**1. Introduction**

Inflammation is a vital host response to the loss of cellular and tissue homeostasis. Inflammation plays a major role in the pathogenesis of metabolic and behavioral abnormalities during cancer, diabetes type II, and neuropsychiatric disorders.

Neuroinflammation is a term used when there is an inflammation of the nervous tissue, in general, and of the central nervous system (CNS), in particular. Neuroinflammation has been suggested as one of the main drivers of neurotoxic symptoms. It is detrimental, and implies that glial cell activation happens prior to neuronal degeneration and, possibly, causes it.

In neuroinflammation, inflammation-like glial responses that do not reproduce classic characteristics of inflammation in the periphery may be initiated in response to a variety of cues, including infection, traumatic brain injury, toxic metabolites, or autoimmunity [1]. Activation of microglia results in their morphological and phenotypical changes and in the release of inflammatory mediators such as cytokines and chemokines [1]. Molfino et al. [2] further suggest that neuroinflammation is both triggered and perpetuated by the activation of microglia, that the release of inflammatory mediators occurs within hypothalamic areas and that the inflammatory response of activated microglia serves to further engage the immune system and initiate tissue repair.

Complex diseases are diseases caused by a combination of genetic, environmental, and lifestyle factors, most of which have not yet been identified. Thus, these diseases do

**Citation:** Asslih, S.; Damri, O.; Agam, G. Neuroinflammation as a Common Denominator of Complex Diseases (Cancer, Diabetes Type 2, and Neuropsychiatric Disorders). *Int. J. Mol. Sci.* **2021**, *22*, 6138. https:// doi.org/10.3390/ijms22116138

Academic Editor: Anne Vejux

Received: 12 April 2021 Accepted: 31 May 2021 Published: 7 June 2021

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not obey the Mendelian pattern of inheritance, and genetic factors represent only part of the risk associated with complex disease phenotypes. A vast majority of diseases fall into this category, including Alzheimer's disease, scleroderma, asthma, Parkinson's disease, multiple sclerosis, osteoporosis, connective tissue diseases, kidney diseases, autoimmune diseases, and more. Intriguingly, a common denominator of many of these disorders is neuroinflammation.
