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Why do swollen lymph nodes form during bubonic plague?

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two plague victims with sores lying in bed, while a robed figure gestures above them. — plague Image depicting victims of the sixth biblical plague.

Swollen lymph nodes, known as buboes, form during bubonic plague because of how the bacterium Yersinia pestis interacts with the human immune system.12

After the bacteria enter the body—typically through a flea bite—they migrate through the lymphatic system to the lymph nodes. Once there, the bacilli produce specific proteins that disrupt the body's normal inflammatory response and prevent them from being destroyed by macrophages, which are cells that usually fight off infections.12

With the immune response weakened, the bacteria multiply rapidly within the lymph nodes. This colonization causes the nodes to become extremely painful and swollen, often appearing as red or darkened lumps under the skin in the groin, armpits, or neck. If the infection is not treated, the bacteria can eventually destroy the tissue of the lymph nodes and spread further into the bloodstream or lungs.123