Organs of the Immune System (Lymphoid Organs)

Good day colleagues and friends, so a couple of days ago, a friend of mine asked me what are the lymphoid organs, and well, that's what we are going to talk about today: the lymphoid organs and how they work!

First of all, we ought to know that the lymphocytes are cells that help protect the body of all animals (well yes, including humans!). The lymphocytes fall into 2 major populations called T cells and B cells based on the organs in which they mature. So therefore the organs that regulate the growth and maturation of these cells are called the primary lymphoid organs.

These include the bursa of Fabricius in birds only, the bone marrow in primates and rodents, and intestinal lymphoid tissues in rabbit, ruminants, and pigs.

The primary Lymphoid organs all develop early in fetal life. As animals develop, newly produced, immature lymphocytes migrate from the bone marrow to the primary lymphoid organs where they mature.

It is very important to remember that the primary lymphoid organs are not sites where lymphocytes encounter antigens and so they do not enlarge in response to antigenic stimulation.

Ok now we are going to mention the Primary Lymphoid organs and what their functions are:

Thymus: is located in the thoracic cavity in front of and below the heart. In horses, cattle, sheep, pigs, and chickens it also extends up the neck as far as the thyroid gland. The size of the Thymus varies, it's relative size being greatest in newborn animals and it's absolute size being greatest at pubrty. It may be very small and difficult to find in adult animals.The main function of the thymus is to produce a development in the immune system's cells called: lymphocytes T (which defend the body by responding to antigens).
There are three types of Lymphocytes T: lymphocytes T cytotoxic,  lymphocytes T auxillaires. and lymphocytes T regulator. And of course we shall talk about theese cells later on.
Back to the thymus, this organ is mainly responsible for the hormons production that helps regulate the growth and the definition of the lymphocytes in their characteristics.
Some of the hormones that are produced by the thymus are peptides such as: thymosins, thymopoietins, thymic humoral factor, thymulin and thymostimulins. The thymulin is especially

Bursa of Fabricius:This organ is only found in birds, it is a round sac located just above the

cloaca. The bursa reaches its greatest size in the chickens about 1 to 2 weeks after hatching and then shrinks as the bird ages. The main function of this organ is maturating and causing the differentiation of B lymphocytes (antibody-forming system), it also produces the hormone bursin, which activates B lymphocytes.
The bursa acts like the thymus insofar as immature cells produced in the bone marrow migrate to the bursa. These cells then proliferate rapidly but 90-95% of these B cells die by apoptosis (physiological cell death or cell suicide mediated by normal body processes).

Peyer Patches: also known as PPs, are lymphoid organs located in the walls of the small intestine. Depending on the specie they can be found in several areas in the intestine walls.
They are divided in two groups: group I includes ruminants, pigs, horses, dogs and humans. Group II includes other mammals such as cats, rabbits and rodents.
In group I, 80-90% of the Peyer's Patches are found in the ileum where they form a single continuous structure that extends forward from the ileocecal junction. Ileal PPs consist of densely packed lymphoid follicles, each separated by a connective tissue sheath, and contain only B cells.
In group II, all PP's are located at random intervals in the ileum and jejunum. In these mamals, the PPs do not develop until 2 to 4 weeks after birth and persist into old age.
The main function of the PPs are to grow and proliferate B cells, althoughmost cells undergo apoptosis (cellular death), the survivors are released into the circulation, then they produce antibodies.

Lymphoglandular complexes: they are submucosal masses of lymphoid tissue penetrated by radially branching extensions of mucosal glands.These glands penetrate both the submucosa and the lymphoid nodule. Their function is unknwon in full, but contain plasma cells, and are sites of antibody production, they may be antigen-sampling sites.

Bone Marrow: Lymphoid cells developing and maturing here are referred to as B cells (B for Bursa or for Bone marrow). This is the site for proliferation for stem cells and for the origin of pre-B cells and their maturation to become immunoglobulin-producing lymphocites. Like thymic selection durng T-cells process withing the bone marrow eliminates B cells with self reactive antibody receptor.
There is no exclusive  B-cell development site in the bone marrow, although it is suggested that precursor B cells develop at the outer edge of the marrow and migrate to the center as they multiply.




Now that we have mentioned the primary lymphoid organs, we can go ahead and mention the secondary Lymphoid organs, which are: lymph nodes, hemolymph node, and the spleen.
 There are other secondary Lymphoid organs, however, these are the most important, and we will get into details with theese in our next post.

Thank you all for your support, and let's keep learning! 
Have a great week everyone.


MVZ Carolina Pruneda