Lymphoid follicles play a major role in the human defense system. They contain a large amount of B lymphocytes, which multiply into immune cells when they come into contact with pathogens.
What are lymphoid follicles?
Lymphoid follicles are a component of the lymphatic system. Under a light microscope, they can be seen as a spherical collection of B lymphocytes. The lymph follicles are involved in certain processes of the defense system. For example, they take on the task of multiplying and specializing the B lymphocytes into plasma cells. They are mainly found in the lymph nodes, where a particularly large number of pathogens are located. In the human body, it is especially the spleen and the pharyngeal tonsil that have to deal with a large amount of antigens. Lymph follicles are also found in the reticular connective tissue of various organs. These include, for example, the mucous membranes of the digestive tract, the organs of the respiratory system, and into the urinary and reproductive organs. Lymphoid follicles arise in response to local stimuli, form transiently into solitary follicles, and occur as solid components of lymphoid organs.
Anatomy and structure
The appearance of lymphoid follicles varies according to their stage of development. Primary follicles, also called primary nodules, are up to one millimeter in diameter. At this stage, the lymphoid follicles have not yet experienced antigen-antibody contact. Instead, they are characterized by an even distribution of small lymphocytes. The secondary follicles or secondary nodules, on the other hand, have a bright center, also known as the germinal or reaction center. Contact with pathogens causes the primary lymphoid follicles to be activated into secondary follicles. The germinal center of secondary follicles is surrounded by a dense cortex that has a high concentration of T lymphocytes. This cortex is also known as the parafollicular space. In the germinal center of secondary follicles are activated B lymphocytes, which are differentiated into plasma cells. Lastly, solitary follicles are those lymphoid follicles located in the tela submucosa. These enlarge, for example, during infections in the mucosa and can even grow to the size of a pin. In individual areas of the human body, so-called solitary follicle formations also occur, which aggregate to form the folliculi lymphatici aggregati. These are found, for example, as Peyer’s plaques in the ileum mucosa.
Function and Tasks
Once pathogens invade specific organs of the body, the body triggers a specific immune response. As a component of the lymphatic system, lymphoid follicles are involved in fighting the invaders. The functions of lymphoid follicles differ according to their functional stage. A high concentration of immature B lymphocytes is formed in the polar caps of primary follicles. These B lymphocytes are also called naive B cells because they have not yet had contact with antigens. After antigen contact, the primary follicle becomes a reaction follicle with a lighter inner zone, which is a cell-poor reaction center. At this stage, lymphoid follicles are called secondary follicles. Now they are surrounded by a dark lymphocyte wall. In addition, there are still undifferentiated B lymphocytes in the lymphoid follicles. If these come into contact with the memory cells and helper cells, they can form specific antibodies. Another function of secondary follicles is to mitotically proliferate and differentiate B lymphocytes after antigen contact. Since the B lymphocytes are already imprinted with specific character traits by the different developmental stages, these are relevant for the later processes in the defense system. Now the proliferated and differentiated B cells mature within the lymphoid follicles. Then, contact of intrafollicular T lymphocytes with follicular dendritic cells gives rise to B lymphoblasts. These eventually migrate out of the lymphoid follicles to develop into antibody-forming plasma cells.
Diseases
Common diseases associated with lymphoid follicles include tonsillitis, appendicitis, and swelling of lymph nodes and the spleen.Tonsillitis, referred to in medical terminology as angina tonsillaris or tonsillitis for short, is an acute bacterial infection of the pharyngeal tonsils, the palatine tonsils or the lingual tonsils. These are all located in the pharynx, whereby the palatine tonsils are most likely to be affected by tonsillitis. If pathogens penetrate the tonsils, they swell and often cause severe pain in the affected person. Tonsillitis is often caused by streptococci, pneumococci, Haemophilius influenzae or staphylococci. This disease mainly affects children and adults with a weakened immune system. If tonsillitis occurs frequently and the patient often has difficulty breathing as a result, surgery is also an option. In appendicitis, the vermiform end of the appendix is inflamed. Although the colloquial term is appendicitis, the entire appendix is not affected by the infection. To avoid life-threatening consequences for the patient, the appendix, which is 10 centimeters long and 1 centimeter thick, is removed after diagnosis. This contains a large amount of lymph follicles, which kick-starts the defense system during an infection, especially in children. The inflammation occurs especially in children and adolescents from the age of 10. Adults are still susceptible to appendicitis until the age of 30. Symptoms of appendicitis include loss of appetite, nausea, vomiting and high fever. When the lymph nodes and spleen swell, it is called mantle cell lymphoma. In this case, not only the healthy B lymphocytes are increased, but also defective ones. These have a similar appearance to such lymphocytes, which are otherwise located in the marginal area of the lymph follicles. These cells are tumor cells that grow in increased numbers in the lymph nodes and spleen and have no function in defense. However, this disease does not arise as a result of an infection like the two mentioned above. There is also no evidence to date of a hereditary cause, although a genetic alteration is present in approximately 85 percent of all patients.
