According to the National Cancer Institute, a subdivision of the National Institutes of Health, clusters of lymph nodes are found in the neck, underarm, chest, abdomen, and groin. Why are lymph nodes located in these places?

Edit: This article claims that about 60% of the lymph nodes are located just under the diaphragm, and breathing (particularly deep breathing) creates a pressure gradient, pulling lymph through the lymphatic system. This could be a reason why most of the lymph nodes are located there.

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    $\begingroup$ "Why" questions of this kind invite "just-so" explanations that are rarely rigorous. As sites of information processing, lymph nodes could be expected to be concentrated near sites of potential infection. Dendritic cells migrate from infection sites to their proximal nodes, to present molecular patterns of interest. I am not sure whether any lymph nodes' locations are adaptations to receiving circulatory boosts from movements, or whether the effect mentioned in your cited article is an exaptation or a spandrel. $\endgroup$
    – Ryan
    Dec 14, 2022 at 1:07
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    $\begingroup$ Reasons I can think of (N.B.: teleological): aggregating lymphocytes further from a source of infection helps fight infection better than WBC's spread out everywhere, and they need to be in soft tissue that can expand without compromising function. Swollen lymph nodes (LN) in your hand would cause a lot of pain (can hardly use hand) and could compromise LN function. But swollen LN in the axilla? Piece of cake and protected. They're especially plentiful below the diaphragm because of all the crud we ingest; they drain the entire GI tract. $\endgroup$ Dec 26, 2022 at 22:15