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The naïve, SARS-CoV-2 specific T cell and B cell precursors that are drawn into the lymph nodes (LNs) following vaccination or infection are small, uninteresting-looking cells that are mostly nucleus and have very little cytoplasm. When their ‘antigen-stimulated’ progeny exit to be found in the blood six or more days later after multiple cycles of cell division, they will be much bigger (activated lymphoblasts) and most will have down-regulated the CD62L molecule that enabled their entry into the LN interior. With all this cell recruitment and proliferation the LNs – which have sensory receptors (feeding back to the brain via the nerves) that detect inflammation and distension – can swell, causing transient pain following a high vaccine dose or an acute infection.