Figure 11.

Innate and adaptive immunity in SARS-CoV-2. [Left circle] The innate immune response functions as the first line of defense against infection. It consists of soluble factors, such as native (n) and monomeric (m) C-reactive protein (CRP) produced by liver and macrophages locally; innate IgM produced by B cells; complement proteins (C1q), and diverse cellular components including granulocytes (basophils, eosinophils and neutrophils), mast cells, monocytes, macrophages, dendritic cells, regulatory T cells (Tregs) and natural killer cells. [Middle intersection] Natural killer T cells and γδ T cells are cytotoxic lymphocytes that straddle the interface of innate and adaptive immunity. [Right circle] The adaptive immune response is slower to develop, but manifests as increased antigenic specificity and memory. It consists of antibodies, B cells that produce adaptive IgM and other antibodies, Th1 CD4⁺, CTL CD8+ T lymphocytes, Th2 cells, Th17 cells, Th9 cells, among others. SARS-CoV-2, severe acute respiratory syndrome coronavirus; Tregs, regulatory T cells; IL, interleukin; Th, T helper; TNF, tumor necrosis factor; IL1RA, IL-1 receptor antagonist; NETs, neutrophil extracellular traps; BETs, basophil extracellular traps; EETs, eosinophil extracellular traps; MCETs, mast cell extracellular traps; FcγRs, Fcγ receptors; C1q, complement 1q; C1qR, C1q receptor; IgM, immunoglobulin M; PC, phosphorylcholine; FcμR, Fcμ receptor; IL-6R, IL-6 receptor; CRP, C-reactive protein; nCRP, native CRP; mCRP monomeric CRP; TGF-β, transforming growth factor-β; TCR, T cell receptor; MHCII, major histocompatibility antigen class II; IFN, interferon; NK cell, natural killer cell; NKT cell, natural killer T cell; GM-CSF, granulocyte-macrophage colony-stimulating factor; iNOS, inducible nitric oxide synthase; COVID-19, coronavirus disease-2019; β2M, β2 microglobulin; MHC class I, major histocompatibility antigen class I; CTL, cytotoxic lymphocyte.