3.1 Cytokine storm in COVID-19
In response to massive amounts of released cytokines, a form of inflammatory reaction would be triggered, namely cytokine storm syndrome, that is characterized by higher IL-6 levels. This is obviously detected in seriously ill COVID-19 patients and may further weaken their immune response (Dal Moro and Livi, 2020).
Because of the positive correlation between high IL-6 level and COVID-19 severity, IL-6 is specifically suggested to be the master marker used for monitoring disease progression (Liu et al., 2020b). There a growing evidence that IL-6 can play a crucial part in the uncontrolled intestinal inflammatory process, proving its role in the pathogenesis of COVID-19-asociated diarrhea. However, another causing factor may be attributed to the direct viral invasion of gut epithelial cells via ACE-2 (Mudter and Neurath, 2007).
As previously reported, IL-6 could prohibit the olfactory signal pathway; proposing that anosmia detected in COVID-19 patients may be due to IL-6-mediated inflammation of the nasal mucosa (Henkin et al., 2013; Luërs et al., 2020). Besides, other additional elements supporting that SARS-CoV-2 may have a neuro-invasive propensity to invade the central olfactory pathway causing an olfactory dysfunction (Marinosci et al., 2020). Jointly, IL-6 was also found to be extremely involved in promoting the ocular inflammation; matching with conjunctivitis that is recently reported to be linked with COVID-19 infection (Ghasemi, 2018).