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Basophils are the least common of the granulocytes, representing about 1% of circulating leukocytes. They contain large cytoplasmic granules which obscure the nucleus under the microscope. However, when unstained, the nucleus is visible and it usually has 2 lobes. A cell in tissues, the mast cell, has many similar characteristics. For example, both cell types store histamine, a chemical that is secreted by the cells when stimulated in certain ways (histamine causes some of the symptoms of an allergic reaction). Like all circulating granulocytes, basophils can be recruited out of the blood into a tissue when needed. Basophils tend to appear in specific kinds of inflammatory reactions, particularly those that cause allergic symptoms. While the exact purpose of basophils has never been proven, they appear often in tissues where parasites are found. They can be found in unusually high numbers at sites of exoparasite infection, e.g., ticks. They also appear in tissues where allergic reactions are occurring and probably contribute to the severity of these reactions. Basophils have protein receptors on their cell surface that bind IgE antibody very tightly. It is the bound IgE antibody that confers a selective response of these cells to environmental substances, for example, pollen proteins. When activated, basophils secrete histamine, several proteoglycans, lipid mediators like leukotrienes, and several cytokines. Histamine and proteoglycans are pre-stored in the cell's granules while the other secreted substances are newly generated. Each of these substances contributes to inflammation. Recent evidence suggests that basophils are an important source of the cytokine, interleukin-4, perhaps more important than T cells. Interleukin-4 is considered one of the critical cytokines in the development of allergies and the production of IgE antibody by the immune system. There are other substances that can activate basophils to secrete which suggests that these cells have other roles in inflammation.
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