1. Al-Mustansiriya University Collage of Pharmacy
Histology- Lecture 4
Histology of Lymphatic System
Al-Mustansiriya University
Collage of Pharmacy
Lecturer Dr Samah A. Jassam BSc MSc PhD MRSB

2. Lymphatic system
It is part of the circulatory system and an important part of the immune system.
It returns fluids that have leaked from the circulatory system back to the blood
Lymphatic system consists of :
1- lymph vessels
2- lymph nodes
3- lymphoid tissues
4-circulating lymph

3. The lymphatic system has three main functions
Lymphatic system maintains the balance of fluid in the blood versus the tissues (fluid homeostasis)
It is part of the immune system and helping defend against foreign bodies such as bacteria
It facilitates absorption of fats and fat-soluble nutrients in the digestive system.

4. Lymphatic system organs
1-The primary or central lymphoid organs, that generate lymphocytes from immature progenitor cells. such as the thymus and the bone marrow.
2-Secondary or peripheral lymphoid organs, which include lymph nodes and the spleen, maintain mature naive lymphocytes and initiate an adaptive immune response.

5. The thymus
The thymus
The thymus is the site of maturation for T cells. The thymus increases in size from birth in response to postnatal antigen stimulation.
Most active in younger children; atrophies with age
Does not contain reticular fiber
Lack B cells, therefore no germinal cells present in the thymus

6. The spleen
The largest lymphoid organ , it sites of lymphocyte proliferation and immune surveillance and response
Functions of the spleen
1- Cleanses the blood by removing old RBCs and platelets, as well as debris from the blood.
2- Stores the breakdown products of RBCs site of erythrocyte production in the fetus
spleen

7. Histology of the spleen
The spleen is surrounded by a fibrous capsule contains both T cells, B cells, RBCs and macrophages
Divided histologically into two regions:
Red pulp – rich in lymphocytes and reticular fibers RBC disposal and recycling
White pulp – rich in macrophages and reticular fibers Immune functions

8. Lymph nodes
A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way back to the blood.
Lymph nodes are located at intervals along the lymphatic system.
afferent lymph vessels bring in lymph to the lymph node, which percolates (filters) through the substance of the lymph node, and is then drained out by an efferent lymph vessel.
There are between lymph nodes in the human body

9. What is the structure of the lymphatic nodes?
1- Capsule sends dense extensions called trabeculae into lymph node , forming lobules
2- The cortex is subdivided into OUTER CORTEX ( B cell dependant lymphoid follicles 
3- active (secondary) lymphoid follicles are pale (inactive are dark) and are in a region called GERMINAL CENTER.

10. Lymphatics (lymphatic vessels)
The lymphatics or lymphatic vessels, also called lymph vessels, conduct the lymph between different parts of the body. Lymphatics are resemble veins in structure except, that their coats are thinner and that these have numerous valves.
The larger collecting vessels are:
A-The right lymphatic duct
B-The thoracic duct (the left lymphatic duct).

11. Tonsils
Tonsils is a secondary lymphoid organ, and the simplest; named according to their location
The location of tonsils follicles make them ideal because they are able to:
Destroy bacteria and prevent pathogens from slipping through the intestinal wall
Generate many “memory” lymphocytes for long-term immunity

12. Lymph
Lymph is the fluid that circulates throughout the lymphatic system.
It is formed, when the interstitial fluid (the fluid which lies in the interstices of all body tissues) is collected through lymph capillaries, then transported through larger lymphatic vessels to lymph nodes, where it is cleaned by lymphocytes, before emptying ultimately into the right or the left subclavian vein, where it mixes back with the blood.
Human lymph, obtained after a thoracic duct injury

13. Formation of lymphocytes

14. Lymphoid cells (Lymphocytes)
Arise in the red bone marrow, they protect the body against antigens
Types of lymphocytes
T-lymphocytes (T cells): mature in the thymus, directly attack and destroy foreign cells
B-lymphocytes (B cells): mature in the bone marrow, produce plasma cells that manufacture antibodies
NK (natural killer ) cells (non-speficic immunity) = they mature in the bone marrow

15. T Cells and Cell Mediated Immunity
T-cell targets virus or parasite infected cells, cancer cells, and cells of foreign grafts
The main types of cells T-cell
1.Cytotoxic T (TC) cells: carry out cell mediated immunity, physically attack foreign cells
2.Helper T (TH) cells: activate B and TC cells
3.Suppressor T (TS) cells: moderate the immune response by inhibiting TC and B cells

16. Main differences between T-cell and B-cell

17. The Immune Response
The immune response: is how your body recognises and defends itself against bacteria, viruses, and substances that appear foreign and harmful
T cells = cellular immunity; function to amplify the inflammatory response
B cells = humoral immunity, antibodies formation

18. How the body responses to Ag (antigen) exposure?
Proliferation (rapid division) of lymphocytes
Production of effector molecules: B cells produce antibodies,
Helper T cells produce cytokines
Cytotoxic T cells produce cytotoxic granules
3) Macrophages and dendritic cells present antigens to T cells.

19. What are the main differences between cellular and humeral immune response?

20. Exercise What are the primary lymphoid organs?
What are the secondary lymphoid organs?
What is the main differences between T-cell and B-cell?
How the body responses to the infection?