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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

Thank you