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As this continues our discussion of the confrontation between the Omicron variant of SARS-CoV-2 and vaccine-induced immunity to the spike protein of the original Wuhan strain of the virus, it’s best to begin by scanning the preceding essay (#90) that summarises key background and defines the technical terms that must inevitably be used here.

Beyond that, the first thing to consider is that adaptive immunity is a highly mobile defence system. While the response to vaccines injected into the upper arm all happens in the axillary lymph nodes (ALNs) of our armpit, the immune effectors and memory B cell and T cell populations begin moving out within a week or so to enter the super-highway system of the blood and disseminate around the body. For the T cells, it’s likely that the clonal expansion phase ends with the elimination of the pMHCI+ and pMHCII+ vaccine-antigen-presenting dendritic cells (APDCs), likely by the emerging CD8+ ‘killer’ T cells. While immune B cells exit the ALNs early, there’s also emerging evidence that maturation of the B cell/antibody/Ig response can continue for much longer in the ALN germinal centres. That is, though, a discussion for later, and it doesn’t change the story we’re developing here.