Lymphoid Tissue

Forms the basis of the immune system of the bodyForms the basis of the immune system of the body LymphocyteLymphocyte, the principal cell of lymphoid tissue, , the principal cell of lymphoid tissue,

is responsible for the proper functioning of the is responsible for the proper functioning of the immune systemimmune system

Classified as Classified as primaryprimary & & secondarysecondary lymphoid lymphoid tissuetissue

Organized into Organized into diffusediffuse and and nodularnodular lymphoid lymphoid tissuetissue

Types of Lymphoid Organs Primary lymphoid organsPrimary lymphoid organs are responsible for are responsible for

the development and maturation of the development and maturation of lymphocytes into mature cells e.g. fetal liver, lymphocytes into mature cells e.g. fetal liver, bone marrow and thymusbone marrow and thymus

Secondary lymphoid organsSecondary lymphoid organs are responsible are responsible for the proper environment in which the for the proper environment in which the immunocompetent cells can react with each immunocompetent cells can react with each other and with the antigens e.g. lymph nodes, other and with the antigens e.g. lymph nodes, spleen, and mucosa associated lymphoid spleen, and mucosa associated lymphoid tissue tissue

Organization of Lymphoid Tissue1. Diffuse Lymphoid Tissue

Occurs throughout the body, especially Occurs throughout the body, especially under wet epithelial membranes where the under wet epithelial membranes where the connective tissue is infiltrated by lymphoid connective tissue is infiltrated by lymphoid cells e.g. cells e.g. lymphocytes, plasma cells, lymphocytes, plasma cells, macrophages and reticular cellsmacrophages and reticular cells

Particularly evident in the Particularly evident in the lamina proprialamina propria of of GIT & subepithelial connective tissue of the GIT & subepithelial connective tissue of the respiratory tractrespiratory tract

2. Nodular Lymphoid Tissue

Includes:Includes: Lymph nodesLymph nodes ThymusThymus SpleenSpleen TonsilsTonsils

Cells of Immune System

Lymphocytes are generally classified into:Lymphocytes are generally classified into: B lymphocytesB lymphocytes (B cells) (B cells) T lymphocytesT lymphocytes (T cells) (T cells) Null cellsNull cells: composed of two types: composed of two types

Stem cellsStem cellsNatural killer cellsNatural killer cells

B Lymphocytes

Small lymphocytes that are formed, and Small lymphocytes that are formed, and become immunocompetent in the become immunocompetent in the bone bone marrow.marrow.

The activated cells The activated cells enter the general enter the general circulationcirculation, , undergo mitosisundergo mitosis, and , and transform into antibody-producing transform into antibody-producing plasma cells plasma cells andand B-memory cells B-memory cells

T Lymphocytes

Histologically identical to B lymphocytesHistologically identical to B lymphocytes Formed in Formed in bone marrowbone marrow, migrate to the cortex , migrate to the cortex

of the of the thymusthymus to become immunocompetent to become immunocompetent There are several subtypes e.g. There are several subtypes e.g. T helper cells, T helper cells,

T cytotoxic cells, T suppressor cells and T T cytotoxic cells, T suppressor cells and T memory cellsmemory cells

Assist B lymphocytes in the performance of Assist B lymphocytes in the performance of their functions their functions

Antigen Presenting Cells

Derived from Derived from monocytesmonocytes They phagocytose, catabolize and process They phagocytose, catabolize and process

antigen and present to T lymphocytesantigen and present to T lymphocytes Include macrophages, dendritic cells Include macrophages, dendritic cells

(Langerhans cells of the epithelium & (Langerhans cells of the epithelium & mucosa), B cells and epithelial reticular mucosa), B cells and epithelial reticular cells of thymuscells of thymus

Stem Cells

Undifferentiated cellsUndifferentiated cells Give rise to the various cellular Give rise to the various cellular

elements of the blood elements of the blood

Natural Killer Cells

Cytotoxic cellsCytotoxic cells Formed in the bone marrowFormed in the bone marrow Become immunocompetent as soon as Become immunocompetent as soon as

they are formedthey are formed They kill some virally altered cells and They kill some virally altered cells and

tumor cellstumor cells

Lymph Nodes Ovoid, kidney shaped Ovoid, kidney shaped

organs organs Each node has:Each node has:

A A convex surfaceconvex surface which receives which receives afferent lymph vesselsafferent lymph vessels

AA hilumhilum where where efferent lymph vesselsefferent lymph vessels leave and drain lymph leave and drain lymph from the organsfrom the organs

Histological Structure of Lymph Node Each lymph node has a dense Each lymph node has a dense

irregular collagenous irregular collagenous connective tissue connective tissue capsule capsule

Derived from the capsule, Derived from the capsule, connective tissue connective tissue septaeseptae (trabeculae)(trabeculae) extend into the extend into the outer part (cortex) of the node outer part (cortex) of the node and divide it into incomplete and divide it into incomplete compartmentscompartments

The framework of the node is The framework of the node is formed by formed by reticular reticular connective tissueconnective tissue

Each lymph node is Each lymph node is divided into three divided into three regions: regions: CortexCortex ParacortexParacortex MedullaMedulla M

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C

Lymph Nodes: CORTEX contains the:contains the:

Subcapsular sinus, Subcapsular sinus, which is continuous which is continuous and drains into the and drains into the

Cortical sinusesCortical sinuses Lymphoid nodulesLymphoid nodules

(primary & secondary) (primary & secondary) composed mainly of composed mainly of B B lymphocyteslymphocytes and and reticular cellsreticular cells

CPC

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Lymph Nodes: PARACORTEX It is the region between cortex and It is the region between cortex and

medullamedulla Contains thymus dependent zone mainly Contains thymus dependent zone mainly

T lymphocytesT lymphocytes High endothelial venules ( HEV) lined by High endothelial venules ( HEV) lined by

simple cuboidal epithelium instead of simple cuboidal epithelium instead of simple squmaus epithelium to facilitate simple squmaus epithelium to facilitate passage of mature T lymphocyte coming passage of mature T lymphocyte coming from thymus to paracortex of lymph from thymus to paracortex of lymph node. node.

Has NO nodulesHas NO nodules

PCC

M

Lymph Nodes: MEDULLA consists of:consists of:

Medullary cordsMedullary cords Medullary sinusoidesMedullary sinusoides..

TheThe medullary cords medullary cords are are composed of composed of T lymphocytes, T lymphocytes, plasma cells plasma cells andand macrophages macrophages

The The medullary sinusoidesmedullary sinusoides are are continuous with the continuous with the subcapsularsubcapsular and and corticalcortical sinuses.sinuses.

There in no lymphatic There in no lymphatic Nodules.Nodules.

MS

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Lymph Nodules Lymph nodules are Lymph nodules are

small masses of lymph small masses of lymph tissue in which tissue in which B B lymphocyteslymphocytes are are produced. produced.

Lymph nodulesLymph nodules may be: may be: Primary nodulesPrimary nodules Secondary nodulesSecondary nodules

PNSN

Primary nodules

which are formed which are formed of virgin (not of virgin (not activated) and activated) and memory memory B B lymphocyteslymphocytes

Secondary NoduleContains:Contains: Germinal centresGerminal centres, central , central

light areas filled with light areas filled with immature lymphocytes. Here immature lymphocytes. Here new lymphocytes proliferate new lymphocytes proliferate from stem cells that originate from stem cells that originate in the bone marrow. Germinal in the bone marrow. Germinal centerscenters contains large contains large lymphocytes, mitotic figures, lymphocytes, mitotic figures, macrophages and plasma macrophages and plasma cellscells

The germinal centre is The germinal centre is surrounded by a darker-surrounded by a darker-staining region called the staining region called the corona corona that contains small that contains small B B lymphocytes ( not activated).lymphocytes ( not activated).

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Lymph Flow Through The Lymph Node

Afferent lymph vesselsAfferent lymph vessels

Subcapsular sinusesSubcapsular sinuses Cortical sinusesCortical sinuses

Medullary sinusesMedullary sinuses

Efferent lymphatic Efferent lymphatic vesselsvessels

Lymph Node: Function

Maintenance and production of Maintenance and production of immunocompetent cells immunocompetent cells

Proliferation of lymphocytes. Activated T- and Proliferation of lymphocytes. Activated T- and B- lymphocytes multiply in the lymph nodesB- lymphocytes multiply in the lymph nodes

Filtration of lymph Filtration of lymph ( not blood)( not blood) Phagocytosis Phagocytosis

Thymus Bilobed lymphoid organ Bilobed lymphoid organ

located in located in thoraxthorax Derived from Derived from endodermendoderm Enclosed in a thin Enclosed in a thin

connective tissue connective tissue capsulecapsule SeptaeSeptae extend from the extend from the

capsule into the organ, capsule into the organ, incompletely subdividing incompletely subdividing it into it into lobuleslobules

Possesses Possesses no lymph no lymph nodules nodules

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L

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Thymus cont’d Each Each lobulelobule is divided into an is divided into an outer outer

cortexcortex and and inner medullainner medulla CORTEXCORTEX: Composed of : Composed of epithelial reticular epithelial reticular

cellscells, , macrophagesmacrophages, and , and thymocytesthymocytes (small T (small T lymphocytes). Here the immature T cells lymphocytes). Here the immature T cells undergo proliferation, reticular cells release undergo proliferation, reticular cells release hormone and factor to activate T lymphocyte hormone and factor to activate T lymphocyte and transform into immunocompetent cells and transform into immunocompetent cells and then migrate to medullaand then migrate to medulla

MEDULLAMEDULLA: consists of : consists of epithelial reticular epithelial reticular cellscells, , large T lymphocyteslarge T lymphocytes and and thymic thymic (Hassall’s) corpuscles(Hassall’s) corpuscles

Microphage will engulf not activated T Microphage will engulf not activated T lymphocyte. lymphocyte.

1 - cortex 1 - cortex 2 - medulla 2 - medulla 3 - Hassal's 3 - Hassal's corpuscle corpuscle 4 - interlobular 4 - interlobular connective tissue connective tissue (septa)(septa)

Hassall’s Corpuscle

Are composed of Are composed of groupsgroupsof of keratinized keratinized epithelial cellsepithelial cells

Increase in number Increase in number with agewith age

Probably represent a Probably represent a degenerative degenerative phenomenonphenomenon

CM

HC

Epithelial Reticular Cells Separate the cortex from the Separate the cortex from the

capsule and the trabeculaecapsule and the trabeculae Surround the cortical capillariesSurround the cortical capillaries

thus forming a thus forming a blood-thymus blood-thymus barrierbarrier

Form a Form a specialized barrier between specialized barrier between cortex & medullacortex & medulla, preventing , preventing medullary material from gaining medullary material from gaining access to the cortexaccess to the cortex

Thus provide an antigen free Thus provide an antigen free environment for the potentiation of environment for the potentiation of the immunocompetent T the immunocompetent T lymphocyteslymphocytes

They also They also form Hassall’s (thymic) form Hassall’s (thymic) corpusclescorpuscles

Thymus

Thymus

Lymph Node

Lymph Node