Antigen: Any substance that causes immune reactions. Antigens are usually protein-compounds such as pollens, foods, mites, etc.
Allergen: Any antigen that causes allergic reactions such as pollens, mites, foods,etc.
Antibody: People produce these molecules in response to antigen challenge. They are capable of producing myriad of different antibodies. As a group, they are called immunoglobulins(Ig). Ig can be sub-grouped into 4 different classes; IgG, IgA, IgM, and IgE. Antibodies to milk can be produced in any of the 4 classes.
IgE antibodies: Only allergic people can produce IgE antibodies.
Mast Cells: Residential cells (reside in the tissues such as nasal cavities and airway) which are involved in allergic reaction. IgE antibodies go to the surface of mast cells.
Hypersensitivity: Hypersensitivity refers to any immunological reaction which may be harmful to the host. Allergic reaction is one of the 4 classes of hypersensitivity.
Allergic reaction: Allergic people are genetically programmed to produce a special class of antibodies known as IgE antibodies. IgE antibodies go to the surface of mast cells that are present in various tissues such as nasal cavity. When allergens such as pollens arrive at the tissue (e.g. nasal cavity), mast cells interact with them through IgE antibodies on their surface. Once the interaction takes place, mast cells get activated and produce various mediators.
The best known and most important of all mast cell mediators are histamine and leukotrienes. These mediators produce allergic reactions; the major components of allergic reactions are;
• Smooth muscle spasm (as occurs in the airways of the asthmatics).
• Tissue swelling (as occurs in the nose of the people with hey fever).
• Mucus secretion (as occurs in the nose of a hey-fever patient.
• Irritation of the nerve endings causing itchiness as in eczematous skin.
• Inflammation resulting from the infiltration of other circulating cells.
• Tissue damage (fibrosis) if inflammation persists.
A different disease state will result depending on where these reactions manifest:
• Lungs > asthma
• Nose > allergic rhinitis (hey fever)
• Skin > atomic dermatitis (eczema) or urticaria(hives)
• Digestive system > allergic gastroenteritis
• Anaphylaxis > when allergic reactions occur involving many sites all at once. The blood pressure may fall because of the fluid may leave the circulation to the tissues. This is an emergency which needs to be treated right away.
Triggers of Allergic Reaction:
- Allergens: Any antigen that causes allergic reactions such as pollens, mites, foods,etc.
- Irritants: When the tissues get inflamed due to allergic reaction, they respond to non-allergic substances such as smoke, viral infection, and temperature change. People do not produce IgE antibodies to irritants. Therefore, there are no skin test or blood tests that can measure the degree of sensitization to irritants.
Allergy Skin testing: Introducing a small amount of allergen on the skin mast cells will cause swelling, if a person is allergic, i.e., if his/her mast cells produce mediators through IgE-mediated reaction.
RAST test: If skin testing is not feasible ( e.g., in eczematous skin), a blood test can be done to detect the amount of IgE antibodies. In general skin testing is considered more sensitive.