This lecture on lymphoid organs explains the different organs involved, their structure, cells and components involved with their functions and other related aspects.
Overview of Lecture
Lymphoid Organs
Components Of Immune System
Lymphocytes
Diffuse Lymphatic Tissue
Lymphatic Nodules
Lymph node
Spleen
Bone marrow
Thymus
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)
Antigen
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
Lymphocytes
B Lymphocytes
T Lymphocytes
Helper
Cytotoxic
Gamma/delta
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:
phagocytosis
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.
Basophils
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.
Granulocytes
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.
Leukocytes
White blood cells. These are the cells which provide immunity, and they can be subdivided into three classes: lymphocytes, granulocytes and monocytes
Macrophages
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.
Monocytes
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
Monocytes
Macrophages
Neutrophils
Basophils
Eosinophils
Reticular
Dendritic
Langerhans
Epithelioreticular
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
Cortex
Medulla
Cortex
Superficial or Nodular
Deep or Para cortex
Paracortex
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
Medulla
Formed by cords of lymphatic tissues surrounded by lymphatic sinuses
Cords contain B lymphocytes
Cells Of Lymph Node
Lymphocytes
Macrophages
Plasma cells
Reticular cells
Follicular Dendritic cells
APC
Filtration of lymph
Subcapsular sinus
Cells of Subcapsular sinus
Flow of blood and Lymph In Lymph Node
Supporting Elements Of Lymph Node
Capsule
Trabeculae
Reticular tissue.
Cells of reticular meshwork
Derived from Mesenchyme
Stellate or elongated
Oval euchromatic nucleus
Small acidophilic Cytoplasm.
Four types;
Reticular cells
Dendritic cells
Macrophages
Follicular Dendritic cells.
Reticular cells of lymph node
Dendritic cells
Thymus
Lympho endothelial organ
Two lobes between the sternum and the heart
Atrophies with age
Structure Of Thymus
General Architecture of Thymus
Capsule
Trabeculae
Thymic lobules
- Developing T cells or Thymocytes.
- Epithelio reticular cells
Thymic lobules
Thymus
Cortex
Small lymphocytes
Epithelial reticular cells
Macrophages
Thymic Cortex
Outer portion of gland
Formed of thymocytes,
macrophages
Lymphocytes
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
Filaments.
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
Tonsils
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
Crypts
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
Spleen
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.
Components
Fibrous capsule
Hilum with artery, vein, efferent lymph vessels
Myofibroblasts
Trabeculae
Red pulp
White pulp
Splenic Capsule
Dense connective tissue capsule
Few smooth muscle cells
Sructure Of Spleen
Capsule
Trabeculae
Red pulp
White pulp
Pulp of spleen
White pulp
Lymphoid Nodule
Pals
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
Comparision
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
