Assignment 1: My Favorite Tissue

THE LYMPH NODES

Reproduced from Lichtman's Atlas of Hematology for Williams Hematology 8e online resources

The Lymph Nodes are a major part of the Lymphatic System, a branch of the Immune System that helps prevent disease and infection in the body

Brief Introduction to the Immune System:

The immune system is a biological system specialized in the detection and protection from infection or disease. In order to properly protect the body, the immune system must have the ability to differentiate pathogens and foreign agents from its own tissues.

The immune system has many defensive mechanisms that aid in the detection and extermination of foreign pathogens that could cause a variety of diseases. The immune system is broken down into two major types:

• Innate Immunity 
• A quick, non-specific response without any immunological memory
• Consists primarily of barriers that resist infection
• Anatomical barriers such as the protective layer of the skin or digestive enzymes in the stomach
• Chemical barriers such as inflammatory response
• Also includes cells that recognize patterns in pathogen groups instead of specific components (i.e. antigens)
• Adaptive (Acquired) Immunity
• A slower, but more effective response involving immunological memory
• Includes Humoral and Cell-Mediated immunity
• Humoral Immunity involves antibody production (by B cells) and effector response in order to facilitate the elimination of a pathogen
• Cell-mediated Immunity involves activation of immune cells (e.g. phagocytes, cytotoxic T cells, or cytokines) that kill the pathogens
• Pathogen's antigen is "remembered" by an antibody created during the first exposure creating a much stronger response that is tailored to the specific pathogen
• Unlike the innate response's pattern recognition, specific components of a pathogen are recognized 

Overview of the Immune Response, Boston University School of Public Health Website, 2012

Here is a "fun" animation about the Immune Response. This animation is fairly complicated and none of the details are important for this blog...not sure why I put this here :) 

Organs of the Immune System: 

Figure 2-11, Kuby Immunology, 2007

• Primary Organs are sites where lymphocytes (central cells of adaptive immunity) mature, they includes the:
• Bone Marrow
• Thymus
• Secondary Organs are sites where mature lymphocytes interact with antigens, they include the:
• Lymph Nodes
• Spleen
• Mucosa-associated Lymphoid Tissues (MALT) and Gut-Associated Lymphoid Tissues (GALT)

The lymph nodes function in the adaptive immune system. They are secondary organs of the immune system. In Lymph nodes, immune responses are mounted in reaction to antigens circulating in the lymph. They are the first organized structure in the lymphatic system to encounter circulating antigens.  

Each lymph node is connected by lymphatic vessels, known collectively as the lymphatic system. (See a diagram here)

Copyright McGraw-Hill Companies, Inc
Figure 3-5b Contemporary Nutrition Chapter 3

The mature lymphocytes circulate in the blood and the lymphatic system (a network of vessels that collect fluid, known as interstitial fluid; that has seeped through capillary vessels). The majority of this fluid is returned to the blood, but some of it enters the primary lymphatic vessels (very thin, blind lymphatic capillaries) and becomes known as lymph 

The lymph travels through progressively larger lymphatic vessels to the regional lymph nodes.

Cancer Research UK Website, 2013

The lymphatic capillaries have blind endings. These thin vessels are composed of simple squamous endothelium.  

Lymph nodes are found all over the body. They are bean-shaped structures that act as filters for foreign particles. Packed tightly with white blood cells, the lymph nodes are the main site where B and T lymphocytes are found. 

Diagram created for "How to Check for Swollen Lymph Nodes"

This diagram shows where the lymph nodes are located within the body. The linked website shows you how to locate your lymph nodes and how to detect whether they are swollen. 

Usually, people think that lymph nodes are primarily found in the head and neck region (known as the cervical region). Although there are quite a few lymph nodes in this area, there are clusters of lymph nodes located all over the body; in the armpit, chest area, pelvic/groin region and behind the knee. 

There are two types of lymph nodes: superficial (somatic) and deep (visceral) nodes. 

•  Somatic lymph nodes are those that receive lymph that has drained from the skin
• Visceral lymph nodes are those that receive lymph that has drained from deeper mucosal surfaces in the body such as digestive or respiratory tracts. 

Structure of the Lymph Nodes: 

Copyright 2001 Benjamin Cummings, an imprint of Addison Wesley Longman Inc

Oklahoma State University Center for Veterinary Studies
Instructional Resources Histology, 2012

A lymph node can be divided into 3 basic regions:

1) The Cortex

2) The Paracortex

3) The Medulla

A fibrous capsule made of collagen and fibroblasts surrounds the entire lymph node structure. The capsule extends internally in the form of trabeculae, made up of fibrous collagen tissue, which are continuous with the reticular network, a framework that supports the lymph node and is made up of reticular and elastic fibers. 

The capsule separates the lymph node from the surrounding connective tissue and is penetrated by many blood vessels. The capsule serves to prevent the diffusion of lymph from the inner lymphoid tissue into the surrounding tissues potentially leading to further infection. 

University of Oklahoma Health Sciences Centre, Histology, Slide 59

The lymphatic channels also known as the sub-capsular sinus lie beneath the capsule. They surround the node except at the hilum, the depression in the node causing its bean shape. Afferent lymphatic vessels open into the sub-capsular sinus, the lymph from these vessels filter through the reticular network and leave the lymph node through the efferent lymphatic vessel. The blood vessels have very thin walls and are lined with endothelium. 

Cortex:

The cortex of the lymph node is made up of lymphatic follicles and is transversed by trabeculae. Follicles that do not contain germinal centers (lymphocyte proliferation zones) are known as primary follicles, whereas those that do contain germinal centers are known as secondary follicles. Eventually, after antigenic stimulation, all follicles develop germinal centers. 

All lymphocytes (white blood cells) arise from stem cells in the bone marrow and function within connective tissue. 

The cortex as a whole is divided into two sections: the outer cortex, which contains immature B lymphocytes arranged into follicles, and the deeper cortex, located between the germinal centers and the medulla that contains mostly T lymphocytes. The deeper cortex is also known as the paracortex

Oklahoma State University Center for Veterinary Studies
Instructional Resources Histology, 2012. 

Mature lymphocytes are found in the medulla, the central part of the lymph node. This area is much less densely packed than the cortex, as you can see from the H&E section in the following picture. 

http://employee.lsc.edu/faculty/BrianBich/Picture%

The medulla contains the medullary sinuses (composed primarily of reticular fibers and reticular cells that secrete reticulin) and the medullary cords (composed of plasma cells and imunoblasts). 

Oklahoma State University Center for Veterinary Studies
Instructional Resources Histology, 2012

The node is composed mostly of lymphocytes and plasma cells known as free cells, but is also densely packed with reticular fibers. 

Lymphocytes enter lymphatic tissues by migrating from the blood and diffusing across the endothelium of  postcapillary venules known as high endothelial venules (HEV). 

Function of the Lymph Nodes: 

I am going to try really hard to explain the function of the lymph nodes without too much "Immunology Jargon" (you can thank me later...)

As previously stated, the lymph nodes are the first organized site of the lymphatic system to encounter circulating antigens. Antigens, as you might know, are specific substances or toxins that are found on foreign pathogens that separate them from other pathogens and allow for recognition and memory. When a primary immune response is mounted, the adaptive immune system produces antibodies that are specific only to a certain antigen, enabling the body to actually remember a foreign pathogen that may have previously infected it. 

Within the confines of the lymph node, antigens that were circulating in the lymph are presented to the T lymphocytes found in the paracortex region. Many factors, including antigen presentation, MHC-T cell receptor recognition, and stimulation from certain accessory molecules (e.g. CD28, CD40-ligand, etc.) contribute to the eventual cooperation of the T cells and B cells that leads to the creation of an antibody-antigen complex. This Ab:Ag complex will then be sequestered within the germinal centers and will indicate to the immune system that there is a foreign pathogen circulating in the body, causing the body to mount a quick defense to destroy the invader.  For a more detailed or lengthy explanation, you can take Biology 4200, or you can read this article! 

Warner Brothers Cartoon, 1930

I hope you guys enjoyed my blog :)

References: 

1. Kindt, T., Goldsby, R., Osborne, B., 2007. Kuby Immunology. 6^th Edition. New York: W. H. Freeman and Company.
2. Lichtman, M.A., Kipps, T.J., Seligsohn, U., Kaushansky, K., Prchal, J.T. 2010. Williams Hematology. 8th Edition. United States: McGraw-Hill Companies, Inc. 
3. Lichtman, M.A., Shafer, J.A., Felgar, R.E., Wang, N.. 2007. Lichtman's Atlas of Hematology. United States: McGraw-Hill Companies, Inc.  
4. Willard-Mack, CL. 2006. Normal Structure, function and histology of lymph nodes. Toxicologic Pathology. 34(5): 409-424. 

a 

     Sources of all images and diagrams have been cited beneath the specific image and in many cases linked. These images are used for an assignment for Biology 3500 at Memorial University of Newfoundland and are in no way attempting to breach copyright laws. 

a