Allergy (steps of development of allergy, allergens and few allergic diseases in humans)

Normally, the immune response eradicates infecting organisms without any serious injury to host tissues.
However, sometimes the responses are inadequately controlled or inappropriately targeted to host tissues, leading to various diseases or unusual conditions.
Literally, the term ‘allergy’ means the condition of being unusually sensitive to particular substances such as foods, pollens, insect stings etc.
Allergy is also called immediate hypersensitivity because it begins rapidly, within minutes of antigen challenge and has major pathologic consequences like in Hay fever, asthma, serum sickness, systemic anaphylaxis, contact dermatitis etc.
Allergy is an immune response / reaction that are triggered by antigen binding to IgE, pre-attached to mast cells which lead to inflammatory mediator release.
These mediators collectively cause increased vascular permeability, vasodilation and bronchial and visceral smooth muscle contraction.

Steps of development of allergic reactions:

Exposure to an antigen or entry of antigen inside the body
Activation of TH2 cells and B cells specific for the antigen
Production of IgE antibody
Binding of IgE antibody to Fc receptors of mast cells
On the re-exposure of antigen, mast cells will be triggered
Finally, activation of mast cells and release of different mediators such as lipid mediators and cytokines that induce inflammations.
Along with mast cells, basophils and eosinophils also play the important role as effector cells on allergic reactions.
Basophils are blood granulocytes with structural and functional similarities to mast cells.
Eosinophils are the bone marrow derived granulocytes that are abundant in the inflammatory infiltrates of late phase reactions and contribute to many allergic diseases

What are allergens?

Antigens that elicit immediate hypersensitivity reactions (allergy) are termed as allergens.
Biochemically, most allergens are proteins or chemicals bound to proteins.
Typical allergens include proteins in pollen, house dust mites, animal dander, different foods and chemicals such as various antibiotics including penicillin etc.

Few allergic diseases in humans include:

Systemic Anaphylaxis

Anaphylaxis is a systemic allergic reaction characterized by edema on many tissues and fall in blood pressure, secondary to vasodilation.
Introduction to antigen by injection, an insect sting or absorption across an epithelial surface may lead to anaphylaxis.
Some anaphylactic reactions may lead to a condition called Anaphylactic Shock, which is often fatal.
Anti-histamines may be beneficial to reduce the effects of anaphylaxis

Bronchial Asthma

Asthma is an inflammatory disease caused by repeated immediate hypersensitivity and late phase reactions in the lungs.
Corticosteroids and Theophylline are commonly used anti-asthma drug.

Allergic Rhinitis:

It is also called Hay fever and is the most common allergic disease.
Most common allergens such as plant pollen, house dust mites etc. can cause allergic rhinitis.
The pathologic and clinical manifestations include mucosal edema, leukocyte infiltration with mucous secretion, coughing, sneezing, difficulty in breathing and allergic conjunctivitis.
Antihistamines are commonly used in this condition too.

Food allergies:

Peanuts and shellfish are the most common allergens in food allergy; however, other different types of foods may also act as allergens.
Clinical manifestations include enhanced peristalsis, increased fluid secretion from intestinal lining cells and may be associated with vomiting and diarrhea.
Most commonly, antihistamines and corticosteroids are used for the treatment.

Allergy may also be beneficial….!!!

Hypersensitivity reactions are almost harmful. However, immediate hypersensitivity reactions provide protections against helminthic reactions by promoting IgE.
Furthermore, mast cells play an important role in innate immune responses to bacterial infections.

Online Science Notes