More and more, researchers are starting to see a connection between allergies and depression. There is evidence of relationships in both directions, so allergies exacerbate depression, while depressive symptoms and affect make allergy symptoms worse. One study found that this connection is partly responsible for an uptick in suicides as winter turns into spring. Part of this is a hope that depression is merely seasonal being dashed—people tough it out through the winter but go into a decline when spring doesn’t bring improvement—but now it appears that allergens increasing in the springtime also bears on this phenomenon.
Depression is thought of as a mental illness, but the brain is part of the body, and there’s no bright line, medically speaking, between physical symptoms and mental ones. One place where this is apparent is allergic reactions causing depressive symptoms. Allergens are normally harmless substances, such as pollen, that trigger an immune response in people who are sensitive to them. Part of this immune response is inflammation. Inflammation can lead to a low feeling called dysthymia; this is why people who are sick feel awful. Dysthymia is also a symptom of depression. Some scientists have proposed that a hitherto underestimated cause of depression itself is inflammation due to allergy.
Meanwhile, stress is both a cause and an effect of depression. Depression can be a response, in part, to stress, but t can also increase it. Stress is also intimately bound up with the immune system The stress hormone cortisol temporarily suppresses the immune system, only for it to come back turned up after the stressor has passed. That means that stress, anxiety, and depression actually make allergic reactions worse, particularly on the second day. That means not only does stress directly lead to depression, it also increases depression as a result o the inflammation from an allergy attack.
In children, scientists have also found a genetic link between allergies and depression, as well as behavioral problems. While allergies themselves exacerbate depression, and allergies and depression alike are partly responsible for children misbehaving, there is evidence that genetics is behind a predisposition to allergies, a predisposition to depression, and a tendency to act out.