Organs of the Immune System (Secondary Lymphoid Ogans)

Good day friends and colleagues, in this post we are going to continue talking about the lymphoid organs, but this time we will talk about the secondary lymphoid organs, what they do, and how they work.

So despite the primary lymphoid organs develop early in fetal life, the secondary lymphoid organs arise late in fetal life and even persist in adults, also they enlarge in response to antigenic stimulation.

Some of the secondary lymphoid organs include the spleen, the lymph nodes, the tonsils, and lymphoid tissues in the intestinal, urogenital and respiratory tracts.

The anatomical structure of these organs is therefore designed to facilitate antigen trapping because they contain dendritic cells that trap and process antigens and lymphocytes that mediate the immune responses.

Now let's see how the organs work:

Lymph nodes: they consist of a reticular network filled with lymphocytes, macrophages, and dendritic cells through which lymphatic sinuses penetrate. Lymph nodes are round or bean-shaped structures that are widely distributed throughout the body. 

Imbedded in connective tissue or fat, they are concentrated in the cervical, axillary, and inguinal regions – the neck, armpits, and groin, respectively. They are typically less than ½ inch in length, depending on the size of the animal.

Lymph nodes are enclosed by a capsule of connective tissue and comprised of several compartments called "lymph nodules." The nodules are masses of T-cells, B-cells, and macrophages. 

 Macrophages are specialized cells that ingest and destroy foreign material.

The nodules are separated by spaces called "lymph sinuses." The vessels that deliver unfiltered lymph are called "afferent vessels," and there are several per node. The lymph is then filtered for antigens and particulate matter, and an immune response is generated, if necessary. The filtered lymph leaves the node through one or two efferent vessels near an indentation called the "hilum." Blood vessels also enter and exit the node at the hilum.

In Lymph node that have been stimulated by antigen, some of the cells within these nodules expand to form foci of dividing cells called germinal centers. These germinal centers have 2 types of zones: light and dark.

the dark zones are sites where B cells proliferate and undergo a process called somatic mutation.

the light zones are sites where immunoglobulin class switching memory B-cell formation occur.

The lymphatic vessels: link together all of the secondary organs and also connect to the cardiovascular system. They provide a route for the one-way flow of lymph from the tissues of the body to the heart. Lymph is the clear, yellowish fluid that is collected from the interstitial spaces (the spaces between the cells of a tissue) into lymphatic capillaries.

The lymphatic vessels are structured similar to veins, with thin walls and valves to prevent backflow. They are not muscular vessels, and external forces such as limb movement regulate the flow of lymph. Once in the capillaries, the lymph moves into progressively larger vessels, passes through the lymph nodes and/or spleen, reaches the large ducts, and enters the blood circulation near the junctions of the jugular and subclavian veins in the upper chest. Thus, the fluid and proteins are eventually returned to the blood, which helps maintain the proper balance of fluid between the blood vessels and the tissues. All of the lymph from the lower body, left arm, and left thorax are drained through the thoracic duct into the junction of the left jugular and subclavian veins. The fluids from the neck, right arm, and right thorax empty into the right lymphatic duct which joins the venous system at the junction of the right jugular and subclavian veins.
Lymphatic capillaries: are interwoven with the blood capillaries. Fluid and proteins are forced out of the arterial end of the blood capillary and into the interstitial space. About 90% of the fluid is reabsorbed in the venous end of the blood capillary, but none of the proteins are able to reenter the blood vessels because they cannot fit through the tight junctions of the cells. The lymph capillaries have extremely loose cell junctions, however, and they are able to absorb the remaining 10% of the fluid along with the plasma proteins. Once inside of the lymph vessels, the fluid is then termed "lymph."

Lymphoid tissues in the intestinal tract: Near the small intestine, where fats are digested and absorbed, the lymphatic vessels have a special function and, therefore, a special name. They are involved in the absorption of digested fat from the small intestine, and are called "lacteals". After a meal the fluid within the lacteals generally has a fat content of 1-2%, and it appears cloudy. This cloudy lymph in the lacteals is called "chyle."

Hemolymph Nodes: are structurally similar to the lymph nodes found in association with the blood vessels of ruminants and other mammals. They differ from conventional lymph nodes in that their lymphatic sinuses contain numerous red cells. They have a cortex containing germinal centers and B cells as well. T cells predominate at the center in association with lymphatic sinuses. There are some differences, however, in the characteristics of these T cells are compared to conventional lymph nodes. They may combine features of both the spleen and lymph nodes.

Spleen: is an organ located in the upper left portion of the abdominal cavity along the outside curve of the stomach that filters blood. The filtering process removes both antigenic particles and aged blood cells. Also stores red cells and platelets and undertakes red cell production in the fetus. It is therefore divided into two compartments: one for antigen trapping and for red cell storage, called red pulp, and one rich in lymphocytes where immune responses occur, called the white pulp.

So generally there is lymphoid tissue also in other organs such as the lungs, the liver and throughout the body. Later we can discuss a little more about how some of theese immune process occurs, such as the simple inflammation or even a bruise.
For now this is in a few words how the lymphoid organs work.
Have a great week everyone!

MVZ Carolina Pruneda