Lymphatic System and Immunity: Structured Study Notes

Lymphatic System: Structure and Function

Overview of the Lymphatic System

The lymphatic system is a network of vessels, nodes, and organs that helps maintain fluid balance, absorbs fats, and provides immune defense. It returns excess tissue fluid to the bloodstream and filters out harmful substances.

• Lymph: Fluid in tissue spaces carrying proteins and other substances back to the blood.

• Lymphatic vessels: Permit one-way movement of lymph, starting from microscopic capillaries.

• Lacteals: Specialized lymphatic capillaries in the intestinal wall for fat absorption.

Lymphatic Vessels and Ducts

Lymphatic vessels transport lymph from tissues to the bloodstream.

• Right lymphatic duct: Drains lymph from the right upper extremity and right side of head, neck, and upper torso.

• Thoracic duct: Largest lymphatic vessel, drains lymph from about three-fourths of the body; contains the cisterna chyli.

Lymphatic System in Fluid Homeostasis

The lymphatic system maintains fluid balance by returning interstitial fluid to the blood.

• Lymphatic capillaries absorb excess fluid from tissues.

• Lymph nodes filter lymph before it returns to circulation.

Lymphatic Disorders

Lymphedema

Lymphedema is swelling caused by blockage of lymphatic vessels.

• Lymphangitis: Inflammation of lymphatic vessels, may progress to septicemia.

• Elephantiasis: Severe lymphedema due to parasite infestation.

Lymphoid Organs

Lymph Nodes

Lymph nodes filter lymph and are located in clusters along lymphatic vessels.

• Lymphoid tissue: Mass of lymphocytes and related cells inside lymphoid organs.

• Functions: Defense and white blood cell formation.

• Lymph flows into nodes via afferent vessels and exits via a single efferent vessel.

• Lymphadenitis: Swelling and tenderness of lymph nodes.

• Metastasis: Cancer cells can spread through lymphatic vessels.

Lymphatic Drainage of the Breast

The breast is drained by an extensive network of lymphatic vessels, which is clinically important in cancer spread and surgery.

Thymus

The thymus is a lymphoid organ in the mediastinum, vital for immunity.

• Produces T lymphocytes (T cells).

• Secretes thymosins, hormones influencing T-cell development.

• Undergoes involution (replaced by fat) during childhood.

Tonsils

Tonsils are masses of lymphoid tissue around the mouth and throat.

• Types: Palatine, pharyngeal (adenoids), and lingual tonsils.

• Function: Defense against pathogens entering through the mouth and throat.

• Chronic infection and enlargement can impair breathing.

Spleen

The spleen is the largest lymphoid organ, located in the upper left abdomen.

• Functions: Phagocytosis of bacteria and old RBCs, blood reservoir.

• Splenomegaly: Enlargement of the spleen.

• Splenectomy: Surgical removal of the spleen.

Lymphoma

Lymphoma is a malignant tumor of lymph nodes.

• Two principal types: Hodgkin disease and non-Hodgkin lymphoma.

• Symptoms: Enlarged, painless lymph nodes, anemia, weight loss, weakness, fever, and spread to other lymphoid tissues.

Immune System: Function and Types

Function of the Immune System

The immune system protects the body from pathogens, foreign tissue cells, and cancerous cells.

• Composed of defensive cells and molecules.

• Includes nonspecific (innate) and specific (adaptive) immunity.

Innate (Nonspecific) Immunity

Innate immunity provides rapid, broad defense against pathogens.

• Recognizes a variety of nonself cells and particles.

• Includes mechanical barriers (skin), leukocytes, tears, mucus, and inflammation.

• Signs of inflammation: Heat, redness, pain, swelling.

Adaptive (Specific) Immunity

Adaptive immunity is characterized by specificity and memory.

• Recognizes and remembers specific harmful substances or bacteria.

• Includes inherited (inborn) and acquired immunity.

Acquired Immunity

Acquired immunity can be natural or artificial.

• Natural immunity: Not deliberate exposure; includes active (disease produces immunity) and passive (maternal antibodies).

• Artificial immunity: Deliberate exposure; includes active (vaccination) and passive (antibodies from another individual).

Immune System Molecules

Antibodies

Antibodies are protein compounds with specific combining sites for antigens.

• Form antigen-antibody complexes (humoral immunity).

• Functions: Neutralize toxins, agglutinate enemy cells, promote phagocytosis.

Complement Proteins

Complement proteins are present in blood in an inactive state and are activated by antigen-antibody complexes.

• Trigger a cascade of reactions, forming protein rings that create holes in foreign cell membranes.

• Result: Cell lysis and enhanced inflammatory response.

Immune System Cells

Phagocytes

Phagocytes ingest and destroy foreign cells or harmful substances.

• Types: Neutrophils (short-lived), monocytes (become macrophages), dendritic cells (APCs).

• Macrophages and dendritic cells act as antigen-presenting cells (APCs).

Lymphocytes

Lymphocytes are the most numerous immune system cells.

• Types: Natural killer cells (innate immunity), B cells, T cells (adaptive immunity).

• Each type has distinct roles in immunity.

Natural Killer Cells

NK cells kill tumor cells and virus-infected cells without prior exposure to antigens.

• Use "kill-activating" and "kill-inhibiting" receptors to distinguish self from abnormal cells.

• Trigger apoptosis in target cells.

B Cells

B cells develop in the liver and bone marrow, then migrate to lymph nodes.

• Activated B cells form plasma cells (secrete antibodies) and memory cells.

• Produce humoral immunity.

T Cells

T cells develop in the thymus and migrate to lymph nodes.

• Activated T cells form effector and memory cells.

• Produce cell-mediated immunity.

• Types: Cytotoxic T cells (kill infected/tumor cells), helper T cells (activate macrophages and B cells), regulatory T cells (suppress immune responses).

Immune System Disorders

Hypersensitivity

Hypersensitivity is an inappropriate or excessive immune response.

• Allergy: Response to harmless environmental antigens (allergens).

• Immediate responses involve humoral immunity; delayed responses involve cell-mediated immunity.

Autoimmunity

Autoimmunity is an excessive response to self-antigens, causing autoimmune diseases.

• Example: Systemic lupus erythematosus (SLE).

Alloimmunity

Alloimmunity is an excessive reaction to antigens from another human, important in pregnancy and tissue transplants.

• Also called isoimmunity.

Immune System Deficiency

• Congenital immune deficiency: Results from improper lymphocyte development before birth (e.g., SCID).

• Acquired immune deficiency: Develops after birth (e.g., AIDS caused by HIV infection of T cells).

Tables

Innate and Adaptive Immunity Comparison

Feature	Innate Immunity	Adaptive Immunity
Synonyms	Nonspecific, native, genetic	Specific, acquired
Specificity	Not specific	Specific
Speed of reaction	Rapid	Slower
Memory	None	Yes
Chemicals	Complement, interferons	Antibodies, cytokines
Cells	Phagocytes, dendritic cells	Lymphocytes (B and T cells)

Mechanisms of Innate Defense

Mechanism	Description
Mechanical and chemical barriers	Physical impediments to entry of foreign cells/substances
Skin and mucous membranes	Continuous wall separating internal/external environments
Secretions	Inhibit entry/survival of pathogens
Inflammation	Isolates pathogens, stimulates immune response
Fever	Enhances immune reactions
Phagocytosis	Ingestion/destruction of pathogens
Neutrophils	Granular leukocytes, short-lived phagocytes
Macrophages	Large phagocytes, often found in tissues
Natural killer cells	Kill any cell lacking self-antigen
Complement	Protein group producing cascade of reactions
Interferon	Proteins released by infected cells, inhibit viral infection

Review Questions

• What is the role of lymph nodes in the body?

• Describe the difference between afferent and efferent lymphatic vessels.

• Why is the thymus important for immunity?

• What are tonsils and their function?

• What is the role of the spleen?

• Identify the two principal categories of lymphoma.

• What is the difference between adaptive and innate immunity?

• Outline the changes in the body's inflammatory response.

• Describe the difference between active and passive immunity.

Additional info:

• Exercise can increase the number and activity of immune cells, especially after trauma or surgery.

• Clinical application: Lymphedema may occur after breast surgery due to removal of lymphatic vessels and nodes.