The immune system is a double-edged sword. While its primary role is to fight infections, it can also become overactive, leading to problems like allergies and autoimmune diseases.
For example, the part of the immune system responsible for resisting parasites acts by releasing white blood cells called eosinophil granulocytes into the blood. But elevated eosinophil levels are also responsible for allergic reactions, including most forms of asthma, gastrointestinal diseases, blood disorders, and cancers.