Granulocytes comprise four main cell types - neutrophils, eosinophils, basophils, and mast cells. They are the first cells to be recruited to the site of an infection and have an essential role in initiating the innate immune response. Neutrophils are the most abundant granulocytes, representing around 40-60% of all circulating leukocytes, and can both phagocytose pathogens or destroy them by releasing granules containing anti-microbial factors such as hydrolase enzymes. Eosinophils make up just 1% of circulating leukocytes and are recruited into tissues at sites of inflammation; here, they release an array of inflammatory mediators to regulate the immune response. Basophils are similar in abundance to eosinophils and are produced in response to parasitic infections; they are also involved in the allergic response, releasing histamine following activation by antigen crosslinking of FcεRI receptor-bound IgE. A main difference between mast cells and other granulocytes is that mast cells reside in mucosal and epithelial tissues rather than in the blood; they are primarily involved in the IgE-dependent allergic response and their granules contain inflammatory mediators that include histamine, heparin, and serotonin.