Eosinophils are a type of white blood cell that play an important role in your immune system. They have various functions in your body similar to other white blood cells. However, two roles stand out the most; they help to fight and destroy parasites and boost inflammation to help your body fight off disease. Eosinophils are the reason you develop hay fever when your body is facing allergies or asthma.

As mentioned, eosinophils are a type of white blood cell, more specifically, they are a subset of granulocytes. There are three types of granulocytes: neutrophils, basophils, and eosinophils. Like all types of white blood cells, eosinophils are produced in the bone marrow. They mature in the bone marrow for 8 days and are then released into the blood. They travel through blood vessels for 8 to 12 hours until they reach the necessary tissues and spend 1 to 2 weeks there.

Eosinophilia, or an increased number of eosinophils in your body, occurs when your bone marrow produces too many eosinophils. It can be caused by various conditions. It can be caused by common and less serious conditions such as asthma or nasal allergies caused by, e.g., pollen, dust mites or animal dander. Eosinophilia can also be connected to more severe conditions like blood disorders or cancers e.g., leukemia, endocrine disorders, parasite or fungal infections, drug allergies, or eczema.

Low levels of eosinophils are less common but can be connected to alcoholism or overproduction of certain steroids, which can occur in Cushing’s disease. If low levels of eosinophils are combined with low levels of other white blood cells, there is a chance that there is a problem affecting the bone marrow. Eosinophil levels also vary throughout the day. Usually, the highest eosinophil count occurs in the evening and the lowest in the morning.