Lymphatic System & Immunity

 1. Introduction

The lymphatic system is a network of vessels, nodes, and organs that returns interstitial fluid to blood, absorbs dietary fats, and participates in immune defense.

Functions:

Fluid balance: Returns 2–3 L interstitial fluid/day to circulation

Fat absorption: Via lacteals in small intestine

Immune surveillance: Filters pathogens via lymph nodes and spleen

Main components: Lymph, Lymphatic vessels, Lymph nodes, Tonsils,  Spleen, Thymus, Bone marrow



2. Lymphatic System Anatomy

2.1 Lymph

Colorless fluid, similar to plasma but lower in proteins, contains lymphocytes and chyle after meals.

Flow: Interstitial space → Lymph capillaries → Lymph vessels → Lymph nodes → Thoracic duct / Right lymphatic duct → Subclavian veins

2.2 Lymphatic Vessels

Structure: Thin-walled, valves prevent backflow, composed of endothelial lining.

Types:

Lymphatic capillaries → blind-ended, permeable to proteins and cells

Collecting vessels → drain into lymph nodes

Trunks: Jugular, Subclavian, Bronchomediastinal, Intestinal, Lumbar

Ducts:

Thoracic duct: Drains most of body into left subclavian vein

Right lymphatic duct: Drains right upper limb, right thorax, head/neck

Clinical correlation: Blockage → lymphedema, e.g., post-mastectomy

2.3 Lymph Nodes

Small, bean-shaped structures filtering lymph, located along vessels.

Structure: Capsule → Cortex → Paracortex → Medulla

Cell types:

Cortex → B lymphocytes

Paracortex → T lymphocytes

Medulla → Plasma cells, macrophages

Major groups: Cervical, Axillary, Inguinal, Mesenteric

Atlas-style diagram placeholder: Label cortex, paracortex, medulla, afferent/efferent vessels.

2.4 Primary Lymphoid Organs

Bone marrow: Hematopoiesis, B cell maturation

Thymus: T cell maturation, located in anterior mediastinum

2.5 Secondary Lymphoid Organs

Spleen: Filters blood, stores platelets, site of immune response

Red pulp → removes old RBCs

White pulp → lymphoid tissue (T and B cells)

Tonsils: Pharyngeal, palatine, lingual → first line of defense in oropharynx

Mucosa-associated lymphoid tissue (MALT): Peyer’s patches, appendix

 

3. Lymphatic Physiology


3.1 Lymph Formation and Flow

Interstitial fluid → Lymph via hydrostatic and osmotic pressures

One-way flow aided by:

Valves

Skeletal muscle contraction

Respiratory movements

Pulsation of adjacent arteries

3.2 Immune Cell Trafficking

Lymph transports antigen-presenting cells (APCs) to lymph nodes

B and T cells enter nodes via high endothelial venules


4. Immunity Overview

4.1 Innate (Non-specific) Immunity

First line of defense, rapid, no memory

Components:

Physical: Skin, mucous

Chemical: Lysozyme, acidic pH, complement proteins

Cellular: Neutrophils, macrophages, NK cells, dendritic cells



4.2 Adaptive (Specific) Immunity

Slower, highly specific, immunological memory

Components:

Humoral immunity (B cells/antibodies)

Plasma cells produce antibodies: IgG, IgA, IgM, IgE, IgD

Functions: Neutralization, opsonization, complement activation

Cell-mediated immunity (T cells)

CD4+ helper T cells → activate B cells, macrophages

CD8+ cytotoxic T cells → kill infected cells

 


4.3 Antigen Presentation

APCs: Dendritic cells, macrophages, B cells

Present processed antigen with MHC molecules to T cells




6. MCQs

Primary site of T cell maturation → Thymus

Main antibody in mucosal secretions → IgA

Lymphatic duct draining left upper limb, left thorax → Thoracic duct

APC presenting antigen to helper T cells → Dendritic cells

Bean-shaped organ filtering lymph → Lymph nodes


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