Immune System

1 Constituents: Lymphocytes in blood vessels, lymphatic vessels and somewhere else, lymphatic vessels and somewhere else, lymphatic tissues and lymphatic organs. lymphatic tissues and lymphatic organs. 2 Function: Destroy the invaders, such as pathogenic orgnisms and other antigens. pathogenic orgnisms and other antigens. Destroy the useless or harmful cells in Destroy the useless or harmful cells in human body, such as old RBC, cancer- human body, such as old RBC, cancer- transformed cells, etc. transformed cells, etc.

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

2. B-lymphocyte B-lymphocyte plasma cell antibody B-lymphocyte plasma cell antibody neutralize antigen neutralize antigen Ag B-lymphocyte: educated in the bone marrow and involved in humoral immune response

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

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

diffuse lymphatic tissue （ 1 ） diffuse lymphatic tissue mainly composed of T lymphocytes

（ 2 ） lymph nodule Also lymphoid follicle,mainly composed of B lymphocytes. After stimulation of antigen ， the germinal center will appear. Classification ： primary lymph nodule secondary lymph nodule

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

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

1. Thymus 1. Thymus Thymus: capsule,thymic cortex and medulla

Thymic cortex Thymocytes: developing T-lymphocytes.about 98% of them undergo apoptosis. 98% of them undergo apoptosis. Thymic cortex contains thymocytes (more ) and epithelioreticular cells (less).

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. involved in the formation of blood-thymus barrier. Type II: Located within the cortex.It is APC and involved APC in thymic cell education in thymic cell education. Type III: Located at the boundary of the cortex and medulla. A kinds of APCs to present Ag. A kinds of APCs to present Ag.

胸腺高倍 thymocytes （ % ） epithelioreticular cells cortex medulla

capsule trabecula epithelioreticular cells thymocyte

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 epithelioreticular cells (more) and T lymphocytes (less). T lymphocytes here are mature ones. There have another 3 types of epithelioreticular cells Type IV: located between the medulla and cortex; Type V: located throughout the 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 Function of the thymus Site of T lymphocyte education Site of T lymphocyte education stem cells maturation and stem cells maturation and differentiation into T lymphocytes differentiation into T lymphocytes

thymus

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

lymph node lymph node

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

Lymph node

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

HEVs

recirculation of of lymphocytes blood vessels lymph nodes lymphatic vessels lymphatic vessels HEVs Bone marrow thymu s

lymph node

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

Lymph node

Medullary cord and sinus

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

spleen Red pulp White pulp Periarterial lymphatic sheath(PALS) Malpighian body （ splenic nodule ） Splenic cord Splenic sinus Paracortex in lymph node Lymphatic nodule in lymph node Cord of lymphatic tissue with lots of blood cells

capsule White pulp trabecula Red pulp

spleen

White pulp Malpighian body: Periarterial lymphatic sheath: A thick accumulation of lymphocytes surrounding the artery. A thick accumulation of lymphocytes surrounding the artery.

White pulp

PALS White pulp Malpighian body

Red pulp It contains large number of blood cells that it filters and degrades. 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 rod- shaped epithelial cells. The processes of macrophages extend into the sinuses.

Splenic sinus red pulp Splenic cord

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 Splenectomy----increasing abnormal RBC in blood Splenomegaly----malaria 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 IType IIType III 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. Located at the boundary of the cortex and medulla. A kinds of APCs to present Ag.

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

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