The Lymphatic System and Immunity: Structured Study Notes

The Lymphatic System

Overview and Main Components

The lymphatic system is a network of vessels, tissues, and organs that plays a crucial role in fluid balance, fat absorption, and immune defense. It consists of two main components:

• Lymphatic vessels: Transport lymph (a fluid containing white blood cells and other substances).

• Lymphoid tissues and organs: Include structures such as lymph nodes, spleen, thymus, and MALT.

Major functions of the lymphatic system:

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

• Absorption of dietary fats: Specialized lymphatic capillaries called lacteals in the small intestine absorb fats and fat-soluble vitamins.

• Immune defense: Filters pathogens and houses immune cells for defense against infection.

Lymphatic Circulation

Lymph flows through a series of vessels and structures before returning to the bloodstream. The pathway is as follows:

• Lymphatic capillaries → collecting vessels → lymphatic trunks → lymphatic ducts → veins

Major lymphatic trunks and their regions:

Lymph Trunk	Region Drained
Lumbar	Lower limbs
Intestinal	Digestive organs
Jugular	Head and neck
Bronchomediastinal	Thorax
Subclavian	Upper limbs

Key structures:

• Cisterna chyli: A dilated sac at the lower end of the thoracic duct.

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

• Right lymphatic duct: Drains lymph from the right upper body into the right subclavian vein.

Mechanisms of lymph movement:

• Valves prevent backflow.

• Skeletal muscle contraction propels lymph.

• Smooth muscle in vessel walls aids movement.

• No central pump (unlike the heart for blood).

Lymphedema

Lymphedema is swelling caused by blockage or removal of lymphatic vessels, often seen after lymph node removal (e.g., during cancer surgery).

Lymphoid Tissues and Cells

Reticular (lymphoid) tissue forms the structural framework of lymphoid organs. Major immune cells include:

• B cells: Produce antibodies (humoral immunity).

• T cells: Mediate cellular immunity.

• Macrophages: Phagocytose pathogens and present antigens.

• Dendritic cells: Activate T cells by presenting antigens.

MALT (Mucosa-Associated Lymphatic Tissue)

MALT protects mucosal surfaces exposed to external pathogens. It is found in:

• Gastrointestinal (GI) tract

• Respiratory tract

• Genitourinary (GU) tract

Key MALT structures:

• Tonsils: Pharyngeal, palatine, and lingual

• Peyer’s patches: Located in the ileum

• Appendix

Lymph Nodes

Lymph nodes are small, bean-shaped structures that filter lymph and initiate immune responses.

• Structure: Capsule → cortex → medulla

• Flow: Afferent vessels → sinuses → efferent vessels (exit at hilum)

• Function: Filter lymph (trap ~90% of pathogens), site of immune activation

• Locations: Cervical, axillary, inguinal, mesenteric

Spleen

The spleen filters blood (not lymph), destroys old red blood cells (RBCs), and provides immune surveillance.

• Red pulp: Destroys RBCs

• White pulp: Contains immune cells

Thymus

The thymus is the site of T cell maturation, is large in children, shrinks with age, and produces hormones such as thymosin.

Immune System Overview

Lines of Defense

The immune system protects the body through three lines of defense:

1. First line: Physical barriers (skin, mucous membranes)

2. Second line: Innate (nonspecific) immunity

3. Third line: Adaptive (specific) immunity

Innate vs Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Speed	Fast (hours)	Slower (3–5 days)
Specificity	Nonspecific	Specific to antigens
Memory	None	Immunological memory
Key Cells	NK cells, phagocytes, complement	B cells, T cells

Surface Barriers (First Line)

• Skin: Keratinized, tough barrier

• Mucus: Traps pathogens

• Cilia: Move debris out of airways

• Stomach acid: Destroys microbes

• Defensins: Antimicrobial peptides

• Normal flora: Compete with pathogens

Innate Immune Cells (Second Line)

• Neutrophils: First responders, kill bacteria

• Macrophages: Phagocytosis and antigen presentation

• Dendritic cells: Activate T cells

• Natural Killer (NK) cells: Kill infected/cancer cells

• Eosinophils: Attack parasites

• Basophils/Mast cells: Release histamine, mediate inflammation

Complement System (Second Line)

The complement system is a group of plasma proteins activated by three pathways:

• Classical

• Lectin

• Alternative

Main effects:

• Cell lysis via membrane attack complex (MAC)

• Inflammation

• Opsonization (enhances phagocytosis)

• Clearance of immune complexes

Cytokines (Second Line)

• Tumor necrosis factor (TNF): Promotes inflammation, can cause septic shock

• Interferons: Antiviral effects

• Interleukins: Regulate immune cell activity

Inflammatory Response

The inflammatory response occurs in two stages:

1. Stage 1: Mediator release

   • Histamine, prostaglandins, and other mediators released

   • Four cardinal signs: redness, heat, swelling, pain

2. Stage 2: Phagocyte response

   • Neutrophils arrive first, followed by monocytes (become macrophages)

   • Phagocytosis and cleanup of debris

Key processes: Chemotaxis, margination, diapedesis

Pus is composed of dead cells and fluid; an abscess is a pus-filled cavity.

Fever

• Caused by pyrogens

• Regulated by the hypothalamus

• Enhances immune response

Anti-Inflammatory Drugs

• NSAIDs (e.g., ibuprofen): Block prostaglandin synthesis

• Corticosteroids: Block prostaglandins and leukotrienes (stronger effect)

Adaptive Immunity (Third Line of Defense)

Key Characteristics

• Specificity: Targets specific pathogens

• Memory: Faster, stronger response upon re-exposure

Cell-Mediated Immunity (T Cells)

• Helper T cells (CD4): Coordinate immune response

• Cytotoxic T cells (CD8): Kill infected or cancerous cells

T cells respond to: Virus-infected cells, intracellular bacteria, cancer cells, transplanted tissue

Antigens

• Antigens: Substances that trigger an immune response

• Immunogens: Antigens that actually cause a response

• Self antigens: Normally ignored

• Non-self antigens: Attacked by the immune system

• Haptens: Small molecules that become antigenic when attached to proteins (e.g., allergies)

Exogenous antigens: From outside cells (e.g., bacteria)

Endogenous antigens: From inside cells (e.g., viruses, cancer)

T Cell Development

1. Origin: Bone marrow

2. Maturation: Thymus

3. Activation: Lymphoid organs

• Immunocompetence: Ability to respond to antigen

• Self-tolerance: Does not attack self

• ~95% of T cells are destroyed during screening

MHC Molecules

Type	Location	Antigen Presented	Activates
Class I MHC	All nucleated cells	Endogenous	Cytotoxic T cells
Class II MHC	APCs (dendritic, macrophages, B cells)	Exogenous	Helper T cells

T Cell Activation

1. Antigen presented on MHC

2. T cell binds specific antigen

3. Co-stimulation required

4. Clonal selection

5. Differentiation into effector and memory T cells

Roles of T Cells

• Helper T cells: Activate macrophages, cytotoxic T cells, and stimulate B cells

• Cytotoxic T cells: Kill infected cells using perforin (creates pores) and enzymes (induce apoptosis)

Transplants and Rejection

Graft Type	Source	Rejection Risk
Autograft	Self	None
Isograft	Identical twin	None
Allograft	Same species	Common
Xenograft	Different species	High

HLA (human leukocyte antigen) matching is important for transplant compatibility.

Antibody-Mediated Immunity (B Cells)

B cells produce antibodies in response to antigens. The process occurs in three phases:

1. Activation

2. Antibody production

3. Memory formation

B Cell Activation Steps:

1. B cell binds antigen

2. Presents antigen on MHC II

3. Helper T cell activates B cell

4. Clonal expansion

• Plasma cells: Secrete antibodies

• Memory B cells: Provide long-term immunity

Antibodies (Immunoglobulins)

• Y-shaped proteins with heavy and light chains

• Variable region binds antigen

Antibody Classes (GAMED):

Class	Main Features
IgG	Most abundant, crosses placenta
IgA	Found in secretions (saliva, milk, mucus)
IgM	First produced, largest (pentamer)
IgE	Allergies, parasites, triggers histamine release
IgD	B cell receptor

Functions of antibodies:

• Agglutination (clumping)

• Precipitation

• Opsonization (enhances phagocytosis)

• Neutralization (blocks toxins/viruses)

• Complement activation

• Stimulation of inflammation

Immunological Memory

• Primary response: Slow (4–5 day lag), IgM first

• Secondary response: Fast (1–3 days), stronger, mostly IgG, often asymptomatic

Vaccines

• Live attenuated

• Inactivated

• Subunit/toxoid

• mRNA

Vaccines create memory cells without causing disease.

Active vs Passive Immunity

Type	Source of Antibodies	Duration
Active	Produced by your body	Long-lasting
Passive	Received from another source (e.g., maternal IgG, antivenom)	Short-term

Immune Disorders

Types of Immune Disorders

1. Hypersensitivity: Overreaction of the immune system

2. Immunodeficiency: Weak or absent immune response

3. Autoimmune: Immune system attacks self

Hypersensitivity Types

Type	Mechanism	Examples
I (Allergies)	IgE-mediated, histamine release	Hay fever, asthma, anaphylaxis
II	Antibodies attack self cells	Transfusion reaction
III	Immune complexes deposit in tissues	Systemic lupus erythematosus
IV (Delayed)	T cell-mediated, delayed response	Poisony ivy, TB test

Immunodeficiency Disorders

• Primary: Genetic (e.g., SCID)

• Secondary: Acquired (e.g., HIV/AIDS)

HIV & AIDS

• HIV attacks CD4 (helper T) cells

• Uses reverse transcriptase to insert DNA into host

• Stages: Acute → Chronic → AIDS

• Results in opportunistic infections and cancers (e.g., Kaposi’s sarcoma)

Autoimmune Disorders

• Caused by loss of self-tolerance

• Examples: Multiple sclerosis, rheumatic fever, type 1 diabetes, lupus

Key Concepts for Mastery

• Trace lymph flow from capillaries to ducts to veins

• Identify all lymphatic organs and their functions

• Compare innate and adaptive immunity

• Explain inflammation step-by-step

• Describe roles of key immune cells

• Explain complement system effects

• Identify tonsils, spleen, thymus, lymph nodes

• Explain fever mechanism

• Understand MALT and lymph node structure

• Compare cell-mediated and antibody-mediated immunity

• Explain MHC I vs MHC II

• Describe T cell activation step-by-step

• Explain B cell activation and antibody production

• List and differentiate 5 antibody classes (GAMED)

• Compare primary vs secondary immune responses

• Identify types of vaccines

• Explain hypersensitivity types I–IV

• Understand HIV mechanism and progression

• Explain causes of autoimmune disease