The Lymphatic System and Immune system

 Introduction

 

• The lymphatic system consist of a fluid called lymph flowing within lymphatic tissue, and red bone marrow, which houses stem cells that develop into lymphocytes.
• The composition of interstitial fluid and lymph are  basically the same, the major difference between the  two is the location. After fluid passes from interstitial  spaces into lymphatic vessels, it is called lymph.

• Lymphatic tissues is a specialized type of connective tissue that contains large numbers of lymphocytes.

Lymphatic System

Network of vessels that parallel the circulatory system + discrete organs
Three primary functions:
fluid recovery – reabsorb excess fluid that filters into interstitial spaces
immunity – production, maturation, and activation of white cells
lipid absorption – specialized vessels within small intestine.

Components of the Lymphatic System

• Lymph
• Lymphatic vessels
• Lymph nodes
• Lymphatic organs (spleen and thymus)
• (bone marrow)   
  

 
Lymphatic Vessels

Lymph = clear fluid similar to plasma but low in protein

• carries bacteria, viruses, cellular debris away from tissues
• circulates lymphocytes throughout the bloodstream

Lymphatic vessels – lymphatic capillaries penetrate nearly all body tissues next to the blood capillaries
 

Structure of lymphatic capillaries:

• closed at one end
• membrane formed by overlapping layers of endothelial cells

• gaps between layers allow bacteria and cellular debris to enter with fluid
• overlapping layers act as one-way valves that open when hydrostatic pressure is higher in tissues
• capillaries join into collecting vessels, then into 6 lymphatic trunks, then 2 collecting ducts, which empty into the subclavian veins.
  

Lymph therefore flows away from tissues and back towards the heart
bundled with arteries in common sheath – arterial pulsation squeezes lymphatic vessels to promote flow; one-way valves keep lymph flowing towards heart
aided by skeletal muscle and thoracic pumps

Lymphatic Tissues

Diffuse lymphatic tissues (collections of lymphocytes) are prevalent in body passages that open to the environment

• respiratory system
• digestive system
• urinary system
• reproductive tracts

Lymphatic Organs

Lymph nodes – permanent collections of lymphatic tissue positioned along the lymphatic vessels in chains

• superficial: cervical, axillary, and inguinal
• deep: thoracic, abdominal, and pelvic

Lymph enters by way of afferent vessels, slowly circulates through the nodal tissue, and leaves by way of efferent vessels

• pathogens contained and destroyed in the nodal tissue
• lymphocytes leave the node by way of efferent vessels and ultimately enter the bloodstream

Structure of the lymph node
 

 

 
 

 

Lymphatic Organs, cont’d

Tonsils – patches of lymphatic tissue at the entrance to the pharynx

• guard against ingested and inhaled pathogens

Thymus – located in the superior mediastinum

• very large at birth and during childhood, shrinks after age 14
• stores lymphocytes and secretes hormones to regulate their activity

Spleen – largest lymphatic organ, located inferior to diaphragm and dorsolateral to stomach (left)

• monitors blood for foreign antigens
• contains lymphocytes that activate immune response, and macrophages that phagocytize cellular debris
• can compensate for excessive blood volume by transferring plasma from bloodstream to lymphatic vessels

 Body’s Defenses Against Foreign Pathogens

 
Pathogens = toxins, living organisms, & other agents that cause disease
Body exposed to countless pathogens in interaction with environment

2 main lines of defense:

• nonspecific resistance - external barriers, antimicrobial mechanisms
• specific immunity – exposure to specific pathogens invokes future protection

Non-specific Resistance

external barriers: skin and mucous membranes

• tough surfaces
• hostile environment (antimicrobial chemicals, acid mantle)
• mucus to trap microbes
• enzymes to destroy cells

leukocytes

• neutrophils – chemically and physically destroy bacteria; die in the process so must constantly be produced
• eosinophils – abundant in the respiratory and digestive tracts; physically destroy allergens, antigen-antibody complexes
• basophils – aid the action of other leukocytes
• lymphocytes – mostly involved in specific immunity, but natural killer (NK) cells attack body cells invaded by cancer or infection
• monocytes – give rise to macrophages
• macrophages – phagocytic cells that circulate in bloodstream or live within tissues
  antimicrobial proteins
•  interferons – secreted by cells invaded by a virus, stimulate neighboring cells to produce anti-viral proteins to contain infection
•  complement system – plasma proteins activated by pathogens and antibodies that contribute to chemical destruction  
•  inflammation – local response to tissue injury
  

• mobilization of defenses – vasodilators increase blood flow for delivery of cells and chemicals
• containment and destruction – fibrinogen traps microbes for attack by cells and chemicals
• tissue cleanup and repair – debris absorbed by lymphatic capillaries, fibrin clot provides matrix for tissue reconstruction

fever (pyrexia) – produced by pyrogens secreted by neutrophils and macrophages

• promotes interferon activity(stimulate neighboring cells to produce anti-viral proteins to contain infection)
• elevates metabolic rate and accelerates tissue repair
• inhibits reproduction of bacteria and viruses
• beneficial up to about 40oC
• excessive temperatures can cause metabolic dysfunction and brain damage

Specific Immunity

“immune system” is not a specific organ system, but a series of defense mechanisms
directed against specific pathogens
information stored so that repeat exposure results in rapid defense reaction
terminology:

• active immunity – production of one’s own antibodies as result of exposure or vaccination
• passive immunity – temporary protection from acquired antibodies (maternal or administered)

Antigens = complex molecules that trigger an immune response

• venom
• toxins
• components of bacterial membranes

Antibodies (immunoglobulins) = proteins produced by immune cells to destroy antigens
Uniqueness allows immune system to distinguish between “self” and “other”

Primary cells involved in immunity are the lymphocytes
T-lymphocytes

• produced and regulated by the thymus
• contact with antigens on foreign cells stimulates direct attack

B-lymphocytes

• produced in the bone marrow
• contact with antigens on foreign cells stimulates production of antibodies

Cellular (cell-mediated) immunity: based on direct action of T-lymphocytes
Humoral (antibody-mediated) immunity: based on antibodies produced by B-lymphocytes and circulated throughout the bloodstream
Two types of immunity interact to contain, attack, and destroy pathogens

Cellular Immunity
Involves 4 types of T-lymphocytes:

• cytotoxic T-cells: carry out the attack
• helper T-cells: promote cytotoxicT-cell action and humoral immunity
• suppressor T-cells: limit response to dangerous pathogens
• memory T-cells:  store information about specific antigens

 Involves 3 steps:

• recognition: antigen-presenting cells (macrophages, etc.) within tissues capture foreign cells for recognition by circulating T-cells
• attack: cytotoxic T-cells release chemicals that destroy foreign cells, helper T-cells attract and stimulate other leukocytes, and suppressor T-cells regulate degree of reaction
• memory: cytotoxic and helper T-cells clone cells programmed against repeated invasion

 Immune response

Response to first exposure to a pathogen = primary response

• time required to activate system
• generates memory B-cells for future immunity

Response to subsequent infection = secondary response
much faster and more powerful

 Types of Immunity

• Active, naturally acquired: response to having the infection
• Active, artificially acquired: vaccination
• Passive, naturally acquired: maternal transmission
• Passive, artificially acquired: exogenous antibodies provide temporary protection

 QUESTIONS TO UNDERSTAND;

1. Describe the lymphatic drainage of the upper limb
2. Describe the lymphatic drainage of the lower limb
3. Describe the lymphatic drainage of the abdomen
4. Describe the Lymphatic drainage of the head and the neck
5. Describe the lymphatic drainage of the thorax and the breast.
6. Describe the Anatomy and physiology of the thymus(location, structures, blood vessels, nerve supply, hormones and its function)
7. Describe the Anatomy and physiology of the spleen(location, structures, blood vessels, nerve supply, hormones and its function)