An allergic response is the body’s overreaction to some substance it believes to be a threat. The substance (called an antigen) may be completely harmless, but the body produces specific antibodies to combat this “threat” regardless. Antibodies are proteins that attach to foreign substances (e.g., bacteria, viruses) and help destroy them.

The allergic cascade refers to the chain of events that takes place when an allergen triggers an allergic response. The cascade includes the allergic response, the cellular and chemical interactions that follow, and the symptoms that usually result.

The key players in an allergic cascade include:

• Allergens. Substances that the body mistakenly perceives to be a threat. In response, the immune system reacts by producing antibodies. Allergens are a type of antigen. Examples of common allergens include pollen, mold, dust, animal dander, food and drugs.
  
  
• IgE (immunoglobulin E). A specific type of antibody that attaches to mast cells and basophils. IgE antibodies are like fuses to mast cells and basophils, waiting to be “lit” by a specific type of antigen.
  
  
• Mast cells and basophils. Types of white blood cells that reside in the mucous membranes and other tissues and contain granules of histamines and leukotrienes. They are like bombs. When IgE antibodies come in contact with an antigen, these cells release chemicals such as histamines and leukotrienes.
  
  
• Histamines and leukotrienes. Released by mast cells into surrounding tissues and the bloodstream, these chemicals directly cause allergy symptoms. They are called chemical mediators because they tell other cells to behave in a certain way. For instance, they tell blood vessels to dilate, which can cause symptoms ranging from nasal congestion to a drop in blood pressure and shock.

Other players include a type of white blood cell called TH2 cell – also called the “allergy cell” because of its central role in allergies. A brief explanation of both TH1 and TH2 cells is as follows:

• TH1 cells. These cells are not usually associated with the allergic cascade. Instead, these cells are used to fight infections by releasing chemicals that destroy microbes. TH1 cells are the body’s normal response to a foreign invader. However, when the body does not have enough TH1 cells, it is often forced to rely more heavily on TH2 cells.
  
  
• TH2 cells. These cells are responsible for sensitizing a person to a specific allergen and later, when the allergen is present in the body again, triggering an allergic response. When there are not enough TH1 cells, the body usually finds itself more sensitive to allergic reactions due to the increased reliance on TH2 cells.

The tendency toward some allergies (e.g., cow's milk, cat dander) appears to be genetic. People with a tendency for allergies are said to be atopic. Scientists are working to understand why some people are more prone to allergic reactions than others. It also appears that some types of allergies (e.g., poison ivy allergy) do not have a hereditary basis.

By understanding how these cells and chemical mediators interact, scientists can target different parts of the chain. For instance, some antihistamine medications work by preventing histamine molecules from binding with receptors in the body. Others may alter the histamine molecules themselves.