Structure of the immune system

 

Structure of the immune system

The immune system are organised into specific structures.

Classified as

ü   Primary organs (bone marrow, thymus)

ü   Secondary organs (lymph nodes, spleen, mucosa-associated lymphoid tissue):

—  Bone marrow: All the cells of the immune system are derived from stem cells in the bone marrow.

1.     Thymus

—  Located in the thorax

—  In the cortex - thymocyte (immature T-cells) dev`t occurs

Immature T-cells enter the cortex, proliferate, mature

It involves T-cell receptor

v  gene rearrangement

v   positive selection

v  and pass on to the medulla.

—  In Medulla – additional negative selection to remove auto-reactive T-cells from the mature cells occurs

—  From the medulla mature T lymphocytes enter  circulation

2. Lymph Node

The node is made up of three components:

a)     Lymphatic sinuses

b)    Blood vessels

c)     Parenchyma (cortex, paracortex, medulla)

—  Lymph passes into the node through the incoming lymph vessel into the marginal sinus, through the cortex to reach the medulla before leaving via the outgoing lymph vessel

—  The  central area of a follicle is known as a germinal centre and this is surrounded by a mantle zone consisting of small, naive B cells and a few T cells.

—  Pathogens in the lymph are removed by antigen presenting cells e.g. MQ which facilitate an immune response.

—  Less than 10% of lymphocytes enter the lymph node thro` the incoming lymph vessel,

—   the large majority enter from the blood via the High Endothelial Venules ( HEVs).

 

                        i.         Cortex – antigen simulation

B cells enter the lymph node via HEVs and pass to the follicles.

v  If activated by antigens, they proliferate and remain in the node.

v  Stimulated mature B cells responding to antigen change into centrocytes

v  The centroblasts (mature) leave the follicle and pass to the paracortex and medullary sinuses, where they become immunoblasts

v  Unstimulated B cells - pass out rapidly from the node to return to the general circulation.

 

                      ii.         PARACORTEX

v  it is the predominant site for T lymphocytes within the lymph node.

v  T cell enter the node from the blood via the HEVs.

v  When activated they form lymphoblasts which divide to produce a clone of T cells responding to a specific antigen.

v  Activated T cells then pass into the circulation to reach peripheral sites.

                     iii.         Medulla

The medulla comprises:

—  large blood vessels

—  medullary cords

—  medullary sinuses

—  The medullary cords are rich in plasma cells which produce antibodies that pass out of the node via the efferent lymphatic.

—  Macrophages are also numerous within the medulla.

3.     Spleen

Two  components -  the red pulp and the white pulp

     Red pulp: Has a complex system of blood vessels within

—  Facilitate removal of old or damaged red blood cells from the circulation.

—  A small % of the splenic blood flow passes through more rapidly without undergoing this process of filtration.

White pulp: Mount an immunological response to antigens within the blood

—  Contains T cells, B cells follicles and accessory cells.

—  Consist of a periarteriolar lymphoid sheath (PALs).

 

 

 

 

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