Lymphoid tissue and lymphoid organs



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Stroma


The supporting tissue of lymph nodes are:

  1. Capsule: lymph node is covered by a dense C.T capsule.

  2. Trabeculae: extend from the capsule forward the substance of the node.

  3. Reticular fibers or tissue = composed of reticular cells and reticular fibers that form of fine supporting meshwork throughout the remainder of the organ.


Two types of lymphatic vessels serve the nodes:

Afferent lymphatic vessels


Convey lymph toward the node and enter at various points on the convex surface of the capsule.
Efferent lymphatic vessels = convey lymph way from the node and leave at the hilum (concave surface).

  • In between there is a system of sinuses along the capsule (marginal sinuses) and medullary sinuses in between medullary cards. Both will drain into efferent lymphatic vessels.



The parenchyma


The parenchyma of the lymph node is divided into cortex and

medulla.


  • The cortex is formed the outer portion of the node except at the hilum.

  • It is composed of: (1) nodules with distinct germinal centers.

  • The cortex adjacent to the medulla is free of nodules and is formed of : (2) diffuse lymphoid tissue, this area is called paracortical or juxtamedullary cortex or deep cortex. Also called thymus-dependent cortex.



Note:

Thymectomy of neonate underdevelopment of this zone.



The medulla

Is the inner part of the node (medullary cords)



  • It consists of cords of lymphatic tissue (medullary cords) separated by lymphatic sinuses called medullary sinuses.

  • Reticular fibers are present in the medullary cords cross-supportive elements. B lymphocytes mainly, plasma cells, macrophages are present in the cords.


Distribution of B + T lymphocytes.

  • B lymphocytes are most common in the cortex (germinal center) and medullary cords.

  • T lymphocytes are common in the deep cortex or para cortex.

  • Both T & B lymphocytes are present where the deep + nodular cortex meet.



Note


Cell mediated immunity (e.g. viral. Chronic TB, skin transplant)

stimulate proliferation of T lymphocytes in the diffuse cortex.


Humoral immunity (acute bacterial infection) stimulates proliferation of B lymphocytes in the nodular cortex (enlargement of the germinal centers).

Function of the lymph nodes


The lymph born antigen originate from the connective tissue

regions of the body are transported by afferent lymphatic

vessels to the regional lymph nodes.

These antigen will be phagocytized by macrophages and

follicular dendritic cells.

These are present in the lymphatic nodule.

They preserve the antigen on their plasma membranes.

The antigen is exposed, thus, they are presented to

immunocompetent memory B cells.

Recognition of an antigen by a B cell may necessitate the involvement of helper T cell to facilitate activation of B cell.



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