7.1. Main conclusions of the results

An association between atopic disorders and depression has now been confirmed with the help of a large, genetically homogeneous birth cohort database. Generally, the existence of atopic disorders increases the risk of both self-reported and hospital-treated lifetime depression about two- to three-fold when compared with an absence of atopic disorders.

This thesis adds some important considerations to the earlier literature, one of them being the observed gender difference with regard to the association between atopy and depression: In atopic females, the association between atopic disorders and depression was present in connection with every level of the depressive symptoms, but the likelihood of depression increased in line with the increased severity of the depressive symptoms. On the other hand, in males an association between atopic disorders and depression was seen only in the most severe manifestations of depression.

Establishing an argument for causation is very difficult in epidemiological research, because of the many potential confounding factors. Due to the study design, the causal hypothesis could not be tested: One can not know whether suffering from atopic disorders is causing a person to suffer also from depression, or vice versa. It is also possible that both atopic and depressive disorders were caused by some third factor, which differs from those that were able to be taken into account as confounding factors. Since it is well established that the roots of atopic disorders exist already during pregnancy and in the first years of a person’s life (Björksten 1999), the finding of this thesis that maternal atopy or atopies in both parents combined with a subject’s own atopy were the most important predictors of a female proband’s depression is worthy of note: Inevitably, the presence of parental atopy precedes the presence of a proband’s depression, and, thus some evidence for the temporal sequence between atopy and depression could be provided. Earlier twin-studies have preferred to assume a common genetic rather than environmental etiology to exist behind these two disorders (Wamboldt et al. 1998, 2000). Since neither parental nor siblings’ atopies predicted any type of depression in male probands, it could be suggested that the putative genetic association (between parental atopic disorders and a child’s depression) involves female probands only. Thus, the results of this thesis are in support of recent findings that genetic factors might play a greater role in the etiology of depression in women than in men (Bierut et al. 1999, Kendler et al. 2001a). It must, however, be reminded that psychosocial familial factors might also explain at least part of the findings of this thesis.

Previous psycho-neuroimmunological studies have deepened our understanding about the body-mind connection. Inevitably one’s feelings have their biological/physiological correspondences, and vice versa. Dysfunction at biological or psychosocial level can cause an atopic disorder or depression of a person, and in the end, genes are involved with all of the functions of the body. Once the homeostasis of the body has been disturbed, the adaptive systems of the human body are starting their work in order to return to the homeostasis. There exist many psycho-neuroimmunological links, which could serve as reasonable explanations for the association between atopy and depression even though the exact mechanisms (or possible dysfunctions of them) are so far incompletely understood in this context.