White blood cells hone in on the site of infection and secrete enzymes to defend the nasal passages from the intruders. In particular, it’s neutrophils that are responsible for the color change. As part of an immune response dubbed the “oxidative burst,” neutrophils secrete an enzyme called myeloperoxidase, which has a green hue to it. Myeloperoxidase is one component of the highly reactive, free radical storm that the immune system unleashes on invading bacteria. Myeloperoxidase catalyzes a reaction to produce hypochlorous acid, a strong oxidizing agent, when it reacts with hydrogen peroxide. The presence of hypochlorous acid, which is one of the ingredients that makes bleach an effective sanitizer, helps eliminate the bacterial infection.

Fun fact: Myeloperoxidase secreted at sites of inflammation can even break down carbon nanotubules implanted in the body.