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HISTOLOGY OF LYMPHOID ORGANS INTRODUCTION • Immune systems consists of : 4 – Lymphoid L h id organs – Heterogeneous group of motile cell types • 2 components of immune systems : 3 – IInnate t Immune I System S t Æ non spesific ifi (complement, ( l t macrophages & Neutrophils, Natural Killer cells/NK cells)) Æ nonclonal defense mechanism – Adaptive Immune System Æ specific (T Lymphocytes, B lymphocytes, Antigen Presenting C ll /APC ) Cells/APCs) • Adaptive immune system : 4 – Humoral immune response : B lymphocytes produce antibodies Æ phagocytosis & digestion of bacteria by macrophages & neutrophils leukocytes – A cell-mediated ll di t d immune i response : T lymphocytes bind to surface of parasites or virus-infected virus infected cells Æ lyse them by secreting a membrane-disrupting protein & a Hydrolytic Enzyme y Lymphoid organs : 1, 3 • Primary/central organs : Thymus & Bone Marrow Æ responsible for development & maturation of lymphocytes y p y • Secondary/Pheripheral y p organs g : Lymph nodes, Spleen, Tonsils, solitary nodules, Peyer’s Patches of ileum, Appendix Origin of Immune System Cells • Pluripotent hematopoetic stem cell in bone marrowÆ growth factors stimulation Æ proliferation and maturation of the cells Æ formed elements of the blood y progenitor p g cell & • Stem cell Æ myeloid lymphoid progenitor cells • Lymphoid progenitor cells Æ B lymphocytes & T lymphocytes Growth factors on hematopoietic p system y • G-CSF, GM-CSF, M-CSF, IL- 1, IL-3, IL-4, IL-6, EPO TPO etc EPO, etc. • Cytokines are a unique family of growth factors Æ messenger molecules that can communicate signals from one cell type to another – Secreted primarily from leukocytes but also produced by various cells of the body Æ interleukin (IL) – IL instruct the receiving cells to proliferate, differentiate, secrete additional cytokines cytokines, migrate or die – IL Stimulate both the humoral and cellular immune responses, as well as the activation of phagocytic cells – The list of identified interleukins grows continuously Origin of the main types of lymphocytes B lymphocytes and lymphocytes. natural killer lymphocytes are formed in the bone marrow and lea e the bone marrow leave marro already alread mature, to seed the secondary lymphoid organs and transit th through h the th blood, bl d epithelia, ith li andd connective tissues. Immature CD4– and CD8– T lymphocyte precursors are transported t t d by b the th blood circulation from the bone marrow to the thymus, where they complete l t their th i maturation t ti andd leave as either CD4+ or CD8+ cells. CLONAL SELECTION OF LYMPHOCYTES • In bone marrow & Thymus Æ primary lymphoid organ • Single type receptor on Lymphocytes can recognize all possible antigens Æ but self tolerance • Lymphocytes with receptors not self tolerance are eliminated li i t d by b apoptosis t i Æ clonal l l deletion d l ti B Lymphocytes Maturation • Bone marro marrow Æ Lymphoid L mphoid stem cell Æ Pro B cell Æ Pre B Cell Æ Immature/naive B cells (Ig M)Æ mature B Cells (Ig M + Ig D) Æ blood stream & circulate Æ secondary lymphoid organ • Proliferation lif i andd maturation i off B-cell ll responses are mediated by cytokines T Cells Maturation • Fetal liver/bone marrow Æ Pre T Cell Æ migrate g to Thymus y Æ Stage g 1: T cells with CD 4- & CD8- (double negative) Æ Stage 2: T Cells with CD 4+ & CD 8+ (Double positive) Æ Stage 3 : mature T Cell with CD4+ or CD 8+ (single positive) Approximate percentage of lymphocytes in l lymphoid h id organs1 Lymphoid organs Thymus T lymphocytes B Lymphocytes % % 100 0 Bone marrow 10 90 Spleen 45 55 L Lymph h nodes d 60 40 Blood 80 20 THYMUS • In superior mediastinum 1, 2 • 2 lobes Æ Thin capsules Æ septa Æ subdivide into incomplete lobules 1, 3 • Each lobule consist of cortex & , medulla:1,3 A.Cortex : • Darker than medulla Æ due to large number of T lymphocytes • Also contain macrophages & E ith li l Reticular Epithelial R ti l Cells C ll • 95-98% of developing T cells p p in cortex Æ die byy apoptosis phagocytosed by macrophages B. Medulla : 1, 3 – Stain lighter than cortex Æ less T cells population & large number b off epithelial ith li l reticular ti l cells – 3 types of epithelial reticular cells in medulla : o Type IV cells o Type V cells o T Type VI cells ll Æ Hassl’s H l’ Body / Thymic Corpuscle (found only in medulla, cornified, even calcified, unknown function) Thymus vascular supply1, 3 • Blood-thymus barrier Æ formed by continuous cappillaries in cortex with thick basal lamina, invested by sheath of type I epithelial reticular cells Æ preventing contact of developing T Cells to blood-borne macromolecules • Self macromolecules crossed barrier Æ to select & eliminate T cells react with self antigens Æ clonal selection & clonal deletion • No barrier in medulla • T cells leave medulla via veins drainning the thymus Hormones in thymus 1, 3 • Epithelial reticular cells produce : – Thymosin – Thymopoietin – Thymulin – Thymic Th i humoral h l factor f t Æ Facilitate T cell proliferation & expression of surface markers • Other hormones influence T cells maturation : – Corticosteroids Æ decrease T cells number in cortex – Thyroxin h i Æ stimulates i l epithelial i h li l reticular i l cells ll to increase i thymulin production – Somatotropin p Æ ppromotes T cells development p in thymus y cortex THYMUS INVOLUTION4 • Start after puberty • Parenchym replaced adipose tissue and connective tissue • Decrease weights : 40 g at puberty, 10-15 g late in life • After involution, thymus still has its function as a maturation place for T cells LYMPH NODE • Kidney shape, encapsulated (capsul of Conn. Tissue ÆTrabeculae) • Location : neck,, axilla,, scrotum,, blood vessels in thorax, etc 1, 2 • Have Afferent lymph vessel & Efferent lymph vessel 1 • Hilum : concave depresion which arteries & nerve enter, veins & lymphatic vessels leave1,2 • Parenchym composed of T cells, B cells, APCs & macrophages3 • On average, naive lymphocyte spend less than ½ hour h iin circulation i l i before b f homing h i to another lymphoid organ • 2 main ports of entry into Lymph Node : – By High Endothelial Venule (HEV) • Specialized type of post capillary venule, lined by cuboid or high endothelial cells • Found F d only l iin secondary d lymphoid l h id organs except spleen • Main site of B & T lymphocytes entry from blood Æ by diapedesis – By afferent lymph vessel Æ Site of some memory cells, free antigens & or antigen-loaded APC BLOOD & LYMPH CIRCULATION OF LYMPH NODE CORTEX 1, 2 o Outer Cortex • Lymphoid nodules – B cells Æ imunocytes – Germinal center/secondary nodules Æ only in response of antigenic challenge • Reticular cells & fiber o IInner Cortex/Paracortical C t /P ti l Area Æ T cells activated & proliferated o Subcapsular Sinus & intermediate/Peritrabekular Sinus Section of a lymph node showing the cortex and the medulla and their primary components. B: (1) Capsule; (2) lymphoid nodule with germinative center; (3) subcapsular sinus; (4) i t intermediate di t sinus; i (5) medullary d ll cords; d (6) medullary sinus; (7) trabecula. H&E stain. Low magnification. (Courtesy of PA Abrahamsohn.) Section of a portion of the outer cortex of a lymph node showing the capsule, subcapsular sinuses, diffuse lymphoid tissue and lymphatic nodules. tissue, nodules H&E stain stain. Medium magnification. (Courtesy of PA Abrahamsohn.) MEDULLA : 1, 2, 3 Medullary Cords : • B cells, plasma cells, macrophages • Reticular cells & fiber • More irregular trabeculae than in cortex Medullary Sinus Æ continue with subcapsular p sinus & intermediate sinus Æend up in efferent lymph vessels SPLEEN • • • • Largestt lymphoid L l h id organ in i body b d 3 Hilum Capsul Æ trabeculae Consist of : 1, 2, 3 A. White Pulp p: • Formed by : – Lymphoid nodules Æ B cells – Peri Arterial Lymphatic Sheath/PALS Æ formed by T cells surrounding A. Centralis • Lymphoid nodules Æ germinal centre due to antigenic challenge B. Marginal i l zone 3 – Separate white pulp to red pulp – Composed of plasma cells, T cells, B cells, macrophages, APCs – Marginal sinuses – Contain an abundance of blood antigens Æ plays l major j role l in i immunologic i l i activities ti iti off spleen 1 3 C Red C. R dP Pulp l :1, • Consist of : Splenic Cords / Billroth’s Billroth s Cords Æ macrophages, T cells, B cells, plasma cells, blood cells Splenic Sinusoids : Endothelial cells fusiform, elongated Discontinuous basal lamina Blood Circulation of Spleen TONSILS • Incompletely encapsulated aggregates of lymphoid nodules 1 • Based on location : p palatine,, pharyngeal, p y g , lingual g tonsils1 • Produce lymphocytes1 PALATINE TONSILS • A pair, in pars oralis pharynx1 1 2, 2 3 • Consist C i off : 1, – Stratified squamous p Epithelium – A band of lymphoid nodule with germinal center – Crypts C t : • Invagination of epithelium • 10-20 crypts/tonsil • Contain food debris, dead leucocytes, desquamated of epithelial cells,bacteria etc – C Capsule l Æ partially ti ll att the th base The palatine Th l ti t il consists tonsil i t off diffuse diff l lymphocytes h t and d lymphoid nodules disposed under a stratified squamous epithelium. One of the crypts of the tonsil is shown; the crypts often contain dead epithelial and inflammatory cells. B: (1) Crypt; (2) stratified squamous epithelium; (3) lymphoid nodules; (4) diffuse lymphoid tissue; (5) germinative center; (6) capsule; (7) mucous glands. Hematoxylin and eosin (H&E) stain. Low magnification. (Courtesy of PA Abrahamsohn.) PHARYNGEAL TONSILS • Single in posterior nasopharynx1, 2 1 22, 3 • Consist C i t off :1, – Pseudostratified ciliated columar epithelium – Lymphoid nodules – No crypts, only shallow longitudinal infolding called pleats l – Thinner capsule than T. Palatina LINGUAL TONSILS • Smaller & more numerous than other tonsils • At base of tongue • Consist of :1,, 2,, 3 – Stratified Squamous Epithelium – Lymphoid L h id nodules d l Æ germinal i l center t – Each lingual tonsils has a single crypts MUCOSA--ASSOCIATED LYMPHOID MUCOSA TISSUE / MALT3 • Non capsulated • Lymphoid nodules in mucosa or submucosa of GI tract respiratory tract tract, tract, urinary tract. Gut-Associated Associated lymphoid • Gut tissue (GALT) Æ peyer’s patches (B Cells surround b T cells by ll & APCs) APC ) • Bronchus-associated lymphoid tissue (BALT) Æ similar to peyer’s patches Section of lung showing a collection of lymphocytes in the connective tissue of the bronchiolar mucosa, an example of mucosa-associated i d lymphoid l h id tissue i (MALT). Pararosaniline—toluidine blue (PT) stain. Low magnification. REFERENCES : 1. 2. 3. 4. Basic Histology Text & Atlas , 10th ed. , L. Carlos Junquira MD, Jose Carneiro MD, Robert O. Kelley PhD, Lange Medical Books, Mc Graw Graw-Hill Hill , 2003. Pp 265 – 290. Essentials of Human Histology, 2nd Edition, William J. Krausse PhD PhD, Little Brown & Company (Inc) (Inc), 1996 1996. Pp 197-228 Color Textbook of Histologi, 2nd edition, Gartner LP, Hiatt JL WB S JL, Saunders d C Company, Phil Philadelphia, d l hi Pennsylvania, P l i 2001. Pp 273-299 Consise Histology, 2nd edition, Don W Fawcett, Ronald P Jensh, Arnold publisher, London, 2002. Pp 148-161