Lymphatic System and Immune System: High-Yield Study Guide (Chapters 20 & 21)

The Lymphatic System

Functions of the Lymphatic System

• Fluid Balance: Returns excess interstitial fluid to the bloodstream, preventing edema.

• Fat Absorption: Absorbs dietary fats and fat-soluble vitamins from the digestive tract via lacteals.

• Immune Defense: Provides sites for immune surveillance and response, housing lymphocytes and macrophages.

Components of Lymph

• Lymph: Clear fluid derived from interstitial fluid, containing water, proteins, lymphocytes, and sometimes pathogens or cell debris.

• Cells: Mainly lymphocytes (T cells, B cells), macrophages, and dendritic cells.

Order of Lymphatic Vessels (Smallest to Largest)

• Lymphatic Capillaries → Collecting Vessels → Lymphatic Trunks → Lymphatic Ducts

• Lymphatic Ducts: Largest vessels, drain lymph into venous circulation.

Lymph Transport and Flow

• Mechanisms: Lymph is propelled by skeletal muscle contraction, respiratory movements, and smooth muscle in vessel walls.

• Valves: Prevent backflow, ensuring unidirectional movement toward the heart.

• No Central Pump: Unlike blood, lymph flow is slow and low-pressure.

Major Lymphatic Anatomy and Structures

• Thoracic Duct: Drains lymph from most of the body into the left subclavian vein.

• Cisterna Chyli: Enlarged sac at the lower end of the thoracic duct; collects lymph from lower limbs and intestines.

• Right Lymphatic Duct: Drains right upper limb, right side of head and thorax.

• Lymph Nodes: Bean-shaped filters along lymphatic vessels; site of immune cell activation.

• Bubo: Swollen, inflamed lymph node, often due to infection.

Structure and Anatomy of a Lymph Node

• Cortex: Contains follicles with germinal centers (B cells).

• Paracortex: Houses T cells.

• Medulla: Contains medullary cords (B cells, plasma cells, macrophages).

• Afferent Vessels: Bring lymph into the node; Efferent Vessels: Carry lymph away.

Peyer's Patches, Tonsils, Appendix

• Peyer's Patches: Aggregated lymphoid nodules in the small intestine; monitor intestinal bacteria and prevent pathogen growth.

• Tonsils: Lymphoid tissue in the pharynx; trap and destroy pathogens entering via mouth or nose.

• Appendix: Contains lymphoid tissue; may play a role in gut immunity.

Islets of Langerhans

• Location: Pancreas.

• Function: Endocrine cells producing insulin, glucagon, and somatostatin (not a lymphatic structure but relevant for immune-related diabetes).

Functions and Histology of Lymphoid Tissue

• Function: Houses and provides proliferation sites for lymphocytes; surveillance for pathogens.

• Histology: Mainly reticular connective tissue.

Thymus Function

• Site of T Cell Maturation: Educates T lymphocytes to distinguish self from non-self (immunocompetence).

• Most Active in Childhood: Atrophies with age.

Spleen Function

• Filters Blood: Removes old RBCs and platelets; recycles iron.

• Immune Surveillance: Site for lymphocyte proliferation and immune response.

• Stores Platelets: Reservoir for blood platelets and monocytes.

The Immune System

Lymphocytes: Function and Physiology

• B Lymphocytes (B Cells): Produce antibodies; mature in bone marrow.

• T Lymphocytes (T Cells): Cell-mediated immunity; mature in thymus.

• Clonal Selection: Process by which specific lymphocytes proliferate in response to antigen exposure.

Burkitt's Lymphoma

• Definition: Aggressive B-cell non-Hodgkin lymphoma, often associated with Epstein-Barr virus.

• Clinical Feature: Rapidly growing tumor, commonly in jaw or abdomen.

Toll-like Receptors (TLRs) Function

• Pattern Recognition Receptors: Recognize pathogen-associated molecular patterns (PAMPs).

• Trigger Innate Immunity: Activate immune cells and initiate inflammatory response.

Inflammatory Response

• Purpose: Localizes infection, removes pathogens, and initiates tissue repair.

• Cardinal Signs: Redness, heat, swelling, pain, and loss of function.

• Key Steps: Vasodilation, increased vascular permeability, leukocyte recruitment.

Antibodies: Characteristics and Anatomy

• Structure: Y-shaped molecules with two heavy chains and two light chains.

• Variable Region: Antigen-binding site; specific for each antigen.

• Constant Region: Determines antibody class and effector function.

• Binding Sites: Two per antibody molecule.

Natural Killer (NK) Cells

• Function: Destroy virus-infected and cancerous cells by inducing apoptosis.

• Innate Immunity: Act without prior sensitization to antigens.

Passive Immunity

• Definition: Immunity acquired by transfer of antibodies from another source (e.g., maternal antibodies, immunoglobulin therapy).

• Temporary: No memory cells produced.

Types of T Cells

• Cytotoxic T Cells (CD8+): Kill infected or abnormal cells.

• Helper T Cells (CD4+): Activate B cells, other T cells, and macrophages.

• Regulatory (Suppressor) T Cells: Suppress immune response to prevent autoimmunity.

• Memory T Cells: Provide long-term immunity.

B Lymphocytes: Function and Clonal Selection

• Function: Produce antibodies (humoral immunity).

• Clonal Selection: Upon antigen exposure, specific B cells proliferate and differentiate into plasma cells (antibody producers) and memory cells.

Antigens and Haptens

• Antigen: Substance that elicits an immune response.

• Complete Antigen: Can stimulate an immune response and react with immune products.

• Hapten: Small molecule that is antigenic only when attached to a larger carrier.

Immune System: General Function

• Defense: Protects against pathogens, toxins, and cancer cells.

• Surveillance: Detects and eliminates abnormal cells.

• Homeostasis: Removes dead or damaged cells.

Chemotaxis and Diapedesis

• Chemotaxis: Movement of immune cells toward chemical signals at infection sites.

• Diapedesis: Passage of leukocytes through capillary walls into tissues.

Phagocytic Cells and Macrophages

• Phagocytes: Cells that engulf and digest pathogens (e.g., neutrophils, macrophages).

• Macrophages: Derived from monocytes; present antigens to T cells.

Interferons

• Antiviral Proteins: Released by virus-infected cells; inhibit viral replication in neighboring cells.

• Activate Immune Cells: Enhance activity of NK cells and macrophages.

Phagocytosis: Sequence of Events

1. Chemotaxis and adherence of phagocyte to microbe

2. Ingestion of microbe by phagocyte

3. Formation of phagosome

4. Fusion of phagosome with lysosome (phagolysosome)

5. Digestion of ingested microbe

6. Discharge of waste materials

Adaptive Immune System: Details

• Specificity: Targets specific antigens.

• Memory: Faster, stronger response upon re-exposure.

• Components: Humoral (B cells/antibodies) and cellular (T cells) immunity.

Types of Tissue Grafts

Type	Definition
Autograft	Tissue transplanted from one site to another in the same individual
Isograft	Between genetically identical individuals (e.g., identical twins)
Allograft	Between individuals of the same species but genetically different
Xenograft	Between individuals of different species

Second Line of Defense Against Microorganisms

• Includes: Phagocytes, NK cells, inflammation, antimicrobial proteins (complement, interferons), fever.

• Non-specific: Responds to a broad range of pathogens.

Cytokines

• Definition: Small proteins released by cells that regulate immune responses (e.g., interleukins, interferons, TNF).

• Functions: Cell signaling, inflammation, cell recruitment, and activation.

Complement System: Function and Role

• Definition: Group of plasma proteins that enhance immune responses.

• Functions: Opsonization, cell lysis, inflammation, chemotaxis.

Fever: Process and Definition

• Definition: Elevated body temperature due to pyrogens (e.g., cytokines, bacterial toxins).

• Function: Inhibits pathogen growth, enhances immune activity.

Immunocompetence

• Definition: Ability of immune cells to recognize and respond to specific antigens.

• Developed in: Thymus (T cells) and bone marrow (B cells).

Primary vs. Secondary Immune Response

Feature	Primary Response	Secondary Response
Speed	Slower (lag phase)	Faster (memory cells)
Antibody Level	Lower	Higher
Duration	Shorter	Longer

Internal Non-Specific Defenses

• Phagocytes, NK cells, inflammation, antimicrobial proteins, fever.

Class I vs. Class II MHC

Feature	Class I MHC	Class II MHC
Location	All nucleated cells	Antigen-presenting cells (APCs)
Present to	CD8+ T cells	CD4+ T cells
Type of Antigen	Endogenous (intracellular)	Exogenous (extracellular)

Complement Pathways: Classical vs. Alternative

• Classical Pathway: Triggered by antigen-antibody complexes.

• Alternative Pathway: Triggered directly by pathogen surfaces.

MAC (Membrane Attack Complex) Physiology/Function

• Function: Forms pores in pathogen membranes, causing cell lysis.

• Formed by: Terminal complement proteins (C5b, C6, C7, C8, C9).

Types of Immunoglobulins and Functions

Type	Main Function
IgG	Main antibody in secondary response; crosses placenta
IgM	First antibody produced; effective in agglutination
IgA	Found in mucosal areas and secretions (tears, saliva)
IgD	Functions mainly as B cell receptor
IgE	Involved in allergic reactions and defense against parasites

Antigen-Presenting Cells (APCs): Function

• Types: Dendritic cells, macrophages, B cells.

• Function: Process and present antigens to T cells, initiating adaptive immunity.

Examples of Autoimmune Diseases

• Systemic Lupus Erythematosus (SLE): Multisystem autoimmune disease.

• Multiple Sclerosis (MS): Immune attack on myelin in CNS.

• Type I Diabetes Mellitus (DM type I): Immune destruction of pancreatic beta cells.

• Glomerulonephritis: Immune-mediated kidney inflammation.

• Myasthenia Gravis (MG): Antibodies against acetylcholine receptors at neuromuscular junction.

• Graves' Disease: Autoantibodies stimulate thyroid gland.

Additional info: Some topics (e.g., Islets of Langerhans) are not primary lymphatic/immune structures but are included due to their relevance in immune-mediated diseases such as Type I diabetes.