[phobosanomaly]

Mucosa has vascular supply.

Naumova EA, Dierkes T, Sprang J, Arnold WH. The oral mucosal surface and blood vessels. Head Face Med. 2013 Mar 12;9:8. doi: 10.1186/1746-160X-9-8. PMID: 23497446; PMCID: PMC3639856.

It also has antibodies (humoral immune response).

Immunoglobulin A (IgA) is the most abundant antibody isotype in the mucosal immune system.

Li Y, Jin L, Chen T. The Effects of Secretory IgA in the Mucosal Immune System. Biomed Res Int. 2020 Jan 3;2020:2032057. doi: 10.1155/2020/2032057. PMID: 31998782; PMCID: PMC6970489.

And the vaccine triggers the development of those antibodies.

We evaluated the serum anti-spike (anti-S) IgG, anti-nucleocapsid (anti-N) IgG and anti-S IgA response following vaccination against SARS-CoV-2 in a cohort of first-responders. Among the 378 completely vaccinated participants, 98% were positive for anti-S IgG and 96% were positive for anti-S IgA.

Montague, B.T., Wipperman, M.F., Chio, E. et al. Elevated serum IgA following vaccination against SARS-CoV-2 in a cohort of high-risk first responders. Sci Rep 12, 14932 (2022). https://doi.org/10.1038/s41598-022-19095-7