The cells capable of transfection are throughout the body.
This image shows the process they undergo when the lipid nanoparticals enclosing the mRNA instructions undergo endocrytosis (a process by which cells absorb external material by engulfing it with the cell membrane) in these host cells:
mRNA delivered in an mRNA vaccine enters cells by endocrytosis and, after release from the endosome, is translated into protein by ribosomes. This process of ribosome translation--called transfection--is only possible in three types of host cells. They will, after administration of an mRNA vaccine intramuscularly, intracutaneously, or subcutaneously, be used by these three types of host cells: (1) non-immune cells (such as muscle cells and epidermal cells) at the injection site, (2) immune cells found in the tissues at the injection site (such as dendritic cells and macrophages) and, (3) immune cells in peripheral lymphoid organs after the injected mRNA is transferred through the lymphatic system to adjacent lymph nodes or the spleen. Translated proteins can then activate the immune system primarily in two ways:
i) Proteins are degraded by the proteasome into peptides subsequently presented as antigens on the cell surface by major histocompatibility complex (MHC) class I molecules which bind to the T cell receptor (TCR) to activate CD8+ T cells to kill infected cells through the secretion of perforin and granzyme.
ii) Proteins secreted extracellularly are engulfed by antigen-presenting cells (APCs) and degraded into peptides subsequently presented on the cell surface by MHC class II molecules for recognition by CD4+ T cells, which can activate both the cellular immune responses by secreting cytokines and the humoral immune responses by co-activating B cells.
In addition, single-stranded RNA and double-stranded RNA delivered in mRNA vaccines bind to Toll-like receptor (TLR) in the endosome to activate the antiviral innate immune responses via the production of type-I interferon (IFN-I) which results in the induction of several IFN-1-stimulated genes involved in antiviral innate immunity, in a mechanism known as the self-adjuvant effect of a sequence-engineered mRNA.
At no time do the ribosome produced Spike proteins remain in their original form. Instead they are degraded into antigen peptides to be used to create T-cells, programmed to attack the COVID-19 viruses' Spike proteins on its cell surface. These T-cells act as a contraceptive to the COVID virus. I am amazed that scientists have been able to understand how these transfection capable cells work and can be made to work for mankind's benefit. Much of the names for the science are not yet in most dictionaries. Through this science, we are on the threshold for a cure to Cancer and other ailments (Google: Can mRNA Treat Diseases?).