Medical Definition of Allergic reaction
Allergic reaction: The hypersensitive response of the immune system of an allergic individual to a substance.
an allergen enters the body, it causes the body's immune system to
develop an allergic reaction in a person with an allergy to it. This
reaction can occur when the immune system attacks a normally harmless
substance (the allergen). The immune system calls upon a protective
antibody called immunoglobulin E or IgE to fight these invading
substances. Even though everyone has some IgE, an allergic person
has an unusually large army of these IgE defenders -in fact, too many
for their own good. This army of IgE antibodies attacks and engages
the invading army of allergic substances of allergens. As is often
the case in war, innocent bystanders are affected by this battle.
These innocent bystanders are special cells called mast cells. When a
mast cell is injured or irritated, it releases a variety of strong
chemicals, including histamine, into the tissues and blood that
promote allergic reactions. These chemicals are very irritating and
cause itching, swelling, and fluid leaking from nearby cells. These
allergic chemicals can cause muscle spasm and can lead to lung airway
and throat tightening as is found in asthma and loss of voice. They
are also what leads to the familiar hay fever or allergic rhinitis and
common pink eye.
Source: MedTerms™ Medical Dictionary
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