A lymph node is a bit like a jungle gym for our microscopic police force. A filter or dragnet, evolved to catch unsavory characters like bacteria and viruses — drifters who are up to no good. Our hundreds of lymph nodes function like district police headquarters, situated around the body with concentrations in the armpits, neck and other areas.
Tissue fluids throughout the body make their way into the lymphatic vessels, carrying any infectious microbes along. Lymph nodes like the one shown above are stationed along those vessels, and here is where the “crooks” meet a concentration of leukocytes, or white “blood” cells. (I add quotes around “blood” because they spend most of their time patrolling through other body tissues. It’s a bit like calling someone a “streetwalker” just because you caught them crossing the street now and then.)
The “jungle gym” in a lymph node is built of reticular connective tissue, specially evolved to provide “networking” opportunities (“reticular” means “net-like”). This facilitates interaction among the cops while also giving them a good chance of nabbing a microbe. Some leukocytes may respond immediately (cop reaching for gun), and will later serve as an antigen-presenting cell to communicate their findings to the others (the “wanted” slide shown at top). After the initial encounter, certain leukocytes with a receptor for the specific microbe (“I was born for this assignment, boss!”) divide into many identical clones that are specifically on the lookout for said crook. The production of clones is what produces the swollen lymph nodes during an illness. Memory cells (the cop with the desk job) remain on high alert (perhaps with the help of coffee and a donut) even after the crooks have been executed, in case they show up again, when the body will be prepared for a more rapid response (adaptive immunity). Some memory cells can live for decades – very old “cops” indeed — and they never forget. Leukocytes communicate with a variety of signals (cops joking around at right), some of which encourage them to “bulk up” for the encounter, like the cop doing pull-ups.
We run into the larger lymph nodes (around 1-2 cm) with every dissection in the cadaver lab. They’re nondescript, and firm, but with a dense, spongy texture that, with a touch of imagination, is suggestive of their “cop station” function.