2.11. Summary of the reviewed literature: what is known and what should be studied?

Both depression and atopic disorders are illnesses of major public health concern in many Western societies. During the last few decades, evidence has been accumulating to indicate that there exist an association between these two disorders. There are several theories attempting to explain the co-occurrence of atopic disorders and depression: Firstly, since atopic allergy itself is an IgE-mediated immune system disorder and depression has been shown to be associated with changes in altered immunity, it could be postulated that certain immune mediators, for example cytokines, could contribute to the mechanism behind both allergic and depressive symptoms. Secondly, in both disorders there exist many other common biochemical features, such as changes in the function of the HPA-axis. Thirdly, psychosocial/environmental factors can also play a role, assuming a common etiology behind these disorders, because it is known that e.g., parental criticism is associated with both disorders. Finally, it has recently been postulated that a majority of the association between atopic disorders and depression is likely due to common genetic effects. On the other hand, the association between atopic disorders and depression might be non-specific, and therefore could be explained by some third factor, because, e.g., increased risks of depression have been noted in connection with many other physical diseases than atopic disorders as well. Any associations between physical disorders other than atopic disorders and depression are, however, beyond the scope of this thesis.

Most of the previous studies that concern themselves with the association between atopic disorders and depression are, however, based on restricted data sample settings, in which the association between the two disorders has been investigated by studying patients suffering from either depressive or atopic symptoms. Thus, there is a need for large epidemiological studies with general population databases to investigate this putative atopy-depression association.

The interpretation between different studies concerning the association between atopic disorders and depression is hampered due to differences in definitions of depression/depressive symptoms; many different rating scales have been used in case definitions. Whether the strength of the association between atopic disorders and depression is dependent on the severity of depression is, in addition, a very sparsely studied area. Therefore, the association should be investigated according to different grades of severity of depressive symptoms, e.g., self-reported depression in the general population versus severe depression requiring hospital admission. Earlier studies are also heterogeneous in terms of defining atopy, and the presence of atopic disorders is not necessarily verified by objective measurements such as skin prick test responses.

Even though there exist a few investigations which show an increased rate of depression especially among mothers of atopic children, and even though it has been suggested that a majority of the association between atopy and depression is due to additive genetic effects, there is a considerable lack of studies investigating the putative genetic linkage between atopy in first degree relatives and depression of a proband.