Lymphatic System

Immune System

immune system 1 Constituents: Lymphocytes in blood vessels, lymphatic vessels and somewhere else, lymphatic tissues and lymphatic organs. 2 Function:

Destroy the invaders, such as pathogenic orgnisms and other antigens. Destroy the useless or harmful cells in human body, such as old RBC, cancertransformed cells, etc.

Lymphocyte Functionally, lymphocytes can be divided into 3 types: T cell, B cell and NK cell.

1.T-lymphocyte: educated in thymus and involved in cell-mediated immune response. Helper T-lymphocyte----(cluster of the differentiation)CD4+T-lymphocyte. it can upregulate both cell-mediated immune

HIV

response and humoral immune response; Cytotoxic T-lymphocyte----CD8+T-lymphocyte. a kind of effector cells cytotoxic/suppressor T- lymphocyte can downregulate or turn off immune system

2. B-lymphocyte B-lymphocyte: educated in the bone marrow and involved in humoral immune response Ag

B-lymphocyte

plasma cell

neutralize antigen

antibody

3. Natural killer lymphocyte NK cell: effector cell. It can kill abnormal cells directly.

antigen-presenting cell (APC) 1. function ： sequestering Ag and presenting it to helper CD4+T lymphocyte to facilitate immune responses. 2. components ： macrophages,Kuffer cells of liver, Langerhans’ cells in epidermis, reticular dentritic cells of spleen and lymph node, type II and type III epithelioreticular cell of the thymus and B lymphocyte

Process of antigen-presenting

MHC (major histocompatability gene complex) MHC I: expressed on the surface of all nucleated cells and platelets. MHC II: expressed on the surface of all APCs.

Lymphatic tissue 1.defination: the tissue which is mainly composed of lymphocyte. 2. classification:(1) diffuse lymphatic tissue (2) lymph nodule: lymph follicle

lymphatic organs Primary (central) lymphatic organs ---- thymus and bone marrow ----site that T-lymphocytes and B-lymphocytes get educated respectively. Secondary (peripheral) lymphatic organs ---- lymph node, spleen and tonsil ---- sites that immune responses are initiated.

1. Thymus Thymus: capsule ,thymic cortex and medulla

Thymic cortex Thymic cortex contains closely packed thymocytes in an extensive meshwork of epithelioreticular cells. Thymocytes: developing T-lymphocytes.about 98% of them undergo apoptosis.

Thymic cortex There are 3 types of epithelioreticular cells in the cortex.

Type I: Located at the boundary of the cortex and capsule. involved in the formation of blood-thymus barrier.

Type II:

Located within the cortex .It is APC with MHC II molecules which are involved in thymic cell education

.

Type III: Located at the boundary of the cortex and medulla. Serve as a barrier that isolate cortex and medulla. A kind of APCs to present Ag.

Blood-thymus barrier It is the structure between thymocytes and blood , a physical barrier to protect developing T-lyphocytes from exposure to Ag. Components:

lining endotheium of capillary wall and its basal lamina; macrophages in the perivascular C.T.; type I endothelioreticular cells with their tight junctions.

Blood-thymus barrier

Thymic medulla It contains lots of epithelioreticular cells and loosed packed T lymphocytes. T lymphocytes here are mature ones. There have another 3 types of epithelioreticular cells

Type IV: located between the medulla and cortex; Create a barrier between the cortex and medulla.

Type V: located throughout the medulla; provide a cellular framework of medulla;

Type VI: form the thymic or Hassall’s corpuscles in medulla

Thymic corpuscle Thymic corpuscle: a characteristic feature of the thymus. It is a isolated mass of closely packed, concentrically arranged type VI epithelioreticular cells . Found in the thymic medulla.

Function of the thymus Site of T lymphocyte education stem cells maturation and differentiation into T lymphocytes

thymus

2 .Lymph node The structure of lymph node: capsule ,cortex and medulla

lymph node

Cortex Cortex consists of superficial cortex, paracortex and sinuses 1. superfacial cortex are orgnized into nodules,mainly composed of B-lymphocytes. 2. Paracortex is the portion of medulla and cortex where is free of nodules,mainly composed of T-lymphocytes. It is also thymus-dependent cortex. 3.Sinuses is divided into subcapsular sinuses and trabecular sinuses where filter the flowing lymph through it.

lymphatic nodule: concentration of lymphocytes contained in the meshwork of reticular cells. mainly containing B cells together with some plasma cells, macrophages,etc.. primary nodule: without germinal center; Ag Secondary nodule: with germinal center (B cells proliferates here rapidly).

Lymph node

Specialized high endothelial venules

(HEVs) Location: in he paracortex Function: entry of circulating lymphocytes into the lymph node---enlarging immune response

recirculation of of lymphocytes HEVs blood vessels

thymu s

lymph nodes

lymphatic vessels

Bone marrow

lymph node

Medulla Medulla consists of medullary cords and sinuses. Medullary cords: cords of lymphatic tissue containing B cells,plasma cells and maceophages Medullary sinuses: similar with the sinuses in the cortex but with more macrophages.

Lymph node

Medullary cord and sinus

Functions of lymph node （ 1 ） filtering lymph ； （ 2 ） initiate immune responses. phagocytosis of macrophage; B cells proliferate and differentiate into plasma cells and memory B cells.

3. spleen the structure of the spleen:capsule,red pulp and white pulp

Capsule of the spleen Capsule: C.T. containing myofibroblasts, blood vessels,etc.

Medullary sinus

Red pulp

Medullary cord Cord of lymphatic tissue with lots of blood cells

spleen

Periarterial lymphatic sheath(PALS)

White pulp

Paracortex in lymph node

Malpighian body （ splenic nodule ） Lymphatic nodule in lymph node

spleen

White pulp A thick accumulation of lymphocytes surrounding the artery.

White pulp

Red pulp It contains large number of blood cells that it filters and degrades. Splenic cord: a meshwork of reticular cells and fibers containing lots of RBCs, macrophages, plasma cells,etc.

Splenic sinus: special sinusoidal vessels lined by rodshaped epithelial cells. The processes of macrophages extend into the sinuses.

Circulation in the spleen Branches of splenic artery enter the white pulp from trabeculae; Central artery :sends branches to white pulp itself and marginal sinuses; continues to penicillar arterioles in splenic cord ; Penicillar arterioles continue with sheathed capillaries which are empty the blood in the cord,then the blood cells enter the splenic sinuses.

Function of the spleen It performs both immune and hemopoietic functions 1. filtering blood: killing pathogenic Ag and destroying senescent, abnormal RBC ,etc.; Splenectomy----increasing abnormal RBC in blood Splenomegaly----malaria 2. initiation of immune responses ； 3. formation of blood cells during early fetal life ； 4. storage of blood.

spleen

Key points 1.T-lymphocytes get mature in thymus while B-lymphocytes get mature in the bone marrow.B-lymphocytes can convert to plasma cells which are responsible for producing Ab. 2. The structure and function of thymus: cortex and medulla . Hassall’s corpuscle and blood-thymus barrier. 3.The structure and function of lymph node;the celular components of lymphatic nodule and paracortex. 4.The structure and function of spleen:red pulp and white pulp ( malpighian body and ).

Thymic cortex There are 3 types of epithelioreticular cells in thymic cortex.

epithelioreticular cells Type I

Type II

Located at the boundary of the cortex and capsule. Serve as a barrier that isolate thymocytes and C.T. Located within the cortex . It is APC with MHC II molecules which are involved in thymic cell education.

Type III Located at the boundary of the cortex and medulla. A kinds of APCs to present Ag.