Fig. 4

Mucosal immune system in the gastrointestinal tract. Macrophages and dendritic cells below the intestinal epithelial layer are specialized antigen-presenting cells (APC). DCs ingest antigen mainly in three ways: ① DCs extend directly out of the intestinal epithelial cell layer to capture the severe acute respiratory syndrome-coronavirus-2; ② M cells in the epithelial cell layer transport external viruses to the PPs to be ingested by DCs; ③ DCs indirectly ingest coronavirus disease 2019 antigen by ingesting infected intestinal epithelial cells. After presenting antigens, the APC moves down the germinal center of the PPs and lymph follicles and activates naive T and B cells. B cells then differentiate into plasma cells to secrete antibodies. Secretory dimeric immunoglobulin A (IgA) are joined by the J chain to polymeric Ig receptor (pIgR) located only at the basement of the enterocyte. Thereafter, enterocytes endocytose the IgA–pIgR complex, transporting it to the upper side. Finally, pIgR is cleaved and IgA is exocytosed (SIgM can be transported to the mucosa by this mechanism). DCs bind to T-cell receptors on the surface of naive T cells (whereas CD40L on the surface of the T cells binds to CD40 on the surface of the dendritic cells), causing them to differentiate into CD4 + T cells. CD28 on CD4 + T cell and antigen presenting MHC on DCs bind to cell surface receptors of naive B cells, causing them to differentiate into plasma cells and secrete antibodies. Fc ___domain-mediated functions: The infected cells can express antigens to help antibodies find them. (I) antibody-dependent cellular cytotoxicity, CD8 + T cells recognize infected cells by Fc ___domain of antibodies, then secret perforin and granzyme to lyse them; (II) antibody-dependent phagocytosis, macrophages (or other APCs) phagocytose antigen-antibody complex and infected cells after binding with the antibodies on them; (III) antibody-mediated complement-dependent cytotoxicity, combining with antibodies, complement proteins form membrane attack complex on the surface of infected cells, which induced cell lysis soon