In this Article

• Who are the "players" in the allergic cascade?
• What about a more detailed look at the "players?"
• What are cytokines?
• What is the "early phase" of an allergic reaction?
• What is the "late phase" of an allergic reaction?
• What are the consequences of the allergic cascade?
• How does understanding the allergic cascade help?
• Allergic Cascade At A Glance
• Allergic Cascade Glossary
• Allergic Cascade Index

What are cytokines?

Cytokines are small proteins that can either step-up or step-down the immune response. One of the cytokines, interleukin 4 (IL4), is essential for the production of IgE. Interleukin 5 (IL5) and others are important in attracting other cells, particularly eosinophils, which then promote inflammation. This spectrum of cytokines is also released by the TH2 lymphocytes, thus further promoting allergic inflammation.

What is the "early phase" of an allergic reaction?

We have seen how the first encounter with ragweed pollen sensitizes the body with the help of lymphocytes and results in the IgE coating of the mast cells and basophils. Subsequent exposure results in the immediate release of the chemical mediators that cause the various symptoms of allergy. This process is the "early phase" of the allergic reaction. It can occur within seconds or minutes of exposure to an allergen. This is also known as an immediate hypersensitivity reaction, which in this case is to the ragweed pollen allergen.

In the context of allergy, hypersensitivity refers to a condition in a previously exposed person in which tissue inflammation results from an immune reaction upon re- exposure to an allergen sensitizer.

What is the "late phase" of an allergic reaction?

About 50% of the time, the allergic reaction progresses into a "late phase." This "late phase" occurs about 4 to 6 hours after the exposure. In the late phase reaction, tissues become red and swollen due to the arrival of other cells to the area, including the eosinophils, neutrophils, and lymphocytes. Cytokines that are released by the mast cells and basophils act as tiny messengers to call these other cells to the area of inflammation. Additional cytokines are released by the TH2 lymphocytes and they attract even more of these cells of inflammation.

The eosinophils appear to be particularly troublesome cells of inflammation. Eosinophils evolved to defend the body against parasites, much like IgE. Nevertheless, they are often present in great numbers in the blood of people with allergies. When they arrive at the site of the allergic reaction, they release chemicals that cause damage to the tissues and continue to promote the inflammation. Repeated episodes of this "late phase" reaction contribute to chronic allergic symptoms and make the tissues even more sensitive to subsequent exposure!