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Lymphoid Organs

This lecture on lymphoid organs explains the different organs involved, their structure, cells and components involved with their functions and other related aspects.

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Overview of Lecture

Lymphoid Organs

Components Of Immune System


Diffuse Lymphatic Tissue

Lymphatic Nodules

Lymph node


Bone marrow


Functions of Immune System

Has the ability to distinguish ‘self’ from non-self

Inactivates/destroys foreign substances

Foreign molecule

Molecules present in viruses, bacteria, parasites.

Microorganisms and other cells (virus-infected cells, cancer cells, cells of transplanted organs)


Any substance that can induce immune response

Examples are foreign protein, foreign tissue, toxin, infectious organism or transformed tissue

Antigens must be processed by cells of immune system before they can initiate a response

Types Of Immune Responses

Non-specific  or Innate immunity

Physical barriers

Secretory substances

Chemical defences

Phagocytic cells

Specific or Adaptive immunity

Antibody mediated

Cellular immune response

Cells Of Immune System


B Lymphocytes

T Lymphocytes




Natural killer cells

Macrophages and dendritic cells
These cells are derived from the bone marrow and have a variety of functions in the immune response:


secretion of cytokines

antigen presentation

The cells performing these various functions have differing microscopic appearances but they are grouped together as the mononuclear phagocytic system.

Polymorphs and mast cells are also involved in the effector side of the immune response

Antigen-presenting cells

Cells which do not have antigen-specific receptors. Instead, they capture and process antigens, present them to T cell receptors.  These cells include macrophages, dentritic cells and B cells.

B cells

Also known as B cell lymphocytes.

B cells spend their entire early life in the bone marrow.  Upon maturity, their job is to travel throughout the blood and lymph looking for antigens with which they can interlock.

Once a B cell has identified an antigen, it starts replicating itself.  These cloned cells mature into antibody-manufacturing plasma cells.


Similar to mast cells, but distributed throughout the body.  Like mast cells, basophils release histamine upon encountering certain antigens, thereby triggering an allergic reaction.

Cytotoxic T cells

Also called cytotoxic T lymphocytes or CTLs

Dendritic cells

Mostly found in the skin and mucosal epithelium, where they are referred to as Langerhan’s cells.  Unlike macrophages, dendritic cells can also recognize viral particles as non-self.


Leukocytes (white blood cells) containing granules in the cytoplasm.  Also known as a granular leukocyte.  They seem to act as a first line of defense, as they rush toward an infected area and engulf the offending microbes.  Granulocytes kill microbes by digesting them with killer enzymes contained in small units called lysosomes

Helper T cells

These cells travel through the blood and lymph, looking for antigens (such as those captured by antigen-presenting cells).  Upon locating an antigen, they notify other cells to assist in combating the invader.

This is sometimes done through the use of cytokines (or specifically, lymphokines) which help destroy target cells and stimulate the production of healthy new tissue.  Interferon is an example of such a cytokine.


White blood cells.  These are the cells which provide immunity, and they can be subdivided into three classes: lymphocytes, granulocytes and monocytes


Literally, “large eaters.”  These are large, long-lived phagocytes which capture foreign cells, digest them, and present protein fragments (peptides) from these cells and manifest them on their exterior.  In this manner, they present the antigens to the T cells.

Macrophages are strategically located in lymphoid tissues, connective tissues and body cavities, where they are likely to encounter antigens. They also act as effector cells in cell-mediated immunity

Mast cells

Cells concentrated within the respiratory and gastrointestinal tracts, and within the deep layers of the skin.  These cells release histamine upon encountering certain antigens, thereby triggering an allergic reaction

Memory cells

Specialized B cells which grant the body the ability to manufacture more of a particular antibody as needed, in case a particular antigen is ever encountered again.


Large, agranular leukocytes with relatively small, eccentric, oval or kidney-shaped nuclei

Plasma cells

Specialized B cells which churn out antibodies—more than two thousand per second.  Most of these die after four to five days; however, a few survive to become memory cells

T cells

Also known as T cell lymphocytes.

Unlike B cells, these cells leave the marrow at an early age and travel to the thymus, where they mature.  Here they are imprinted with critical information for recognizing “self” and “non-self” substances.

Among the subclasses of T cells are helper T cells and cytotoxic (or killer) T cells.

Cells Of Immune System










Differentiation Of Lymphocytes

Antigen independent differentiation occurs in thymus, GALT and bone marrow

Antigen dependent differentiation occurs in Lymphatic Nodules ,spleen,tonsil and lymph node . Formation of effector lymphocytes and memory cells

Types Of Immune Response

Primary———-Ist encounter  IgM antibodies memory cells formation

Secondary ——-More intense,IgG antibodies, hypersensitivity reactions

Specific ———-Antibody or cell mediated

Lymphoid Organs

Widely distributed in body

Lymphatic vessels collect lymph from various parts of body and deliver it to blood circulation mainly through thoracic duct

Lymphoid Tissue

Lymphoid tissue is CT with rich supply of lymphocytes

Exists free within regular CT or is surrounded by capsules.

Very little cytoplasm so stain dark blue with H&E.

Rich network of reticular fibrils produced by fibroblast whose many processes rest on fibrils.

Lymph  Node

Small Bean shaped

I mm to I-2 cm.

Although widely distributed may be concentrated in certain areas.

Lymphatic vessels are either afferent or efferent.

Lymph Nodes

Functions include:

Filtration of particles and microorganisms to keep them out of general circulation.

Interaction of circulating antigens in lymph with lymphocytes to initiate immune response.

Activation, proliferation of B lymphocytes and antibody production.

Activation, proliferation of T lymphocytes.

Structure of Lymph Node

Two zones




Superficial or Nodular

Deep or Para cortex


Also called thymus dependent cortex due to abundance of T cells

Histological Structure

Important site of B cells proliferation and their transformation into plasma cells

High endothelial Venules. (HEVs.)

Located in Deep Cortex

Absorb fluid and electroytes

Cause migration of lymphocytes into lymph nodes


Formed by cords of lymphatic tissues surrounded by lymphatic sinuses

Cords contain B lymphocytes

Cells Of Lymph Node



Plasma cells

Reticular cells

Follicular Dendritic  cells


Filtration of lymph

Subcapsular sinus

Cells of Subcapsular sinus

Flow of blood and Lymph In   Lymph Node

Supporting Elements Of Lymph Node



Reticular tissue.

Cells of reticular meshwork


Derived from Mesenchyme

Stellate or elongated

Oval euchromatic nucleus

Small acidophilic Cytoplasm.

Four types;

Reticular cells

Dendritic cells


Follicular Dendritic cells.

Reticular cells of lymph node

Dendritic cells


Lympho endothelial organ

Two lobes between the sternum and the heart

Atrophies with age

Structure Of Thymus

General Architecture of Thymus



Thymic lobules

  • Developing T cells or Thymocytes.
  • Epithelio reticular cells

Thymic lobules



Small lymphocytes

Epithelial reticular cells


Thymic Cortex

Outer portion of gland

Formed of thymocytes,



Reticular cells

Epithelio Reticular Cells

Type I

Barrier between developing T cells and connective tissue of the organ.

Type II

Have pale staining nucleus

Compartmentalize cortex

Involved in Tcell education

Type III

Barrier between Cortex and Medulla.

Type IV

Barrier at Corticomedullary junction.

Type V

Have cellular processes to compartmentalize the medulla.

Type VI

Forms Hassall’s corpuscles

Keratohyaline granules.

Lipid droplets


Epithelio reticular cells

Thymic medulla

Inner portion of thymus

Contains epithelio reticular cells and loosely packed T lymphocytes

Hassalls corpuscles are main feature

Thymic Medulla

Contains large lymphocytes with pale staining nuclei

Epithelio reticular cells

Hassalls Corpuscles

Closely packed concentrically arranged type VI epithlio reticular cell

TEM shows keratohylin granules and lipid droplets

Produce interleukins for T cells education and differentiation

Epithelial reticular cells and thymic lymphocytes

Comparision between cortex and medulla

Adult thymus

Blood thymus barrier


Multiple groups of large lymphatic nodules

Location – mucous membrane of the oral and pharyngeal cavities

Palatine tonsils

Lateral walls of the oropharynx

Pharyngeal tonsil

Posterior wall of nasopharynx

Lingual tonsils

Base of tongue

Palatine tonsil

Histological structure

Stratified squamous epithelium


Diffuse and Nodular  lymphoid tissue

Capsule infiltrated with lymphocytes

Pharyngeal tonsil

Pseudo stratified epithelium

No crypts

Capsule present

Lingual Tonsils

  • Stratified squamous epithelium
  • Lymphatic nodules
  • Wider crypts


Largest mass of lymphoid tissue in body

Between stomach and diaphragm

Functions Of Spleen

Abundant phagocytic cells—defense against antigens in blood

Site of destruction of aged erythrocytes.

Production site of activated lymphocytes which are delivered to the blood.

An important blood filter and antibody-forming organ.


Fibrous capsule

Hilum with  artery, vein, efferent lymph vessels



Red pulp

White pulp

Splenic Capsule

Dense connective tissue capsule

Few smooth muscle cells

Sructure Of Spleen



Red pulp

White pulp

Pulp of spleen

White pulp

Lymphoid Nodule


White pulp


B lymphocytes

Central artery

White Pulp


Central arteriole originates from trabecular arteries

Marginal Zone

Surround lymphoid nodule

Consists of loose lymphoid tissue and blood antigens

Major role in immunological activities of spleen


Red pulp spleen

Splenic cords

Splenic sinuses

Red pulp spleen

Red appearance is due to large no of RBCs

Red Pulp of Spleen

Blood circulation

Mucosa associated lymphoid tissue

Gut associated lymphoid tissue




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