Thymus, Lymphoid Tissues and Lymph

Introduction

• The immune system relies on specialized lymphoid organs for defense and surveillance.
• The thymus supports T cell maturation, while lymph nodes and spleen coordinate immune responses, filter blood, and signal disease through enlargement during infection or abnormal cell proliferation.

Thymus

• The thymus is a primary lymphoid organ located in the superior mediastinum.
• It is well developed at birth, weighing about 10 to 15 grams, and enlarges to 30 to 40 grams at puberty.
• After puberty, it undergoes involution, with gradual replacement by adipose tissue.
• Despite regression, it continues to support T lymphocyte maturation throughout life.

Structure

• Structurally, the thymus has two lobes connected by fibrous tissue.
• Each lobe is enclosed by a connective tissue capsule that sends septa inward, forming multiple lobules.
• Each lobule measures approximately 2 millimeters and has an outer cortex and inner medulla.
• The cortex is densely packed with immature lymphocytes, while the medulla contains fewer lymphocytes and more epithelial cells.
• The medullary regions of adjacent lobules are continuous.
• The thymus has a rich vascular supply for cellular exchange.
• It lacks afferent lymphatic vessels but provides efferent lymphatic drainage. ·  The organ plays a crucial role in immune system development and central tolerance.

Epithelial Cells

• Epithelial cells of the thymus, also called epitheliocytes, form a structural and functional framework.
• Type 1 cells line the capsule, septa, and blood vessels, contributing to the blood thymus barrier.
• Type 2 and Type 3 cells are located in the outer and inner cortex respectively.
• Type 4 cells are present in deep cortex and medulla, forming a network supporting lymphocytes.
• Type 5 cells surround Hassall corpuscles in the medulla.
• Cortical epithelial cells act as thymic nurse cells, eliminating self-reactive lymphocytes.

Lymphocytes of the Thymus

• Thymocytes are developing T lymphocytes within the thymus.
• They are densely packed in the cortex and less numerous in the medulla.
• As they mature, they migrate from cortex to medulla.
• Mature thymocytes exit via blood vessels and efferent lymphatics.

Macrophages

• Macrophages are part of the mononuclear phagocyte system in the thymus.
• Subcapsular macrophages show strong phagocytic activity.
• Deeper macrophages function as dendritic cells, contributing to immune regulation and antigen presentation.

Corpuscular Hassall

• Hassall corpuscles are rounded structures located in the medulla of the thymus.
• They consist of a central core formed by degenerating epithelial cells, producing a hyaline appearance.
• This core is surrounded by concentrically arranged epithelial cells.
• The central region may also contain degenerating macrophages.
• These structures are associated with maturation and regulation of T lymphocytes within the thymus.

Functions of the Thymus

• The thymus is the primary site for development and maturation of T lymphocytes from precursor cells.
• Bone marrow derived precursor cells migrate to the thymus and differentiate into functional T cells.
• Epithelial cells provide structural support and signals essential for T cell maturation.
• The thymus establishes immune tolerance by eliminating self-reactive lymphocytes.
• Although it undergoes involution in adults, the medulla remains functionally active.

Role in Lymphopoiesis

• The thymus plays a central role in lymphopoiesis, particularly in T lymphocyte development.
• Bone marrow derived stem cells migrate to the thymic cortex and undergo rapid proliferation to form immature lymphocytes.
• During maturation, lymphocytes develop diverse antigen recognition ability through genetic rearrangements.
• Many cells that recognize self antigens are eliminated by negative selection.
• Nearly 90 percent of developing thymocytes undergo apoptosis within a few days.
• Surviving lymphocytes are immunologically competent T cells that recognize foreign antigens without reacting to self.
• These cells enter the circulation and migrate to secondary lymphoid organs such as lymph nodes and spleen.
• On antigen exposure, they proliferate and differentiate, contributing to adaptive immune responses and long term immunity.

Thymus as a Primary Lymphoid Organ

• The thymus is a primary lymphoid organ, along with bone marrow, essential for T lymphocyte development.
• Developing thymocytes are protected from antigen exposure by the blood thymus barrier, preventing premature immune activation.
• Therefore, immature lymphocytes do not form plasma cells or lymphoid nodules within the thymus.
• The thymic medulla acts as a distinct compartment where selected thymocytes encounter self antigens presented by antigen presenting cells.
• This interaction is crucial for establishing central tolerance and self recognition.
• Maturation and proliferation of T lymphocytes are regulated by thymic hormones and direct interaction with epithelial cells.
• These factors promote differentiation into immunocompetent T cells capable of responding to foreign antigens.
• Thymic hormones also influence lymphocyte development in peripheral lymphoid organs, especially during early life.
• Removal of the thymus in childhood impairs normal immune system development.
• After puberty, the effect of thymus removal is less significant as lymphoid tissues are already established.

Thymic Hormones

• The thymus produces several thymic hormones that regulate T lymphocyte development and function.
• Thymulin enhances functional activity of T cells, especially regulatory subsets.
• Thymopoietin promotes differentiation of cytotoxic T lymphocytes and helps maintain immune balance.
• Thymosin alpha one stimulates lymphocyte proliferation and supports antibody responses.
• Thymic humoral factor regulates multiplication and activity of helper and regulatory T cells.
• These hormones collectively ensure proper immune maturation and functional competence of T cells.

Clinical Correlation

• Enlargement of the thymus is commonly associated with myasthenia gravis, an autoimmune disorder.
• This condition is characterized by weakness of skeletal muscles due to antibodies against neuromuscular junction proteins.
• Thymic hyperplasia or tumor may be present.
• Thymectomy can improve symptoms in selected cases, highlighting the role of thymus immune dysregulation.

Lymphatic Tissues

• The lymphatic system comprises lymphatic vessels and lymphoid tissues that maintain fluid balance and immunity.

Lymphatic Vessels

• During capillary exchange, part of plasma forms tissue fluid, most of which returns to blood, while the remainder enters lymphatic vessels as lymph.
• Lymphatic capillaries are the smallest vessels and merge to form larger collecting vessels.
• These vessels transport lymph back to the venous circulation.
• The thoracic duct is the largest lymphatic vessel and drains lymph from most of the body.
• It empties into the junction of the left subclavian vein and internal jugular vein.
• The right lymphatic duct drains the right upper quadrant of the body and joins the venous system on the right side.
• Lymphatic vessels play a key role in fluid homeostasis, lipid transport, and immune surveillance.

Lymphoid Tissues

• Lymphoid tissues are specialized structures involved in immune defense and lymphocyte activity.
• They are broadly classified into diffuse and dense lymphoid tissues based on organization.

Diffused Lymphoid Tissue

• Diffuse lymphoid tissue consists of scattered lymphocytes and plasma cells within mucosal linings.
• It is commonly found in the respiratory, gastrointestinal, and genitourinary tracts.
• These cells provide local immune surveillance and rapid response to antigens.

Dense Lymphoid Tissue

• Dense lymphoid tissue contains organized aggregates of lymphocytes forming nodules.
• These may exist as encapsulated organs or as clusters associated with mucosal surfaces.
• Discrete lymphoid organs include thymus, lymph nodes, spleen, and tonsils.
• The thymus and bone marrow are primary lymphoid organs, while others are secondary.
• Mucosa associated lymphoid tissue includes lymphoid aggregates in mucosal regions.
• In the respiratory tract, it is termed bronchus associated lymphoid tissue.
• In the gastrointestinal tract, it is called gut associated lymphoid tissue, including Peyer patches and appendix.
• These structures play a crucial role in immune surveillance, antigen processing, and lymphocyte activation.

Lymph Node

• Lymph nodes are small, encapsulated lymphoid organs located along lymphatic vessels, commonly in cervical, axillary, and inguinal regions.
• They receive lymph through afferent lymphatic vessels and drain it via efferent vessels at the hilum.
• A connective tissue capsule surrounds the node and sends inward projections called trabeculae.
• The internal framework consists of reticular tissue organized into cortex and medulla.
• The cortex contains lymphoid follicles with germinal centers, which are sites of B lymphocyte proliferation.
• The surrounding darker zone represents the B cell region.
• The paracortex contains T lymphocytes and is important for cell mediated immunity.
• The medulla contains lymphoid cells arranged in medullary cords and sinuses.
• Lymph flows from subcapsular sinus to cortical and medullary sinuses before exiting.
• Lymph nodes function in filtration of lymph, removal of pathogens, and activation of immune responses.

Spleen

• The spleen is the largest lymphoid organ and plays a major role in blood filtration and immune function.
• It is enclosed by a capsule containing elastic and smooth muscle fibers, with trabeculae extending inward.
• The internal framework consists of reticular tissue and vascular sinusoids.
• The functional tissue is divided into white pulp and red pulp.
• White pulp appears as nodular regions surrounding a central artery.
• It contains lymphoid follicles rich in B lymphocytes and periarteriolar regions rich in T lymphocytes.
• Germinal centers are sites of active lymphocyte proliferation.
• Red pulp consists of splenic cords and venous sinusoids.
• It contains red blood cells, leukocytes, and macrophages within a reticular network.
• Blood passes through open and closed circulatory pathways, allowing efficient filtration of blood cells.
• Old, damaged, or abnormal red cells are identified and removed by macrophages.
• The spleen also serves as a reservoir for blood cells and contributes to immune surveillance and response.

Functions of Spleen

• The spleen plays a vital role in immune defense by filtering blood and exposing antigens to immune cells.
• Splenic macrophages perform phagocytosis of microorganisms and cellular debris.
• It supports development of B and T lymphocytes, with germinal centers aiding lymphocyte proliferation and antibody production.
• The spleen removes senescent and abnormal blood cells, including red cells, leukocytes, and platelets.
• Cells of the mononuclear phagocyte system identify and eliminate defective cells.
• During fetal life, the spleen participates in hematopoiesis, and it may resume this function in certain diseases after birth.
• It serves as a blood reservoir, regulating circulating blood volume.
• The spleen also recycles iron from degraded hemoglobin, facilitating its reuse in erythropoiesis.

Mucosa-associated Lymphoid Tissue

• Splenomegaly refers to enlargement of the spleen and indicates increased immune or hematological activity.
• It commonly occurs in infections, blood disorders, and malignant conditions affecting lymphoid tissues.
• In infections, enlargement results from heightened phagocytic activity of splenic macrophages.
• Chronic parasitic diseases such as malaria produce sustained stimulation of the immune system, leading to marked enlargement.
• In hematological malignancies such as leukemias and lymphomas, abnormal proliferation of cells infiltrates the spleen.
• This infiltration contributes to both enlargement and altered splenic architecture.
• The spleen also enlarges due to increased destruction of abnormal or aged blood cells, a process known as hypersplenism.
• This may lead to reduction in circulating red cells, white cells, and platelets.
• Clinical evaluation of splenomegaly includes assessment of size, consistency, and tenderness.
• A soft spleen is often associated with acute infections, whereas a firm or hard spleen suggests chronic disease or malignancy.
• Progressive enlargement may indicate worsening disease, while reduction in size may reflect therapeutic response.
• Thus, careful examination of the spleen provides valuable information about disease activity and progression.

Mucosa-associated Lymphoid Tissue in Respiratory System

• Mucosa associated lymphoid tissue in the respiratory system is present in the walls of the trachea and bronchi.
• These aggregates are termed bronchus associated lymphoid tissue and provide local immune defense.

Mucosa-associated Lymphoid Tissue in Alimentary System

• In the alimentary system, it is called gut associated lymphoid tissue.
• It includes tonsils, Peyer patches, and appendix nodules, which protect against ingested pathogens and support mucosal immunity.

Lymph And Lymphatic Circulation

• Lymph is a fluid derived from plasma that contains proteins in lower concentration and mainly lymphocytes.
• Its composition varies with the tissue from which it originates.
• Lymphocytes are added as lymph passes through lymph nodes, while some arise from local tissues.
• The lymphatic system plays a key role in immune surveillance and transport of cells.
• Dietary fats absorbed from the intestine enter lymph as chylomicrons.
• After a fatty meal, lymph becomes milky and is termed chyle.
• This fat rich lymph is prominently seen in mesenteric lymphatic vessels.

Functional Anatomy

• Lymphatic vessels collect lymph from tissues and return it to the venous system at the junction of the subclavian and internal jugular veins.
• They begin as blind ended lymphatic capillaries that absorb tissue fluid.
• In the intestine, specialized capillaries called lacteals absorb dietary lipids.
• Lymph flows from capillaries into interconnected channels and then into larger vessels.
• These vessels progressively unite to form major lymphatic trunks.
• Larger lymphatic vessels contain smooth muscle like cells that aid lymph propulsion.
• Lymph nodes are positioned along these pathways and filter lymph before it reenters circulation.

Types of Lymphatics

• Lymphatics are classified into initial lymphatics and collecting lymphatics based on structure and function.

Initial Lymphatics

• Initial lymphatics include lymphatic capillaries and small interconnected vessels.
• They have thin endothelial lining with loosely arranged junctions, allowing easy entry of tissue fluid.
• Basal lamina is absent or minimal, and there are no valves or smooth muscle.
• Fluid entry is aided by tissue movements and nearby vascular pulsations.
• These vessels efficiently absorb interstitial fluid, proteins, and particulate matter.

Collecting Lymphatics

• Collecting lymphatics are larger tubular vessels that receive lymph from initial lymphatics.
• They contain valves to ensure unidirectional flow and prevent backflow.
• Their walls possess smooth muscle, producing peristaltic contractions that propel lymph forward.
• Lymph flow is further supported by skeletal muscle activity and negative intrathoracic pressure during inspiration.
• Increased capillary permeability enhances lymph formation; such agents are termed lymphagogues.
• Together, these lymphatic types maintain fluid balance, immune transport, and efficient return of lymph to circulation.

Formation and Composition of Lymph

• Lymph is formed from tissue fluid produced by capillary filtration.
• It represents a modified interstitial fluid entering lymphatic vessels.
• Formation depends on transcapillary exchange and movement into lymphatics.
• Lymph flow is slow, about 1 milliliter per minute in the thoracic duct.
• It has lower colloid osmotic pressure than plasma.
• High lipid content gives a milky appearance, termed chyle.

Table 18.1: Composition of lymph.

Component	Features in Lymph
Proteins	Low concentration, about 2–6 mg percent; higher in hepatic lymph
Lipids	Increased in intestinal lymph due to absorption
Carbohydrates	Lower than plasma levels
Clotting factors	Present, especially in lymph from liver
Cells	Predominantly lymphocytes, few monocytes
Electrolytes and water	Contains major ions and high water content

Functions of Lymphatic Circulation

• Lymphatic circulation returns excess interstitial fluid to the bloodstream, preventing edema formation and maintaining fluid balance.
• It contributes to plasma volume by reabsorbing filtered fluid from tissues.
• Lymphatics transport significant amounts of plasma proteins from tissues back to circulation, helping maintain oncotic pressure.
• This mechanism is especially important in the liver and intestine.
• The system carries long chain fatty acids, cholesterol, and fat soluble vitamins from the intestine to the bloodstream via lacteals.
• Lymph nodes filter lymph and remove microorganisms through phagocytic cells.
• This provides an important protective immune function.
• Overall, lymphatic circulation supports fluid homeostasis, nutrient transport, and immune defense.