Figure 6.

Origin of secretory IgA (SIgA), secretory IgM (SIgM) and IgG antibodies specific to SARS-CoV-2 in human milk. (A) In the mammary gland tissues’ interstitial space, plasma cell secretes IgA, which can bind to the polymeric Ig receptor (pIgR). (B) IgA binds via their Fc region to pIgR to form the pIgR–IgA complex. (C) The pIgR–IgA is transported in a vesicle across the mammary epithelial cells (MEC) to the alveolar lumen. (D) SIgA is released from the pIgR–IgA complex by proteolytic cleavage into the alveolar lumen, and then SIgA is diffused in human milk. (E) IgM is produced by plasma cells in the interstitial space and can bind to pIgR to form the complex pIgR–IgM. (F) pIgR–IgM is transported in a vesicle across the MEC to be released in the alveolar lumen. (G) SIgM is released from the pIgR–IgM complex by proteolytic cleavage into the alveolar lumen, and then SIgM is diffused in human milk. (H) IgG can diffuse from maternal blood after binding to neonatal Fc receptor (FcRn) (I) on the basolateral membrane of the MEC. (J) FcRn transport IgG via a vesicle to (K) the alveolar lumen. (L) Deactivated or intact viruses in human milk could activate the adaptive immunity to recognize the viral proteins from coronaviruses, including S1, S2, RBD, and nucleocapsid. Created with BioRender.com.