**3. Mechanism of immune systems in the human body against COVID-19**

Both innate and adaptive immunity are required to fight against SARS-CoV-2. The innate immunity includes monocytes, dendritic cells (DCs), granulocytes, and natural killer (NK) cells, while the adaptive immune system includes B and T cells. Severe patients are characterized by the following: Lymphopenia with a drop in CD4+ and CD8+ T cells, lymphocyte activation and dysfunction, an increase in circulating neutrophils with the appearance of circulating neutrophil precursors, and loss of the monocytes function and impaired the function of NK and DCs [19, 20].

Inflammatory cytokine levels are elevated, particularly interleukin-6 and IL-1 [21]. On the other hand, the interferon response is delayed, while the levels of immunoglobulin G (IgG) and total antibodies are increased [22, 23]. Immune disorders, which are characterized by an elevated inflammatory profile, are frequent in severe infections and sepsis and finally ends up with immunosuppression. For severe COVID-19, a similar method has been proposed [24, 25]. Owing to the absence of the effective antiviral therapy, the immunological response of the body is a critical element in disease severity and clinical outcome. Thus, clearer picture regarding the cellular immune response through the disease progression is highly required for establishing diagnostic indicators and potential therapeutic strategies against COVID-19.

During the SARS-CoV-2 infection, the cellular and molecular cascades orchestrate the activation, recruitment, and resolution of the antiviral immune response. These cascades fine-tune the balance between viral eradication and immune injury. Multiple innate immune identification pathways fight viruses during infection [26]. Within the first few hours, The innate immune system inhibits virus replication by releasing type I/III interferon [27], pro-inflammatory cytokines (including: IL-1, IL-6, and IL-18), and chemokines (including: CCL2 and CCL7 and then adaptive immunity is activated.)

Following the SARS-CoV-2 infection, T cells are highly involved in the viral clearance process, whereas humoral immune response is mainly involved in the production of neutralizing antibodies to block the viral entry. T lymphocytes directly attack the infected cells, and they stimulate the cytokines production to boost the immune response of T lymphocytes and other immunocompetent cells such as B lymphocytes and macrophages. To defend the host from nonspecific injury, the body then diminishes innate immunity [28].

Innate immune cells (DCs and macrophages) and adaptive mediated- cell types (regulatory B cells and T cells) establish an inflammation repair status when viruses have been eradicated.
