LYMPHOID LYMPHOID ORGANSORGANS

The Immune SystemThe Immune System

Functions of Immune Functions of Immune SystemSystem

Has the ability to distinguish ‘self’ from Has the ability to distinguish ‘self’ from non-selfnon-self

Inactivates/destroys foreign substancesInactivates/destroys foreign substances Foreign moleculeForeign molecule Molecules present in viruses, bacteria, Molecules present in viruses, bacteria,

parasites.parasites. Microorganisms and other cells (virus-Microorganisms and other cells (virus-

infected cells, cancer cells, cells of infected cells, cancer cells, cells of transplanted organs)transplanted organs)

Where is the Immune Where is the Immune SystemSystem

Cells of the immune system are:Cells of the immune system are: Distributed throughout the body in the blood, Distributed throughout the body in the blood,

lymph, epithelial and CT.lymph, epithelial and CT. Arranged in small spherical nodules (lymphoid Arranged in small spherical nodules (lymphoid

nodules) found in CT and inside various nodules) found in CT and inside various organs.organs.

Found in the mucosa of digestive (tonsils, Peyer’s Found in the mucosa of digestive (tonsils, Peyer’s patches), respiratory, reproductive, urinary systems patches), respiratory, reproductive, urinary systems are MALT (mucosa-associated lymphoid tissue. are MALT (mucosa-associated lymphoid tissue.

Organized as differently sized organs—Organized as differently sized organs—lymphoid organs—the lymph nodes, spleen, lymphoid organs—the lymph nodes, spleen, thymus, bone marrow.thymus, bone marrow.

Lymphoid TissueLymphoid Tissue

Lymphoid tissue is CT with rich Lymphoid tissue is CT with rich supply of lymphocytessupply of lymphocytes

Exists free within regular CT or is Exists free within regular CT or is surrounded by capsules.surrounded by capsules.

Very little cytoplasm so stain dark Very little cytoplasm so stain dark blue with H&E.blue with H&E.

Rich network of reticular fibrils Rich network of reticular fibrils produced by fibroblast whose many produced by fibroblast whose many processes rest on fibrils.processes rest on fibrils.

Lymphatic SystemLymphatic System

Lymphatic CapillariesLymphatic Capillaries

Lymph NodeLymph Node

Structure of Lymph NodeStructure of Lymph Node

Medullary sinus of a lymph node containing reticular cells with long processes and elongated nuclei, macrophages, and many lymphocytes. (1) Macrophage; (2) reticular cell; (3) trabecula. H&E stain. High magnification. (Courtesy of PA Abrahamsohn.)

Lymph NodesLymph Nodes

Functions include:Functions include: Filtration of particles and microorganisms Filtration of particles and microorganisms

to keep them out of general circulation.to keep them out of general circulation. Interaction of circulating antigens in Interaction of circulating antigens in

lymph with lymphocytes to initiate lymph with lymphocytes to initiate immune response.immune response.

Activation, proliferation of B lymphocytes Activation, proliferation of B lymphocytes and antibody production.and antibody production.

Activation, proliferation of T lymphocytes.Activation, proliferation of T lymphocytes.

Cells of Lymph NodeCells of Lymph Node

Lymphoid cellsLymphoid cells Macrophages and other phagocytic Macrophages and other phagocytic

antigen processing cellsantigen processing cells Lymphatic and vascular endothelial Lymphatic and vascular endothelial

cells and fibroblasts responsible for cells and fibroblasts responsible for lymph node supporting framework.lymph node supporting framework.

Lymph NodeLymph Node

Section of a lymph node showing the cortex and the medulla 1 Capsule;2 lymphoid nodule with germinative center 3

subcapsular sinus; (4) intermediate sinus; (5) medullary cords; (6) medullary sinus; (7) trabecula. (Courtesy of PA Abrahamsohn.)

Medulla of Lymph NodeMedulla of Lymph NodeLymphocytes predominate

Medullary sinusesMedullary cords

SpleenSpleen

Largest accumulation of lymphoid tissueLargest accumulation of lymphoid tissue Abundant phagocytic cells—defense Abundant phagocytic cells—defense

against antigens in bloodagainst antigens in blood Site of destruction of aged erythrocytes.Site of destruction of aged erythrocytes. Production site of activated lymphocytes Production site of activated lymphocytes

which are delivered to the blood.which are delivered to the blood. THUS, an important blood filter and THUS, an important blood filter and

antibody-forming organ.antibody-forming organ.

SpleenSpleen

SpleenSpleen

SpleenSpleen

Lymphoid NoduleLymphoid Nodule1. Germinative Center2. Central artery

Red Pulp of SpleenRed Pulp of Spleen

Macrophages

Fenestrated endothelium