Written by Joel Robert Thompson
Hello everyone. I am a third year Doctor of Physical Therapy student who has had the privilege of training here at North Carolina Physical Therapy since the beginning of August. My main research interest is around autoimmunity, namely a disease called Ankylosing Spondylitis, as well as how exercise and physical therapy can positively influence the immune system as a whole. For this reason, I have told many of our patients here at NCPT that I am interested in lymphedema and NCPT because the lymphatics are part of the immune system and so I want to take a little time to explain what exactly I mean by that. How do the lymphatics play a role in immune function after all?
It is pretty straightforward to say that a large chunk of your immune cells live in the lymph vessels and lymph nodes – which they do – but that does not really answer how the lymphatic system acts as part of the immune system and why that even matters or helps the body. I will steal from Philipp Dettmer and give the most succinct description of the lymphatic system as “The Immune System Superhighway.” We talk a lot about how the vessels in the body are roads to bring nutrients, oxygen, and all the good stuff to the cells. It all comes from your heart via the arteries, which you may all recognize as the red vessels on a body map. From there, the veins (blue vessels) and lymph system (green/yellow vessels) return all that stuff – the blood that needs an oxygen refill, extra nutrients, byproducts of energy creation, etc – to the heart, liver, kidneys to either be returned to circulation and removed from the body.
When explaining the lymphatic system, we give the common estimate that about 80% of the fluid originally pumped by the heart is returned with the veins, about 10% sticks around in the space between the cells, and the remaining 10% is drained and returned to circulation via the lymphatic system. Although this is absolutely true – give or take a few percentage points – it does leave out the distribution of what is in that 10% that is described as Lymph fluid. Lymph is described as white or clear because most of the blood (not to mention the excess nutrients and all the other good stuff) goes into the veins. Rather, the lymph system is a bit more gross: it drains the stuff that is otherwise ‘stuck’ in the space in between cells. This certainly includes some nutrients unabsorbed by the cells and plasma and platelets that went unused. However, the larger chunk of this includes cellular debris, waste products (from converting the raw nutrients into energy), dead cells, and – most relevant here – any signs of bacteria or viruses that may be plaguing the system.
And when I say “signs” of infection, it is regrettably a bit darker than that. When a cell is about to die, it will send out a warning sign that simultaneously warns other cells to be careful and attracts immune cells (for context, Rabies is so insidious because the infection deactivates this warning signal in affected cells). One of the immune cells in particular, called dendritic cells, act almost like intelligence officers. These cells will swallow the infected cells whole and study what specifically is infecting your body. It does this by isolating the surface protein chains on the invading cells. In a microscopic world where things do not have eyes, all cells have little protein chains on their surface that acts to identify the cell. This is very helpful to identify what is self and what is foe (and is a major part of the problem with autoimmune diseases).
The dendritic cell then uses the protein chains it gathered from eating the invading cells and uses them to recruit more help. In short, your body already has an immune cell that is specifically designed to fit almost any infection and your body maintains cells that ‘remember’ each infection that has attacked your body. This is why many people argue it is important to expose yourself to stuff as a kid via eating dirt and bathing in mud (this is not entirely true, of course, but the spirit is common). We call these cells Helper T-cells and they mostly hang out in the lymph nodes. (You have probably heard of T-cells because these are the cells specifically that the HIV virus primarily targets.) So the dendritic cell enters the lymphatic system and goes node by node until it finds a T-cell that matches the infection that it has identified. Once identified, the T-cells can direct the immune system and create more of the specific cells, antigens, and antibodies needed to combat the specific infection.
The question, of course, is why do the T-cells and ‘dormant’ immune cells hang out in the lymphatic system rather than circling the blood stream, constantly guarding against infection. To be fair, many of them do, especially in the immediate periods after infection. However, the storage of immune cells in the lymphatic system is, in short, to keep the system from being too active. Many immune cells cause widespread damage and have no system for differentiating good cells and bad cells. Thus, these cells need to be kept in reserve until they are truly necessary. Much of your crummy feeling from when you are sick is actually just your immune system trying to fight the infection rather than the infection itself. Fever, inflammation, achiness; that is all your body’s immune response to infection, not necessarily the infection itself.
And while hanging out in the lymph vessels, these immune cells gather in lymph nodes. Thus, any substance that is moving along the lymph system back to the heart to reenter circulation must push through these collections of immune cells. Although not a proper detoxifying and filtration system such as the kidney, the lymph nodes do identify harmful cells and gobble them up or neutralize them so they cannot return to circulation. A colleague of mine refers to this process as the lymph nodes acting like a sort of TSA checkpoint that inspects, analyzes, and scans what is coming out of the space between cells.
So naturally if the lymphatic system is not operating properly – or, to continue the TSA metaphor, acting short-staffed because some lymph nodes are not acting properly – the immune system has fewer opportunities to catch foreign invaders before they begin to set up shop in the body, in our skin, and in the space between the cells. I am excited, then, for the opportunity to work with patients to help promote better movement of lymph fluid and ultimately help the immune system function better in order to help the patients at North Carolina Physical Therapy.