Overview of Lymphoid System

-It consists of capsulated and 

  diffuse lymphoid tissue.

-Lymphoid tissue is usually

  recognized by a concentration

  of lymphocytes along with

  other cell types.

-The thymus and bone marrow

  are called primary(central)

  organs and represent sites of

  lymphocyte programming.

-Other lymphoid tissues such as

  tonsils, lymph nodes, etc. are

  called secondary(peripheral)

  lymphoid structures.

Diffuse Lymphoid Tissue

-Diffuse lymphoid tissue is often found in a mucosa.

-A mucosa is a moist membrane that lines the lumen of a number of internal organs. It consists, in most organs, of an epithelium and a lamina propria. The majority of the diffuse lymphoid tissue is found in the lamina propria.

M Cells

-are specialized antigen processing cells associated with the epithelium

Lymph Nodules

-have germinal centers  and represent site of proliferation of B cells

Lymph Flow

ending lymph capillaries to

afferent lymphatic vessels

that have valves to direct the

flow of lymph toward a lymph

node.

-Lymph percolates through the

node in a series of sinuses

(sinusoids). It then leaves the

node via efferent lymphatic

vessels. Lymphocytes also

leave the node through the

efferent vessels.

-The efferent vessels may go to

other nodes, but eventually

the lymph, containing some

lymphocytes, will be returned to

large veins in the neck through

the thoracic or right lymphatic

duct (not illustrated).

Lymph Node

LYMPH FLOW

-afferent lym.

-subcapsular s.

-cortical s.

-medullary s.

-efferent lym.

CORTEX

Outer cortex-

location of lymph nodules

Paracortex-

concentration of t-cells

MEDULLA-

Medullary cords-

concentrations of plasma cells, macrophages, lymphocytes

Postcapillary,high endothelial, venules(v)

-located in the paracortex. 

-Lymphocytes (L) entering the paracortex

from the high endothelial venule.

(thin section)

Thymus

-The thymus is the organ in which T-cells  differentiate. 

-It has NO afferent lymphatics or lymph nodules

-divided into a cortex and medulla.

Stroma

-support cells, of the thymus consist in large part of epithelial reticular cells. 

blood thymus barrier

-The barrier consists of the continuous endothelial lining of the capillaries in the cortex and a sheath of epithelial reticular cells. 

Hassal’s corpuscles

-peculiar structures in the medulla of thymus that increase with age 

Spleen

-Belongs to lymphatic system and produces lymphocytes

-Destroys old red cells, which in turn provide material for bile or new red cells

- Has Red and White Pulp

Red and White Pulp of Spleen

-The splenic artery gives rise to trabecular arteries which in turn branch to form central arteries (actually small arterioles and most often not centrally located in the white pulp).

-The central arteries are surrounded by a sheath of lymphoid tissue that forms the white pulp.

-The white pulp can be divided into a T-cell rich area, periarteriolar lymphatic sheath (PALS) and a B-cell rich area, the peripheral white pulp (PWP).

-The marginal sinuses lie between the white and red pulp. The white pulp is the area where the spleen first screens the blood for foreign material.

Open VS Closed Circulation of Lymph in Spleen

-The central artery gives rise to several smaller branches some of which have sections ensheathed by macrophages.

-The smallest branches either empty their blood directly into sinusoids (closed circulation) or directly into the area between the sinusoids called splenic cords (open circulation).

-The splenic cords have many macrophages, reticular cells and blood cells. The sinusoids are rather porous and allow blood cells to squeeze in and out.

-If  the blood cell is aged, it can not move through easily and is broken down.

-Macrophages process these red cells and send breakdown products to trabecular veins to the splenic vein then through the portal vein to the liver and then to the general circulation. 

Tonsils

-Tonsils are collections of lymphoid tissue that guard the naso and oro pharynx. They have lymphoid nodules and diffuse lymphatic tissue. They also have crypts or pleats that are extensions from the surface epithelium.

Three Types:

Pharyngeal, Palatine, Lingual