Types of allergies
Allergies are classified into IgE mediated and non-IgE mediated allergies.
IgE mediated allergy
In IgE mediated allergy the immune system produces great amounts of a class of antibodies known as IgE antibodies that are, specific for the particular offending allergen. These IgE antibodies bind to the surface of cells in the body called “mast cells” which become IgE-sensitized. These cells can then identify particular allergens the next time they come in contact with the body. This process is called sensitization, and at this stage there are no physical symptoms of an allergy.
Mast cells are present in the skin, eyes, nose, mouth, throat, stomach and gut. The next time that we get in contact with the same allergen the mast cells identify it as an enemy and produce histamine and other chemicals. The release of these substances from mast cells induces the allergic symptoms. In the nose histamine release results in symptoms of runny nose, itchy nose, sneezing that are commonly associated with itchy red eyes.
In the skin symptoms include redness and nettle rash. In the breathing tubes allergies cause wheezing, cough and shortage of breath, whereas in the gut symptoms such as abdominal discomfort, nausea, vomiting and diarrhea may occur. Severe allergic reactions are also known as anaphylaxis, and can be life-threatening.
Non-IgE mediated allergy
Non-IgE-mediated reactions, which are poorly defined both clinically and scientifically, are believed to be T-cell-mediated. This mechanism is related to disorders such as contact eczema (allergic contact dermatitis). Whereas symptoms of IgE mediated allergies occur rapidly and soon after exposure to the allergen, this may not be the case with non-IgE mediated allergies where symptoms may appear much later, usually 24-48 h.
Acute response: this is what we usually call allergy. The immediate reaction occurs within 15 - 30 minutes of exposure to the allergen. During the early phase reaction chemical mediators released by mast cells including histamine, prostaglandins, leukotrienes and thromboxane produce local tissue responses characteristic of an allergic reaction. In the respiratory tract for example, these include sneezing, edema and mucus secretion, with vasodilatation in the nose, leading to nasal blockage, and bronchoconstriction in the lung, leading to wheezing.
Late-phase response: Occurs 4-6 hours after the disappearance of the first phase symptoms and can last for days or even weeks. During the late phase reaction in the lung, cellular infiltration, fibrin deposition and tissue destruction resulting from the sustained allergic response lead to increased bronchial reactivity, edema and further inflammatory cell recruitment. These observations suggest that IgE is instrumental in the immune system's response to allergens by virtue of its ability to trigger mast cell mediator release, leading directly to both the early and late phase reactions.
Which elements are involved in the allergic reaction:
Allergen Usually a protein, that can generate an allergic reaction.
Immunoglobulin (IgE) An antibody involved in allergic reactions.
Mast cell Are immune system cells which are located in the skin, respiratory tract and digestive tract. IgE molecules are attached to their surface. Histamine and other mediators are produced by mast cells, which are released during the allergic reaction inducing allergy symptoms.
Histamine Is stored within the mast cell and released during the allergic reaction. Has the ability to dilate blood vessels (vasodilation), increase permeability of blood vessels (fluid leak) and stimulates the nerves. This results in redness, swelling and itching.
Causes of Allergies.
Real allergies are almost always triggered by large protein molecules. Proteins that are known to have the capacity to cause allergies are called allergens. Substances in the environment known as allergens are triggers of an allergic reaction. Nearly anything can be an allergen for someone.
The most common causes are: