1. Lecture 4 Histology Dr.Widad.J.H.
The Lymphatic System – returns fluids that have leaked from the circulatory system back to the blood; protects the body by removing foreign material; provides a site for immune surveillance

2. The Immune Response
T cells = cellular immunity; function to
amplify the inflammatory response
B cells = humoral immunity
T Cells and Cell Mediated Immunity
- targets virus or parasite infected cells, cancer
cells, and cells of foreign grafts
3 main types of cells:
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

3. Lymphoid cells
Lymphocytes
Arise in the red bone marrow
Protect the body against antigens
Two types
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

4. Macrophages – phagocytize foreign substances and activate the T cell response
Dendritic cells – initiate the immune response
Reticular cells – produce the reticular fibers that form the soft skeletal structure of lymphoid organs

7. Components of the Lymphatic System
Lymphatic vessels – return to the blood any fluids that have escaped from the circulation
Distribution of lymphatic vessels
Lymphatic vessels travel alongside blood vessels
Lymphatic vessels are absent from bones, teeth, bone marrow, and the central nervous system
Lymphatic capillaries – microscopic blind-ended tubes that are interwoven between the tissue cells and the blood capillaries
Lacteals – specialized lymphatic capillaries of the intestinal mucosa
Fatty lymph (chyme) containing fats and fat-soluble substances is absorbed in the lacteals

8. Lymph transport
The lymphatic circulation is a low-pressure system
The lymphatic system lacks a pumping organ; must utilize the valves, respiratory pumps and muscular pumps to promote lymph flow toward the heart
The movement of surrounding tissues is also important in propelling lymph through the lymphatics
The lymphatic capillaries converge into larger vessels
Lymph flows from the lymphatic capillaries to lymphatic collecting vessels to lymphatic trunks to the right lymphatic duct/thoracic duct

9. The right lymphatic duct drains lymph from the right upper arm and the right side of the head and thorax
The thoracic duct receives lymph from the rest of the body

10. Lymphoid cells and tissues
Lymphocytes
Arise in the red bone marrow
Protect the body against antigens
Two types
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

11. Macrophages – phagocytize foreign substances and activate the T cell response
Dendritic cells – initiate the immune response
Reticular cells – produce the reticular fibers that form the soft skeletal structure of lymphoid organs

12. Lymph nodes
Only the lymph nodes filter lymph
Cluster along the lymphatic vessels of the body
Lymph is filtered through the lymph nodes before it is returned to the bloodstream
Lymph nodes are embedded in connective tissue
Large clusters of lymph nodes appear near the body surface in the inguinal, axillary, and cervical regions.
Functions of lymph nodes
Filters lymph
Assist in activating the immune system

13. Anatomy of a lymph node
Most are kidney-shaped
Surrounded by a fibrous capsule
Strands of connective tissue (trabeculae) divide the node into compartments
Two histologically distinct regions
Cortex
Contains densely packed follicles with many germinal centers
Deeper portion of the cortex primarily houses T cells
Medulla
Contain both types of lymphocytes

15. Circulation in the lymph nodes
Afferent lymphatic vessels – lymph enters here
Once inside the nodes, the lymph moves through a series of sinuses and then exits at the hilus
Efferent lymphatic vessels – lymph exits here

16. Spleen
The largest lymphoid organ
Site of lymphocyte proliferation and immune surveillance and response
Functions of the spleen
Cleanses the blood by removing old RBCs and platelets, as well as debris from the blood.
Stores the breakdown products of RBCs
Site of erythrocyte production in the fetus

17. Anatomy of the spleen
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

20. Thymus – the site of T cell maturation
Most active in younger children; atrophies with age
Does not contain reticular fiber
Lack B cells, therefore no germinal cells enters are present in the thymus

21. Tonsils – the simplest lymphoid organs; named according to their location
Palatine tonsils
Lingual tonsils
Pharyngeal tonsil
Tubal tonsils

22. Aggregates of lymphoid follicles
Location of these 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
Examples
Peyer’s patches – found in the distal portion of the small intestine
Appendix – an off-shoot of the cecum (the first part of the large intestine)

23. Thank you