ATTACK OF THE KILLER POLLEN: CAN CHRONIC INFLAMMATION FROM ALLERGIES LEAD TO THE DEVELOPMENT OF DEPRESSION?

Abstract

Allergies are very common world-wide, affecting between 10-30% of the population. (American Academy of Allergy, Asthma & Immunology [AAAAI], 2024b). In the United States, more than 1 in every 4 individuals has some form of allergy (Centers for Disease Control and Prevention [CDC], 2023a). A Type I hypersensitivity reaction is the mechanism underlying many allergic diseases, including allergic rhinitis and asthma (Cohen, 2023). During this reaction, the body releases inflammatory mediators, such as histamine, and other signaling molecules called cytokines. This is an incorrect immune response by the body to harmless allergens (substances that cause allergies). Chronic exposure to allergens may also lead to a persistent release of these substances, causing chronic inflammation (Galli et al., 2008). Interestingly, histamine and cytokines have also been implicated in the development of depression (Hersey et al., 2022; Osimo et al., 2020). Histamine release increases the permeability of the blood-brain barrier (BBB), potentially allowing for substances like histamine itself to cross into the brain (Abbott, 2000). In the brain, histamine can lower available levels of serotonin and dopamine, which are both associated with depression (Cowen & Browning, 2015; Dunlop & Nemeroff, 2007; Threlfell et al., 2004; Varaschin et al., 2018). Additionally, pro-inflammatory cytokines can also cross the BBB and have been associated with dysregulation of systems in the brain, ultimately resulting in symptoms of depression (Yang et al., 2022). Current rates of depression and other mental illnesses are very high, while access to mental health services in the U.S. remains low (Rapfogel, 2022; Word Health Organization, 2023a). Treating allergies may be one beneficial target in the prevention of depressive symptoms or as an intervention for the treatment of depression.