COVID-19 Vaccines

The mRNA COVID-19 vaccine contains mRNA encoding the SARS-CoV-2 spike protein that is responsible for inducing host immune responses. The mRNA is protected by a lipid coating which preserves it in an intact form protecting it from natural enzymes. Once the vaccine is administered by intramuscular injection in the upper arm of the patient, the vaccine particles bump into dendritic cells and infiltrate them; crossing of the cell membrane is facilitated by the lipid coating. Upon release of the mRNA inside the cytoplasm of the vaccinated cell, ribosomes translate it into the viral spike protein. The mRNA from the vaccine is eventually destroyed so no permanent traces are left in the vaccinated cells. Peptide fragments of the viral spike protein are bound to other molecules called MHC-I and MHC-II and migrate to the surface of the vaccinated cell. There can be other cells transfected. Dendritic cells can make whole spike proteins that are both presented to B-cells and processed. Dendritic cells can take up the spike protein or dead transfected cells and process for MHC presentation, but this is likely very minor. B cells bind spike via surface Ig and interact with T cells to expand and produce antibody. When a vaccinated cell dies, fragments of the cell including the spike protein are taken up by antigen presenting cells to form MHC-II complexes and exposed on the cell surface. When helper T cells detect the exposed fragments recruitment of other immune cells is triggered.