

By Adem Lewis / in , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , /

Allergy refers to abnormal reactions of the
immune system to otherwise harmless substances. Normally, the immune system raises immune
response to protect the body from foreign invaders, such as bacteria or viruses, but
does not react to non-infectious environmental antigens. In people with allergies, however, the immune
system also reacts to these substances, producing allergic reactions. Such substances, called allergens, can come
from the patient’s natural environment, foods, medications, latex products, or insect
bites. Most allergies are mediated by a class of
antibody called immunoglobulin E, IgE. IgE is produced when the body is first exposed
to an allergen. Production of IgE is activated by a subtype
of T-lymphocytes, known as type 2 helper T-cells, TH2. IgE molecules then bind to their receptors
on the surface of mast cells and basophils. The first exposure is usually asymptomatic,
but the body is now sensitized. Upon reexposure to the same antigen, the antigen
binds to adjacent IgE molecules, bringing their receptors together, triggering a signaling
cascade that induces the release of histamine and other inflammatory chemicals. These chemicals cause dilation and increased
permeability of blood vessels, mucus secretion, stimulation of sensory nerves, smooth muscle
spasms, and are responsible for allergic symptoms, which can range from mild to severe. Mild symptoms usually consist of watery eyes,
runny nose, sneezing and a mild rash; while severe reactions may include swelling, hives,
difficulty breathing due to bronchospasm, and digestive problems due to increased gastrointestinal
motility. When released systemically, these chemicals
can cause extensive vasodilation and smooth muscle spasms which may lead to anaphylaxis,
a life-threatening condition in which blood pressure drops and airways narrow to a dangerous
level. The reactions are immediate, within minutes
of contact with the allergen. There is also a late phase response, due to
subsequent tissue infiltration with eosinophils and other inflammatory cells. People who are sensitized to a specific allergen
may also react to other substances that contain similar antigens. This is called cross-reactivity. For example, people who are allergic to birch
pollen may also have reactions to certain fruits and vegetables such as apples or potatoes,
consumption of which can cause itching and swelling of the lips and oral cavity. Both genetic and environmental factors contribute
to the development of allergic diseases. Allergies tend to run in families. What is inherited is the susceptibility to
allergic reactions, due to irregularities in the makeup of the immune system. Early childhood exposures to bacterial and
viral infections are thought to suppress TH2 cells and are therefore protective against
allergic diseases. This theory, known as hygiene hypothesis,
implies that living in too sterile an environment is a risk factor for allergic diseases. While still a hypothesis, it does partly explain
the higher prevalence of allergies in developed countries. Other risks factors include exposure to allergens
and stress. Diagnosis is usually based on symptoms and
patient’s history. Potential allergens may be identified with
skin prick test or intradermal test, where small amounts of common allergens are introduced
into the skin and local reactions are observed. A blood test, called allergen-specific serum
IgE test, can also be performed. In this case, patient’s blood sample containing
IgE is tested for binding to common allergens. If binding occurs, the person is allergic
to that allergen. Antihistamines are effective for treatment
of mild allergies. Other drugs include mast cell stabilizers,
corticosteroids and leukotriene modifiers. Severe reactions require immediate injection
of epinephrine. The best way to prevent allergies is to avoid
the offending allergens. People with serious reactions to unavoidable
allergens may benefit from immunotherapy. In immunotherapy, patients are injected weekly
with gradually increasing doses of the allergen, starting with a tiny amount. This process desensitizes the immune system,
reducing reactions to the allergen, but may take several years to complete.

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