Lymphatic and Immune System

OT Dude Academy’s Free NBCOT® Exam Prep Course Medical Terminology, Anatomy, and Physiology Lymphatic and Immune System
Contents Hide
1 Introduction
2 Lymphatic Capillaries
3 Lymphatic Vessels, Trunks, Ducts
4 Primary Lymphoid Organs
4.1 Bone Marrow
4.2 Thymus
5 Secondary Lymphoid Organs
5.1 Lymph Nodes
5.2 Spleen
5.3 Lymphoid Nodules
6 Immune System
7 Inflammatory Response
8 Benefits of the Adaptive Immune Response
9 Antibodies
10 Active vs. Passive Immunity

Source 1  licensed under Creative Commons Attribution 4.0 International License.

Introduction

  • The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it to the blood.
  • The lymphatic vessels are also used to transport dietary lipids and cells of the immune system. Cells of the immune system, lymphocytes, all come from the hematopoietic system of the bone marrow.
  • Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes proliferate and mature.
  • Secondary lymphoid organs are the sites in which mature lymphocytes congregate to mount immune responses.
  • Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.
  • The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream.
  • Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen.

Lymphatic Capillaries

  • An important function of the lymphatic system is to return the fluid (lymph) to the blood.
  • Lymph may be thought of as recycled blood plasma.
  • Blood pressure causes leakage of fluid from the blood capillaries, resulting in the accumulation of fluid in the interstitial space.
  • If the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, interstitial fluid accumulates in the tissue spaces, causing a condition called lymphedema.

Lymphatic Vessels, Trunks, Ducts

  • Lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves.
  • In general, superficial lymphatics, follow the same routes as veins, whereas deep lymphatic vessels of the viscera generally follow the paths of arteries.
  • The superficial and deep lymphatics eventually merge to form larger lymphatic structures known as lymphatic trunks.
  • On the right side of the body, the right sides of the head, thorax, and right upper limb trunks drain lymph fluid into the right subclavian vein via the right lymphatic duct.
  • On the left side of the body, the trunks from the remaining portions of the body drain into the larger thoracic duct, which drains into the left subclavian vein.
  • The thoracic duct itself begins just beneath the diaphragm in the cisterna chyli.

Primary Lymphoid Organs

Bone Marrow

  • All blood cells, including lymphocytes, are formed in the red bone marrow.
  • The B cell undergoes nearly all of its development in the red bone marrow.
  • An immature T cell, called a thymocyte, leaves the bone marrow and matures largely in the thymus gland.

Thymus

  • The thymus gland is where T cells mature.

Secondary Lymphoid Organs

Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules.

Lymph Nodes

  • Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the “filters of the lymph”.
  • Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node.
  • Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body.
  • The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system.

Spleen

  • Called the “filter of the blood” because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells.
  • Functions as the location of immune responses to blood-borne pathogens.

Lymphoid Nodules

  • Located in the respiratory and digestive tracts.
  • Are routinely exposed to environmental pathogens.
  • Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens.
  • Bronchus-associated lymphoid tissue (BALT) is effective against inhaled pathogens.
    • Found along the bifurcations of the bronchi, and between bronchi and arteries
  • Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with the mucous membrane.
    • Makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes.
    • Important for immune responses against ingested substances.
    • Adaptive immune response to potential pathogens.

Immune System

Barrier defenses are part of the body’s most basic innate defense mechanisms.

  • Skin: Keratinized cells of the surface are too dry for bacteria to grow and are continuously sloughed off, along with pathogens that are on their surfaces. Lower pH than pathogens prefer may contain substances that are toxic to pathogens, washing action.
  • Oral Cavity: Lysozyme is an enzyme that destroys bacteria.
  • Stomach: Low pH which is fatal to many pathogens.
  • Mucosal: Traps both microbes and debris and facilitates their removal.
  • Flora: Prevents pathogens from growing on mucosal surfaces.

Whereas barrier defenses are the body’s first line of physical defense against pathogens, innate immune responses are the first line of physiological defense.

  • Innate responses occur rapidly but with less specificity and effectiveness than the adaptive immune response.
  • Within the first few days of an infection, a series of antibacterial proteins are induced, each with activities against certain bacteria.
  • Interferons are induced that protect cells from viruses in their vicinity.
  • Does not stop when the adaptive immune response is developed. Both can cooperate and one can influence the other in their responses against pathogens.
  • Innate immune responses (and early induced responses) are in many cases ineffective at completely controlling pathogen growth but they slow pathogen growth and allow time for the adaptive immune response to strengthen and either control or eliminate the pathogen.
  • The innate immune system also sends signals to the cells of the adaptive immune system, guiding them in how to attack the pathogen.
  • Cells of the innate immune response: macrophages, neutrophils, monocytes, natural killer cells.
  • A cytokine is a signaling molecule that allows cells to communicate with each other over short distances.
  • The hallmark of the innate immune response is inflammation.
    • Heat, redness, pain, and swelling.

Inflammatory Response

  • The released contents of injured cells stimulate the release of substances from mast cells including histamine, leukotrienes, and prostaglandins.
  • Histamine increases blood flow to the area by vasodilation, resulting in heat and redness. Histamine also increases the permeability of local capillaries, causing plasma to leak out and form interstitial fluid, resulting in swelling.
  • Leukotrienes attract neutrophils from the blood by chemotaxis.
  • When local infections are severe, neutrophils are attracted to the sites of infections in large numbers, and as they phagocytose the pathogens and subsequently die, their accumulated cellular remains are visible as pus at the infection site.
  • Prostaglandins cause vasodilation by relaxing vascular smooth muscle and are a major cause of the pain associated with inflammation.

Benefits of the Adaptive Immune Response

  • Specificity
  • Immunological memory
  • Self-recognition

Antibodies

B cell (lymphocyte) that has been activated through exposure to an antigen and produces antibodies against that antigen.

  • IgM promotes chemotaxis, opsonization, and cell lysis, making it a very effective antibody against bacteria at early stages of a primary antibody response.
  • IgG is the one that crosses the placenta to protect the developing fetus from disease and exits the blood to the interstitial fluid to fight extracellular pathogens.
  • IgA is the only antibody to leave the interior of the body to protect body surfaces. IgA is also of importance to newborns, because this antibody is present in mother’s breast milk (colostrum), which serves to protect the infant.
  • IgE is associated with allergies and anaphylaxis.

Active vs. Passive Immunity

Immunity to pathogens, and the ability to control pathogen growth so that damage to the tissues of the body is limited, can be acquired by:

  1. The active development of an immune response in the infected individual.
    or
  2. The passive transfer of immune components from an immune individual to a non-immune one.

The downside to this passive immunity is the lack of the development of immunological memory. Once the antibodies are transferred, they are effective for only a limited time before they degrade.

Vaccinations create an artificial active immune response.

 

  1. Betts, J.G., Young, K.A., Wise, J.A., Johson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J.E., Womble, M. & DeSaix, P. (2013). Anatomy and Physiology. OpenStax. http://cnx.org/content/col11496/latest/[↩]
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