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Why can lymph nodes swell during an infection?

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Diagram of the human lymphatic system highlighting lymphatic vessels and major lymphoid organs, including lymph nodes, adenoids, tonsils, thymus gland, spleen, appendix, bone marrow, and ducts. — lymphatic system The human lymphatic system, showing the lymphatic vessels and lymphoid organs.

Lymph nodes swell during an infection primarily due to the immune system's response to foreign invaders, such as bacteria or other microorganisms, that have entered the body's tissues.1

When these foreign particles are carried into the lymph nodes via the lymphatic vessels, they are trapped by lymphoid tissue. In response to the presence of these antigens, lymphocytes—the white blood cells responsible for defending the body—begin to proliferate rapidly. This massive increase in the number of immune cells, combined with the accumulation of foreign material, causes the lymph node to enlarge significantly.12

This process, known as lymphadenopathy, is a vital part of the body's defense mechanism, as it provides a specialized environment where immune cells can mature, produce antibodies, and directly attack the invading pathogens.12