Eosinophilia is a condition in which the quantity of eosinophils in the blood is excessively high. The white blood cells are an essential component of the cellular immune system. Eosinophils are white blood cells that are a part of the cellular immune system. They help with normal physiologic processes and host immunity, as well as allergic reactions and parasite infection resistance.
When there are more than 500 eosinophils per microliter, eosinophilia is considered to occur, though the exact cutoff varies by laboratory. Eosinophilia can be mild, moderate, or can even be severe. Usually, less than 5% of the circulating white blood cells are eosinophils.