Organ of Immune System

Organ of immune system

The parts of body which comprises individual cells, entire organs and organ systems that help in protection from infection of pathogens (foreign bodies).

Picture courtesy: https://clinicalinfo.hiv.gov/en/glossary/immune-system

The organs of immune system are.

• Skin       
• Mucous Membrane 
• Lymphatic System

NOTE: Skin and Mucous Membrane are first line of defence and act as a physical barrier.

Some of Physical barriers are.

• Antibacterial substance: enzyme present in the Saliva, tears and their ways of respiratory system.

• Mucus: present in the bronchi that helps to move out germ, bacteria via the hair like projection structure called Cilia.

• Stomach acid; hydrochloric acid (HCL) present in stomach helps to excrete out or dilute the bacteria and germs.

• Helping hands bacteria; Bacteria over the skin that helps to maintain the PH and temperature of the body and also kill the other bacteria and organism to protect the body.

Organ of Lymphatic System

• Primary lymphoid organs
• Bone marrow
• Thymus
• Secondary lymphoid organs
• Lymph nodes
• Spleen
• Tonsils

Primary Lymphoid Organs: 

Differentiation and maturation of lymphocytes takes place.

• Stem Cell Niche (SCN): The Microenvironment where any stem cells self-generate and differentiate to perform cellular functions very well.

Bone Marrow  

The spongy like a structure in the bone where differentiation and maturation of all Erythroid Cells, Myeloid Cells and B-lymphocytes cells occur.

• Residence of hematopoietic stem cells (HSC).

• At birth, many bones contain red bone marrow wherein adulthood only Ribs, Breastbone and Pelvis have it.

Almost all bones contain marrow but particular marrow containing bones.

1. Long bones (Femur, Humerus)
2. Hips bone (ileum)
3. Sternum.

In Birds, the haematopoiesis occurs at the site of Bursa of Fabricius where the development of b-cell occurs.

Bone marrow develops and renew blood cells throughout the life of an adult vertebrate.

With the increasing age, the fat cells replace 50% and more bone marrow and efficiency of haematopoiesis prices decreases.

The microenvironment in bone marrow

• Endosteal Niche: The area directly surrounds the bone and in contact with bone producing Osteoblasts.

Endosteal Niche >>>>>occupied by Quiescent hematopoietic stem cell>>>>>close association with Osteoblast>>>>>regulate stem cell proliferation.

• Vascular Niche: The area directly surrounding the bone and in contact with endothelial cells.

  

Vascular Niche >>>>>occupied by hematopoietic stem cells>>>>>leave endosteal niche to differentiate or circulate.

Thymus

A flat bilobed organ situated above the heart and beneath the breastbone.

It has two lobes. Each lobule are separated by connective tissue called Trabeculae and covered by a capsule.

Each lobule consists of two compartments.

• Cortex (outer)
• Medulla (inner)

 

T-cell development (Maturation).

Entry of immature t-cells.

The T-cell precursor which give rise to multiple hematopoietic cell types of travel to blood from bone marrow to thymus.

Immaturity cell travels from a specific thymic environment they become functional T-cells.

The thymus is a specialised environment where an immature cell (Thymocytes) generates unique antigen receptor (T-cell rereports). This TCRs selected on the basis of reactivity to self-MHC expressed on the surface of thymic stromal cells.

Reactivity of TCRs to self-MHC

Courtesy: https://www.researchgate.net/figure/Positive-and-negative-selection-of-T-cells-The-processes-of-thymic-selection-result-in_fig1_47933374

Negative Selection: TCRs binds to the self MHC with too high affinity that induce to die T cells it happens because of intercellular signal is so strong that leads to the death(apoptosis) of TCR.

TCR has function to recognise self MHC class I &II not its own body antigen.

Positive selection: The Thymocyte (TCRs) binds to self MHC with intermediate (Moderate) affinity undergo positive selection.

Positive selections result their survival, maturation and migration to thymic medulla.

95% thymocytes die in transition process from cortex to periphery because of too low affinity for self-antigen MHC and fail to positive selection.

Transition of thymocytes

• The T-cell precursors in the thymus from the blood enters at corticomedullary junctions present between the thymic cortex (outer) and thymic medulla(inner). In this stage TCRs called double negative cells (DN cells).

• DN cells travels at subcapsular cortex from where TCRs generates and express CD4 and CD8 receptors and become Double positive cells (DP cells).

• Mature thymocytes express only CD4 or CD8 receptors are referred as single Positive Cells (SP cells).

• These Cells leave thymus via blood through corticomedullary junction and reach to the periphery where it explores antigen presented secondary lymphoid tissue including spleen and lymph nodes.

Secondary lymphoid organs:

1. Highly Organised SLOs:

• Lymph nodes
• Spleen

Lymph Nodes:

A bean shaped, covered structures that includes network of lymph vessels packed with lymphocytes, macrophages and dendritic cells.

Lymph nodes are the first organised lymphoid structure which encounters antigens.

Lymph node provides microenvironment to encounter between antigen and lymphocytes that generates cellular and humoral immune responses.

Lymph Nodes divides into three regions.

• Cortex (outermost layer)   

B-lymphocytes, Macrophages, and Dendritic Cells found in this region.

• Paracortex (Middle layer)   

Maturity of T-cells along with dendritic cells occur here. Which migrate from tissue to nodes.

• Medulla (Innermost layer)   

 It is the site where lymphocytes exit(egress) from the lymph nodes through efferent lymphatic vessels. 

It is populated with plasma cells that secretes antibodies.

Antigen from infected area enters the cortex of lymph nodes via incoming afferent lymphatic vessels. Which pierces lymph node capsules at numerous sites. And emptied lymph into subcapsular sinus (SCS).

Antigen trapped by antigen presenting (APC) cells in SCS or Cortex.

Spleen:

It is a large, ovoid secondary lymphoid organ situated high in the left abdominal cavity.

While lymph nodes are specialised for trapping antigen from local tissues, spleen specialises in filtering blood and trapping blood-borne antigens.

Spleen is the home of immunocyte, and generate immune response that produces some active substance such as complement system et cetera.

Blood-borne antigens and lymphocytes are carried into the spleen through splenic arteries and out via splenic veins.

Spleen is surrounded by capsule from which number of projections called trabeculae extended providing structural supports.

In spleen, two main microenvironments compartment are.

• The Red Pulp

The splenic red pulp consists of a network of sinusoids populated by RBCs, macrophages and some lymphocytes.

It is the site where old and defective RBC are destroyed and removed because of that spleen is known as Graveyard of RBCs.

• The White Pulp

The site in splenic area where pathogen first gain access to lymphoid rich region of spleen.

Its surrounds the branches of splenic arteries and consists of Periarteriolar lymphoid sheath (PALS) populated by T-lymphocyte and b-cells follicles.

The marginal zone borders white pulp populated by specialised macrophages and b-cells.

2. Less Organised SLOs:

The entry for microbes into the body is through mucosal surfaces. Hence lymphoid tissue located within the lining of the respiratory, digestive and and genitourinary areas.

Thus, these lymphoid organs are collectively known as Mucosal-associated lymphoid tissue (MALT). It includes peyer's patches, tonsils, appendix etc.

Based on location, MALT is divided into,

• BALT (Bronchus-associated lymphoid tissue) 

• NALT (Nasal-associated lymphoid tissue)

• GALT (Gut-associated lymphoid tissue)

The Structure of MALT in small intestine have,

• Peyer's Patches: it is an aggregated lymphoid organ which is found in the lining of intestinal villi which contains well defined follicles and t-cell zones. Named after the Swiss anatomist Johnn Peyer.

• Tonsils: It plays an important role in defence mechanism against antigen entering through the mouth and nasal route.

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