The pulmonary circuit oxygenates and the systemic arterial circuit circulates several Liters of blood per minute. As this blood trickles through the capillaries, some of it oozes out through the thin endothelial lining, becoming interstitial fluid.

Most of this flud gets back into the capillaries and returns to the heart in the venous return. About 20% of the fluid drains into the LYMPHATIC SYSTEM, through the Lymph nodes, and back into the thoracic duct, into the vena cava, and eventually back into the general blood supply, which recirculates.

The Return of the residual interstitial fluid by the Lymphatic vessels back to the general circulation is important, and if BLOCKED by such things as surgery and parasites, produces a swelling known as LYMPHEDEMA.

But the major importance of the lymphatic vessels is their characteristic cells, LYMPHOCYTES, and their accessory structures, The THYMUS, SPLEEN, LYMPH NODES, AND LYMPHATIC TISSUES is in the development and training of the IMMUNE SYSTEM. The THYMUS can be regarded as an organ to TRAIN Lymphocytes to be functional members of the immune system.

SomeT-cells operate to Help B-cells to produce antibody. These are called Th-4 cells. Others, called Th-8 cells. function to delete cells,

The thymus POSITIVELY selects cells that are prepared to recognize pathogens, and gets rid of cells (NEGATIVELY selects) cells that recognize the body’s own cells.

Failure to do so would result in “horror autotoxicus”, or AUTOIMMUNITY. The Thymus performs most of its functions when we are young.

The LYMPH NODES act as a type of filter for the Lymphatic channels, and intercept most bacteria and other pathogens coming through the lymphatic channes to keep them out of the blood stream. Lymph nodes are present at predictable locations throughout the body.

The Spleen acts as a big lymph node to filter the blood directly, and can intercept organisms that have escaped the lymph nodes. The Spleen can enlarge with certain infections, as with the EB Virus that causes mononucleosis.

Rupture of the spleen is not uncommon with direct blunt trauma to the abdomen. Removal of the spleen can increase susceptibility to blood stream infection. Lymph tissue is scattered throughout the body, like in the Tonsils, adenoids and Intestinal tract.

This Lymphocyte-containing tissue can expand with exposure to infection, and can become cancerous with Lymphomas. It can also enlarge enough to produce local Blockage, as with Adenoids. Lymph tissue should not be removed wantonly, since it may provide protection. There are 2 arms of our immune system.

I have been discussing the ADAPTIVE immune system which recognizes the pathogen, and stores memory of this interaction in “memory T-cells”. The helper T-cells( Th-4) start to multiply and stimulate the B-cells to “clonally” proliferate and differentiate into Antibody-producing Plasma Cells. The Cytotoxic T-cells, which can directly kill Pathogens, also multiply.

The first time this happens with a new Pathogen, Like SARS CoV-2, this adaptive process takes a week or so. But if the same Pathogen comes later, The adaptive immune system is ready, via the Memory T-cells, and springs rapidly into action. The 2nd arm of our immune system, shared with all of our vertebrate relatives, is the INNATE immune system.

The innate system is prepared IMMEDIATELY to recognize certain foreign molecules common to many invaders, and foreign our own bodies. It usually does not have the Power of the innate immune system, but in healthy people does an amazingly good job.

Some people just don’t get sick very often. Do the best you can to be one of these healthy people by getting proper SLEEP, DIET. AND EXERCISE!