lymphoid system

Lymphoid System
The lymphoid system all cells, tissues and
organs that aggregates. The lymphocytes are
distributed through out the body .
The major lymphoid organs are the:-
1-Tonsils,
2-lymph nodes
3- thymus
4- spleen
These organs are important components of
the immune system .
Tonsils
Are paired , ovoid structures that
consist of dense accumulations of
lymphatic tissue located in the
mucous membrane of the junction
of oropharynx and oral cavity.
Lymph Nodes
• Lymph nodes are small, bean shaped, encapsulated
lymphatic organs.
They are widely distributed through out the body .
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
capsule.
• Efferent lymphatic vessels convey lymphatic away from
node and leave at the hilum, a depression on the concave
surface of the node that also serves for the and exit of
blood vessels and nerves.


The supporting elements of the lymph node are:
1-Capsule:- composed of dense connective tissue
rounds the node
2-Trabeculae:- also composed of dense
connective tissue which extend from the
capsule into the substance node , forming a
gross framework
3-Reticular tissue:- composed of reticular cells
and reticular fibers that form a fine supporting
network extend throughout the remainder of the
organ.

Thymus
The thymus is a bilobed organ located in the
superior mediastinum anterior to the heart and
great vessels.
It consist of :-
1-Cortex , the outer portion of the parenchyma that
contains a high concentration of lymphocytes.
2-Medulla, the inner portion of the parenchyma that
contains a lesser number of lymphocytes.
3-Capsule, the external connective tissue that
extends trabeculae to the margin of the cortex and
medulla.

the thymus contains variable numbers of
plasma cells ,granulocytes, lymphocytes ,
mast cells, fat cells and macrophages.
The capsule and trabeculae also contain
blood vessels, efferent (but not afferent)
lymphatic vessels, and nerves ,the cortex is
basophilic ,in contrast ,the medulla stains
less .
Spleen
The spleen ,which is about the size of
clenched fist, is the largest lymphatic
organ. It is located in the upper left
quadrant of the abdominal cavity and has
a rich blood supply .
The substance of the spleen , other than
the capsule and trabeculae , consists of
splenic pulp . This , in turn, is divided into:
1-white pulp The white pulp appears as circular or
elongated whitish – gray areas surrounded by
the red pulp ,and it consists of lymphatic tissue.
2-red pulp red pulp has a red appearance
because of its content of large numbers of red
blood cells.
The spleen is surrounded by a capsule of dense
connective tissue from which trabeculae extend
into the substance of the organ .
The hilum , located on the medial surface of the
spleen , provides for the passage of the splenic
artery and vein , nerves and lymphatic vessels.

Lymphatic vessels
In addition to blood vessels, there is a set
of vessels that circulates fluid, called lymph,
through certain parts of the body . These
lymphatic vessels are an adjunct to the blood
vessels .
Lymphatic vessels :are unidirectional
conveying fluid only from tissues, the
smallest called
.
1-lymphatic capillaries
they are specially numerous in the loose
connective tissues under the epithelium of
the skin and mucous membrane