Without any doubt, one of the most important roles of the immune system is protection against various pathogens, including bacteria, viruses, fungi and parasites. Currently, it is well known that any changes in its activity lead to an increase in the frequency of infectious diseases. There are some data available, suggesting that in patients with psychiatric disorders, such as depressive disorder, schizophrenia or bipolar disorder, immune system reactivity is modified. In this review, the activity of some innate immunity elements, such as the number and the cytotoxic activity of NK cells, the activity of the complement system, the number of neutrophils, the activity of monocytes/macrophages, the concentration of reactive oxygen species and reactive nitric species, the concentration of acute phase proteins (e.g. C-reactive proteins, mannose-binding lectin) in patients with mental disorders is presented. Also, cytokine and chemokine concentrations (IL-1β, IL-4, IL-6, IL-8, IL-10, IP-10, MCP-1, TNF, IFN-γ) in patients with psychiatric disorders are discussed. Finally, the alterations in adaptive immunity response, including B cell and T cell subpopulation number, and immunoglobulin levels (IgG, IgM, IgA, IgE) are demonstrated. It should be stressed, however, that data concerning the immune system reactivity in patients with different mental disorders are still scarce and ambiguous. Further studies are needed, therefore, to fully explain the relationship between psychiatric disorders and immune system functioning.