Immunology Lecture 4th

Organs & Tissues of

Immune system

BY

Dr. Humera Kausar

23rd Sep 2014

V

Organ of Immune System

Leucocytes and erythrocytes are derived from bone

marrow stem cells. Lymphocytes, the dominant cells

of immune system mature in the bone marrow (B-

lymphocytes) and Thymus (T-lymphocytes),Hence

these are called primary lymphoid organ or

generative lymphoid organ.

1) Bone Marrow

2) Thymus

Primary lymphoid organ

BM is the Largest tissue of a mammalian body.

An adult contain 3kg of bone marrow.

BM has two distinct kind red and yellow marrow

Primary lymphoid organ

Bone Marrow

Flat, bilobed organ situated above the heart.

function of thymus is to generate & select a repertoire of T-Cells.

Each lobe is surrounded by a capsule and divided in to lobules, which is separated by strands of connective tissues called trabeculae.

THYMUS Cortical Epithelial

Cells

Medullary Epithelial Cells

Macrophages

Dendritic Cells

Thymocytes

Cortex and medulla is crisscrossed by 3-dimensional network of stromal cells composed of epithelial cells, dendritic cells and macrophages.

Many of these stromal cells interact physically with developing thymocytes.

THYMUS

Thymic epithelial cells secrete several hormonal factors necessary for the differentiation & maturation of T-Cells.

As thymocyte develop, an enormous diversity of T cell receptor is generated by a series of gene rearrangement.

THYMUS

Thymus induces apoptosis of those T-cells that fails to recognize MHC complexes or react with MHC complexes.

More than 95% of thymocytes die by apoptosis in the thymus.

THYMUS

Thymic functio

n is known

to decline

with age

Thymic Function

Secondary lymphoid organ

are the sites where mature

lymphocytes respond or interact

to foreign antigens that Includes

1.lymph nodes

2.spleen

3.Lymphoid tissues

LYMPH & LYMPHATICS

•Lymph is a clear body fluid which clots like blood.

•Lymph forms when dissolved proteins and solutes filter out of venules and capillaries. (due to hydrostatic & osmotic

pressure)

Thoracicduct

Overview of blood and lymph system and how leukocytes travel from one system to another

Site of exchange between theblood and lymph systems

Lymph carrying antibodiesand immune cells collect in thoracic duct where the lymph empties back into the blood circulatory system

•Lymph flows uni directionally towards lymph nodes.

• Efferent lymph from regional lymph nodes may drain into one or more additional nodes before flowing into major efferent lymphatic.

• Thoracic duct carries lymph draining from the gut and lower half of the body.

•Lymphatic system is "garbage collector”

LYMPH & LYMPHATICS

1) Lymph nodes

are small nodular

aggregates of

lymphoid tissue

situated along

lymphatic channels

throughout the body

LYMPH NODE

Lymphnodes are bean shaped connective tissue bags, filled with mobile cell (lymphocyte, macrophages, dendritic cells) organized in functional compartments.

Trap Ag & Cells containing Ag.

Provide a site for clonal expansion.

Cortex (B-cells, macrophages, follicular DC)

Paracortex (T-cells, DC)

Medulla (sparsely populated lymphoid cells, many plasma cells)

LYMPH NODE

Posses distinct regions of T-cell and B-cell activity and are surrounded by a fibrous capsule.

Follicular dendritic cells, of the germinal center, trap Ag-Ab complexes on membrane for long time.

Antigenic stimulation in node can increase the no. of lymphocytes 10-50 fold, resulting in visible swelling of the node.

LYMPHOID FOLLICLE

Primary follicles (comprises a net work of follicular dendritic cells & resting B-cells)

Secondary follicles (a ring of concentrically packed B lymphocytes surrounding a germinal center).

SPLEEN

The spleen is an organ found in virtually all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter.

Functions of spleen

The site of immunocytes residence. The site of immune response. Produce some active

substances,such as complement,et al.

Function of filtration.

SPLEEN PLAYS A MAJOR ROLE IN

MOUNTING IMMUNE RESPONSES TO

ANTIGENS IN BLOOD

LYMPHNODES ARE SPECIALIZED FOR

TRAPPING ANTIGENS FROM LOCAL TISSUES

Spleen is surrounded by a capsule from which a number of projections extend in to the interior to form a compartmentalization.

Red zone & White zone are separated by marginal zones.

Red pulp populated by macrophages, numerous RBC,s and few lymphocytes.

Red pulp is the site where old & defective red cells are destroyed & removed.

SPLEEN

Splenic white pulp surrounds the branches of splenic artery, forming a Periarteriolar lymphoid sheath (PALS).

SPLEEN

PALS rich in T cells.

Marginal zones rich in B cells & Dendritic cells.

SPLEEN

PALS rich in T cells.

Primary lymphoid follicles are attached to the PALS.

These follicles are rich in B cells & some contains germinal centers.

SPLEEN

Marginal zones, peripheral to the PALS are populate by lymphocytes and macrophages.

SPLEEN

Blood born Ag. & lymphocytes enter the spleen through splenic artery, empty in to marginal zone.

Ag trapped by DC, carry it to the PALS.

Blood lymphocytes also migrate from marginal zone to the PALS.

SPLEEN

Initial activation of T & B cells takes place in the T-cell rich PALS.

Activation of DC T helper B-cell

Activated B cells with Th Cells migrate to primary follicles in marginal zone.

SPLEEN

On antigenic challenge, primary follicles develop into characteristic secondary follicles containing germinal centers.

Where rapidly dividing B cells & plasma cells are surrounded by dense cluster of lymphocytes.

SPLEEN

Lymphoid tissues Besides the secondary lymphoid organ, there

are some structures which consist of aggregates of lymphocytes, macrophages, and other accessory cells. All such anatomically well defined structures are collectively called mucosa -associated lymphoid tissues.(MALT).

They contain aggregates of immune cells

MALT

GALT

BALT Surface area 400 m2

Having lose clusters of lymphoid cells (B-cells, T cells, macrophages)

Population of Plasma cells in MALT is far exceeds that of in spleen, lymph nodes & bone marrow combined.

MALT includes,

Peyer’s patches

Tonsils

Appendix

PEYER’S PATCHES

Peyer's patches (or aggregated lymphoid nodules) are organized lymphoid tissues, named after the Swiss anatomist Johann Peyer. They are aggregations of lymphoid tissue that are usually found in the lowest portion of the small intestine, the ileum, in humans.

PEYER’S PATCHES

Payer,s patches found within the intestinal lining, contains well organized follicles from which germinal centers developed upon antigenic stimulation.

Germinal Centres

M Cells

Lymphocytes

Tonsils play a role in defense against antigens entering through the nasal and oral epithelial routes.

Nodular structure consisting of a meshwork of reticular cells and fibers interspersed with lymphocytes, macrophages, granulocytes and mast cells.

B-cell are organized in to follicles and germinal centers.

Germinal centers are surrounded by regions showing T-cell activity.

TONSILS

Summary