Before an allergic cascade can be put into motion, an allergic person must first have come into prior contact with an allergen (e.g., pollen, dander, food protein) and become sensitized to that allergen. This sensitizing process involves the following:

• The immune system encounters the allergen and misinterprets it as a threatening invader to the body. Lymphocytes are the specific immune system cells that actually encounter the allergen.
  
  
• The lymphocytes react to the invader by releasing a specific type of antibody known as IgE (Immunoglobulin E). The IgE antibodies contain specific information about proteins in the invading allergen – essentially a description – so that the body can remember if it ever encounters the same allergen again.
  
  
• The IgE antibodies flow through the bloodstream and eventually attach themselves to mast cells and basophils (volatile cells that can suddenly self–destruct and release powerful chemicals) and wait for months or years for another encounter with the allergen. Essentially, the IgE operates as a fuse and the mast cells and basophils operate as bombs.

This process of sensitizing a person to an allergen usually takes seven to 10 days. However, it does not produce any allergic symptoms. People do not become aware that this process has occurred until the next time that they encounter that allergen, which triggers an allergic cascade. This chain of reactions has several links, including:

• The allergen to which a person is sensitized enters the body. It may be inhaled through nasal passages, come in direct contact with the skin, be injected under the skin, or be ingested. The allergen flows through the bloodstream and encounters the IgE antibodies, which are attached to mast cells and basophils.
  
  
• The IgE antibodies recognize the invader and bind to the allergen molecule. This triggers the mast cells and basophils to which they are attached to self-destruct.
  
  
• Powerful chemicals from granules inside the mast cells and basophils are released into surrounding tissues and the bloodstream. These include histamines, leukotrienes and other allergy stimulators.
  
  
• Allergy symptoms begin to appear. These may be localized (only in the area where these chemicals were first released) or systemic (throughout the entire body). These chemicals mainly affect the blood vessels, mucous glands and bronchial tubes.

The allergens are therefore the match that lights the fuse (IgE) that triggers the bomb (mast cells and basophils) to explode.

Sometimes one substance is similar enough to another that the immune system will mistake it for a known allergen and trigger the allergic cascade. For example, a protein in latex is similar to a protein found in fresh fruits, vegetables and nuts. People with an allergy to latex often have allergic reactions to these foods. This phenomenon is known as cross reactivity.