Body guards: fighting a germ
A germ gets in through a cut. Watch the lymph nodes catch it and send help.
Meet the guards
Your body has hundreds of tiny guard posts called lymph nodes, each about the size of a pea or a bean. They sit in your head and neck, your armpits, your chest, your belly, and your groin and legs. They are joined by thin tubes that carry a clear liquid called lymph. Bigger helpers are the spleen (behind your stomach), the thymus (behind your breastbone) and the tonsils (at the back of your throat). Watch each group light up.
The lymphatic system is a one-way drainage network. Fluid and protein constantly leak out of the blood capillaries into the tissues; lymphatic vessels collect that fluid as lymph (a few litres a day), pass it through the lymph nodes and return it to the blood at the base of the neck. There are close to 800 lymph nodes in a human body, about 300 of them in the head and neck. Each is a small bean-shaped filter (2 to 25 mm long, most under 10 mm) packed with white blood cells. The spleen filters the blood itself, the thymus is where T cells mature, and the tonsils guard the entrance of the throat. The lymph vessels themselves are not in this model: only the nodes and organs are shown, with an outline of the skeleton for scale.
A germ gets in
You scrape your little finger and a germ (purple) sneaks in. It is washed into the lymph, which slowly flows up your arm. It has to pass the guard posts on the way: first a node just above your elbow, then the nodes in your armpit. Each node grabs the germ and starts making fighters. When a node is busy fighting, it fills up with cells and swells: that is the sore lump you can feel under your arm or in your neck when you are ill. Past the armpit, the lymph pours back into the blood near your collarbone.
Tissue fluid from the cut, carrying bacteria and their antigens, drains into lymphatic vessels that run beside the veins of the arm (the lymph vessels are not modelled here; the dots follow the basilic vein). From the little-finger side of the hand one typical route passes the supratrochlear node above the elbow, then the lateral, central and apical axillary nodes in the armpit; from there the subclavian lymphatic trunk runs beside the subclavian vein to the right lymphatic duct, which empties into the veins at the venous angle behind the collarbone. Drainage varies from person to person, and the cubital, infraclavicular and supraclavicular nodes shown here are on side routes. Inside a node, dendritic cells present the antigens to T cells and B cells pick antigens straight out of the lymph; the matching lymphocytes are activated and multiply, which is why nodes swell during an infection. There is no central pump like the heart: the collecting vessels squeeze lymph along with muscle in their own walls, helped by the movement of the muscles around them and by breathing, and one-way valves stop it flowing back.
Calling for help
Two kinds of helpers go to the cut. The first are the scavenger cells that live in your blood: they arrive within hours and simply eat germs. The second are the trained fighters the nodes have been making. They cannot swim back down the lymph, so they take the fast road: they join the blood near your collarbone, ride through your heart, and come racing down the arteries of your arm (white). Some of them make antibodies, sticky tags that cling to the germ so the scavengers can find and eat it. The spleen, a blood filter behind your stomach, sends fighters too.
Innate immunity comes first: neutrophils and macrophages leave the blood at the injured tissue within hours and engulf bacteria. The adaptive response takes days to build but is specific to that germ. Lymphocytes activated in the nodes leave through the efferent lymph, enter the bloodstream at the venous angle and are pumped by the heart through the aorta, brachiocephalic trunk, subclavian, axillary, brachial and radial arteries to the palm and finger. T cells coordinate the response or kill infected cells; activated B cells become plasma cells whose antibodies travel in the blood to the wound and tag the bacteria for destruction. The spleen filters the blood, removes old red cells and produces immune cells that reach the tissue the same way, through the splenic vein, portal vein, liver and heart.
The training camp
Where do the trained fighters come from? Some are trained in the thymus, a small soft organ behind the top of your breastbone. Baby fighter cells go to school there and learn one important lesson: never attack your own body. Only the ones that pass are let out. The thymus works hardest when you are a child. From about the time you become a teenager it slowly gets smaller, but it keeps sending out a few new fighters all your life.
The thymus and the bone marrow are the primary lymphoid organs: they make lymphocytes without needing an antigen. B cells mature in the bone marrow (there is no marrow mesh in this model; it fills the inside of the bones). T cells are made in the marrow too but mature in the thymus, hence the T, where the ones that would react to the body’s own tissues are eliminated. The thymus is most active in childhood; its gradual, age-related involution becomes pronounced from around puberty, when much of it is replaced by fat, though it keeps producing some T cells in adults. The lymph nodes, spleen and tonsils are the secondary lymphoid organs, where mature lymphocytes meet antigens and the adaptive response begins.
Remembering
When the fight is over, a few of the fighters that beat this germ stay alive for years as memory cells. If the same germ comes back, they recognise it and your body can fight it much faster, so you are often only mildly ill or not ill at all. A vaccine is a safe practice run: it shows your body a weakened or broken-up germ, a harmless piece of one, or the instructions for making that piece, so the memory cells are ready before the real one turns up.
Adaptive immunity is slower than innate immunity on first contact but leaves immunological memory: long-lived memory B cells and memory T cells specific to that antigen, made in the lymph nodes, spleen and tonsils (the lit parts), not in the thymus. On a second exposure they respond faster and more strongly, which usually makes the illness milder or prevents it, though protection can fade with time. Vaccination triggers the same memory with a weakened or inactivated pathogen, one of its antigens, or the genetic instructions for an antigen, without the disease. The rest of the immune system, the cells in the blood and tissues, is too small to model.