Lymphatic System
The lymphatic system is formed from the lymphatic organs, lymphatic vessels, lymph & small masses of lymphoid tissue called lymph nodes. The major lymphatic organs are the lymph nodes, tonsils, thymus & spleen.Function:
1- Drain the protein containing fluids from the tissue spaces, such proteins can not be directly reabsorbed by the capillaries of the blood.2- Transport fats from the digestive tract to the blood.
3- Produce lymphocytes & antibodies (immunoglobulins) forming part of the immune system of the body).Lymph:
Is an exudates from the capillaries to the tissue space, it is similar to the blood (formed of plasma + cells) but some differences are:Lymph contains lymphocytes & monocytes cells.
No granular leukocytes.
No R.B.C"s.
Platelets are absent .
Albumin and globulin proteins are higher than that of the blood.
Fibrinogen is less.
Coagulation is slower than blood.
Oxygen level is less than blood.
Carbonic acid is higher than blood.
Fats are seen in the lymphatics of the intestine & appear milky (chyle) due to the presence of fat globules.
Lymphatic vessels:
1-They begin first as a network of lymphatic capillaries which are similar to blood capillaries but with some differences:
Irregular in diameter but wider than blood capillaries.
Thinner than blood capillaries.
Lined by a layer of flat endothelial cells with incomplete or no basal lamina.
Have no pericytes & perithelial tissue.
Lie deeper than the blood capillaries in skin & muscularis mucosa.
More permeable than blood capillaries, thus the colloid material can pass through the lymphatic capillaries while only soluble materials can pass through the blood capillaries.
2-The lymphatic vessels have thinner wall than veins & they have more valves, the small lymphatic vessels measuring less than 0.2 mm in diameter are lined by endothelial cells while the larger lymphatic vessels measuring more than 0.2 mm in diameter are lined by thicker wall formed of 3 layers:
Tunica intima: formed of endothelial cells + thin layer of connective tissue.
Tunica media: formed of circularly arranged smooth muscle fibers.
Tunica adventitia: thickest layer formed of bundles of smooth muscle fibers with elastic & collagen fibers in between.
3-All lymphatic vessels end in 2 main ducts: right lymphatic duct & thoracic duct.
4-The lymphatics are absent in the sclera & lens of the eye, avascular tissue, uncalcified cartilage, in the C.N.S, splenic pulp, fetal part of the placenta, internal ear and in the bone marrow.Lymphocytes:
are the cells that carry out the immune response, 2 main types of lymphocytes are present found in the blood, lymph & lymphatic organs: T-lymphocytes & B- lymphocytes they appear similar morphologically but different functionally, they are distinguished according to the receptors present on their cell membrane, both cell types originate from stem cells in the bone marrow then they enter the blood stream.
Types of lymphocytes:
1-T-lymphocytes: responsible for cellular immunity (i.e. sensitized T- lymphocyte itself has the capacity to invade the foreign agent & destroy it) they are circulating in the bloodB- lymphocytes: responsible for humeral immunity (i.e. they produce antibodies & these antibodies attack the invading antigen) they are the fixed type.
Function of T-lymphocyte:
Four groups of T- lymphocytes can be recognized:
1-Helper T-lymphocytes:
They assist other T- lymphocytes by secreting some immune chemicals that stimulate proliferation & differentiation & maturation of B-lymphocytes into plasma cells which then produce antibodies.
2-Cytotoxic T- cells ( T- killer):
They can recognize foreign cells, malignant cells & virus- infected cell, on activation & binding to the target cells they secret cytotoxic substances that perforate the target cell membrane causing cell death .Secreting a substance called ( MAF ) macrophage activating factor that attract the macrophage to the site of invasion of the micro-organism.
It is effective in slowly developing diseases in T.B. brucellosis, cancer and bacterial diseases.
3-Suppressor T- lymphocyte: they inhibit the function of helper T-lymphocyte & cytotoxic T-lymphocytes & inhibit the development of B- lymphocyte into plasma cells.
4-Memmory cell: are long-lived progeny of T-lymphocytes, when sensitized by foreign antigen, they will print that antigen on its surface & respond rapidly to the same antigen when second invasion occur.
Function of B- Lymphocyte:
B-lymphocytes become mature in the bone marrow then carried by the blood to the lymph node, spleen & connective tissue, specific B- lymphocytes are present for each specific antigen they become activated when:The invading antigen binds to specific Ag- receptors present on the surface of their cell membrane.
The response is more intense when Ag- presenting cells such as T- Helper present that Ag to the B-lymphocytes.
Activates B - lymphocyte enlarges, divide, proliferate, & differentiate into:
Plasma cells: which produce antibodies specific for the invading Ag then Ag-Ab reactions will occur forming immune complexes.
Memory cell: some activated B-lymphocytes not become plasma cells instead they persist as memory B-lymphocytes which produce a more rapid immune response when exposed to the same antigen.
Lymph node:
The L.N are encapsulated bean or kidney shaped organs composed of lymphoid tissue, inactive nodes are only few millimeter long but may increase greatly in size after stimulation & active immunological response, distributed throughout the body along the coarse of the lymphatic vessels, they are found in the axilla, groin, around the aorta ( paraaortic lymph nodes), along the great vessels of the neck, in the thorax, abdomen especially in the mesentery, each L.N characterized by a convex & concave side, the afferent lymphatic vessels enter through the convex side while on the concave side there is identation called hilum through which the arteries & nerves penetrate & the veins with the efferent lymphatic vessels leave.
Characters of the lymph node which differentiate it from other lymphatic organs:
The only lymph organ that lies along the coarse of lymphatic vessels.
The only organ that contain lymph in the lymphatic sinuses.
The only lymphatic organ that have afferent & efferent lymphatic vessels.
The only organ deal with filtration of lymph.
Usually present in groups or chains ( paraaortic L.N ).
Histological structure:
It is surrounded with a C.T capsule formed of collagen, elastic fibers, & smooth muscle cells present only at the point entrance & exit of lymphatic vessels, the C.T. capsule send C.T trabeculae that divide the lymph nodes into compartments called cortical trabeculae which extend to the medulla & called medullary trabeculae, they carry the trabecular vessels that deliver the nutrition & oxygen supply to the cortex & medulla.
Each lymph node has 3 regions:
1- cortical zone: lies at the periphery below the capsule.2- medullary zone: lies in the central part of the lymph node.
3- paracortical or juxtamedullary or deep cortex: lies between the outer cortex & inner medulla.
Cortical zone:
It is formed of reticular tissue infiltrated with lymphocytes, it contain 3 components:Lymphatic nodules.
Marginal sinuses (or subcapsular sinuses).
Cortical trabeculae.
The "lymphatic nodules" are spherical aggregation of lymphocytes, diameter (0.2-1) millimeter in sections stained with H&E, the lymphocytes are mostly B- lymphocytes, they could be:
Primary nodules containing inactive or nave B-lymphocytes distributed uniformly throughout the nodule.
secondary nodules containing a peripheral dark-stained area crowded with small B-lymphocytes with a central light area called germinal centre containing lymphoblasts which are large activated B-lymphocytes with large light nucleus contain uncoiled chromatin (heterochromatin ), it give rise to the 2nd type of cells called plasmoblasts (immunoblasts) this cells divide to give rise to plasma cells which are immunoglobuline-producing cells. The germinal center also contains follicular dendretic cells characterized by long processes with large basophilic nuclei, its function is to trap the antigen antibody complex on their surface for a long period of time then the antigens are recognized by the B-cell.
The "marginal (subcapsular) sinus" lies below the capsule is continuous with the cortical (trabecular) sinuses which extend between the trabeculae & the aggregations of the cortical nodules, the cortical sinuses are lined with fenestrated endothelial cells (with gaps in between them) associated with reticular cells & macrophages which are more in the side of the sinus facing the cortical nodule.
The "cortical trabecula" are C.T. septa arise from the C.T capsule & run in the cortex surrounded by the cortical trabecular sinus.
Paracortical region), (deep cortex), ( juxtamedullary zone):
consist of dense lymphoid tissue, it contain T-lymphocytes& high endothelia venules (HEV) which are lined with tall cubical endothelium having receptors on their surface which can be recognized by the circulating T-lymphocyte to enter the lymph node.
Medulla: consist of
Medullary cords: contains numerous B-lymphocytes, plasma cells that migrate from the cortex & macrophages.Medullary sinuses: continuous with the cortical lymphatic sinuses drain the lymph into the efferent lymphatic vessels.
Medullary trabeculae: are C.T trabeculae continuous with the cortical trabeculae carrying the medullary blood vessels.
Circulation of the lymph:
From the afferent lymphatic vessels the lymph pass to the subcapsular sinuses then to the cortical sinuses then to the medullary sinuses then it will be collected by the efferent lymphatics & leave through the hilus.
Afferent lymph vesselsEfferent lymph vessels1- enter the lymph node at the convex side1- leave the lymph at the hilus2- contains few lymphocytes, no plasma cells, no antibodies 2- contains large number of lymphocytes, plasma cells & antibodiesFunctions of the lymph node :
1-filteration of lymph.
2-production of lymphocytes.
3-production of antibodies.
4-phagocytosis of foreign bodies.