Immunology: Lymphoid System Histology

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Lymphoid System

Development of Lymphoid System

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-fxn protective/immunologic; source of immunocompetent cells which have capacity to react/neutralize foreign substances or Ag
-lymphocytes, plasma cells, & macrophages

-origin: mesenchyme-derived yolk cells
-after vasculature: multipotential cells migrate to liver & spleen (proliferate & differentiate
-later bone marrow source of stem cells

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Primary Lymphoid Organs

Secondary Lymphoid Organs

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-Thymus: Tcells mature (true primary lymphoid organs)

-Bursa Equivalent: origin gut of birds bursa of Fabricus-derived, mature Bcells
-includes Gut Assoc. Lymphoid Tissue (GALT), tonsils, peyers patches of ileum & appendix

-T & B cells migrate to: Lymph nodes, spleen, GALT (also fxn as secondary organs)

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T Cells vs. B Cells

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-are morphologically similar but fxnally distinct:

1. T Cells: cell mediated immunity
-eliminate Ag by release toxic substance (lymphokines) & also activate other cells (neutrophils, monocytes, & macrophages)

2. B Cells: Humoral
-form plasma cells that secrete Ab

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Parenchyma

Stroma

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functional tissue (ex: in thymus = diffuse tissue only)

non-fxnal tissue, the structural (CT framework of lymphoid) = reticular fibers

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Diffuse Lymphatic Tissue

Nodular Lymphatic Tissue

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loose aggregates; T Cell Zones: Cell mediated

denser, more highly organized form; dense tissue w/ light central region (germinal center); B cell Zones - humoral

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Germinal Center

Periphery of Nodule

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-center of nodule contain many large lymphocytes/lymphoblasts esp. when active where

-periphery of nodule consists of tight-packed lymphocytes

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Primary Lymphoid Organs vs. Secondary Lymphoid Organs

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STROMA
-primary lymph node = stroma has no reticular fibers
-secondary lumph node = stroma rich in reticular fibers *and reticulo endothelial cells (highly phagocytic)

PARENCHYMA
-primary: only diffuse tissue
-secondary: diffuse & nodular tissue

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Thymus

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-fully developed: paired organ located in midline of superior mediastinum, beneath sternum
-highly developed @ time of birth (babies thymus dwarfs heart & lungs)
-surround by thin CT capsule has trabeculae extending from it; separate thymus into partial lobules (cortex & medulla)
-cortex = aggregated T cells
-medulla: light in appearance, few Tcells, more reticulo-endothelial cells (NO reticular fibers in thymus) - macrophages, dendritic process. cells

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Thymic Corpuscles

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Hassal's Corpuscles:
-#s increase w/ person's age
-responsible for cell death of T Cells
-fxn unknown although suggested to be site of T cell death in medulla
-macrophages present in large #s in medulla w/ plasma cells & granulocytes (present bc of circulation)
-lymphocyte population replaced w/ adipose tissue & Hassal's Corpuscles become enlarged

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Thymopoietin

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hormone-like substance
-extensive proliferative activity of lymphocytes
-at puberty: thymus begins to undergo involution (mediated by adrenocortical & sex hormones)

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Bursa Equivalent

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-mammal equivalent to Bursa of Fabricus (avian) - generator of lymphocytes
-exists w/ GALT which acts as central lymphoid & more commonly as secondary lymphoid organs

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Secondary (Peripheral) Lymphoid Organs-Lymph Nodes

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-small, encapsulated organ which lymph flows
-most prominant in inguinal & axillary region but also found throughout body in small groups, assoc. w/ regional lymphatic drainage & fxn as lymph filter
-lymph-borne Ag trapped by lymph node

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Lymph Nodes

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-covered by dense CT capsule w/ trabeculae (extend from organ to capsule)
-inner stroma: delicate network of reticular fibers & reticular cells (phagocytic in Ag trapping)
-afferent lymphatics penetrate capsule & confluent w/ system of sinuses
-efferent & blood supply located at hilus (thick region of CT)
-parenchyma (fxnal tissue) composed of lymphoid tissue organized into outer cortex & inner medulla
-cortex=nodules
-paracortical region = diffuse

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Medullary Cords

Path of Lymph

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-sinuses: medulla composed of lymphoid cells organized into strands
-macrophages usually found in/around in large #

-lymph enters sinuses: travel afferently along capsule, trabeculae & medullary cords
-exits via efferent at hilus
-most drain into collect. vessels to bring to neck (venous circulation)

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Lymph Node Fxn

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-soluble & particulate Ag phagocytized by macrophages & phagocytic reticular cells (dendritic cells) which trap & maintain Ag on plasma mem (99% impurities removed)
-maintenance of Ag allows Bcell activation, proliferate germinal centers, plasma cells differentiate-->produce Ab
-Ab inactivate Ag by elicit cells such as neutrophils, or activate complememnt

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Lymph Node Clinically

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-presence of Ag, node rapidly proliferate & node increase to several times its normal size becoming hard = lymph adenapathy
-lymph nodes connected along vessels (chain), so lymph goes 1 node to next = infection & malignant cells may spread to other parts of body

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Spleen

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-largest lymphoid organ in body
-acts as blood filter
-fetal: fxns temporarily as hematopoietic organ & elastic reservoir for blood in mammals
-filtration mxn of spleen traps senescent or old RBC & blood-borne Ag
-encapsulated by dense CT w/ branching trabeculae extend into organ
-made up of elastic & reticular fibers
-trabeculae carry blood vessels into/out of parenchyma from hilus (convex)

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Splenic Pulp

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-islands of white pulp in sea of red pulp
-white pulp composed of diffuse & nodular lymphoid tissue
-red pulp reflects predominance of RBC
-white pulp organized around arteries of parenchyma

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PALS

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Periarteriolar Lymphoid Sheath
-diffuse lymphoid tissue forms a cuff around arterial branches
-nodular lymph extends from sheath
-parenchyma of red pulp composed of diffuse tissue organized in cords which are highly infiltrated w/ RBC

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Marginal Zones

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-bw red and white pulp
-recieve much of blood entering spleen
-zones imp in Ag trapping & distribution of incoming cells (traffic cops)
-fxn: essential for trap Ag & old RBC
-contain large # of macrophages & phagocytes that degrade trapped cells/particles
-diffuse lymphoid tissue (PALS) populated by Tcells

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Spleen Organization

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-PALS & Lymph Nodules = white pulp
-marginal zone separate red from white pulp: primarily composed of plasma cells, T & B cells, macrophages & dendritic (reticular) cells
-Red Pulp = sinuses & cords; contains reticular fibers which contain reticular cells & macrophages

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Erythroid Cell Destruction

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-machrophages monitor RBC as migrate from splenic cords into sinuses
-globin disassembled into aa--> aa pool
-iron: taken to bone marrow vie transferrin
-heme converted to bilirubin & excreted by liver in bile

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Gut Layers

GALT

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1. Mucos: lamina propria
2. Submucosa
3. Musculara (smooth muscle)
4. Serosa
(Stomach, small intest, colon, appendix, gallbladder)

-lamina propria of digestive & respiratory tract often considered lymphoid tissue
-3 areas of body which consist of diffuse & nodular lymphoid tissue
1. Peyer's Patches: ileum of small intestine (Nodules surrounded by diffuse confined in lamina propria
2. Tonsils: form ring of tissue around throat entrance (paired palantine & lingual & single pharyngeal) - epithelium is stratified squamous (crypts), reticular fibers = inner framework

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vēritās

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truth

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vēritātis, F.