Inflammatory response at injection site

The "inflammatory response at injection site" refers to the local activation of immune and inflammatory pathways following the administration of a drug or vaccine, resulting in classical signs of inflammation such as pain, redness, swelling, and, sometimes, delayed tissue reactions. The process is initiated by immediate mast cell degranulation (often via MRGPRX2) and further amplified by innate immune cell recruitment (neutrophils, macrophages) or cytokine/chemokine release triggered by pattern recognition receptor (PRR) activation on resident and infiltrating cells. This response is mostly beneficial (initiating immunity), but can also produce adverse events, including injection site reactions of varying severity.

Other names

Injection site reactionLocal inflammatory responseLocal injection reaction

Mast cell activation via MRGPRX2 triggers immediate pseudo-allergic reaction; PRR activation by DAMPs/PAMPs → cytokine/chemokine release → recruitment of granulocytes, monocytes, macrophages; Complement cascade activation (local C3a/C5a production); Endothelial activation/vasodilation, oedema, nociceptor sensitization (pain)