Lymph node histology

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Lymph node histology

USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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 Two biopsies were taken from the lymph nodes of two different children to examine the changes in the follicles present in the outer cortex. The first specimen (Specimen A) and second specimen (Specimen B) are shown below. Which of the following is true regarding the difference in appearance between the follicles in specimen A and specimen B?  
 Specimen A 
 Specimen B 


The lymphatic system is an essential part of the immune system and it consists of a network of lymphatic vessels, tissues, and organs.

The lymphatic vessels drain interstitial fluid or lymph from peripheral tissues back into the blood.

Lymphoid tissue and organs contain a lot of lymphocytes and other white blood cells.

The primary lymphoid organs include the thymus and bone marrow.

And the secondary lymphoid organs include the tonsils, lymph nodes, spleen and mucosa-associated lymphoid tissue or MALT for short.

Lymph nodes are small secondary lymphoid organs that are found along lymphatic vessels throughout the body.

They’re encapsulated, bean-shaped structures that usually have a diameter of about 1 cm along the short axis and 2.5 cm along the long axis.

And they support the immune system by filtering the lymph, in order to identify and fight infections.

If we zoom closer, we can more easily identify the outer capsule of connective tissue, as well as the three functional regions of the lymph node.

Just beneath the capsule is the outer cortex, which contains spherical nodules or follicles of B cells, each with a germinal center, similar to the follicles of the spleen.

The germinal center is where B cells differentiate into plasma cells.

The next region is the inner cortex or paracortex, which doesn’t have any nodules.

And finally, the innermost region of the lymph node is the medulla.

The distinction between the inner cortex and medulla is hard to see at this magnification, but the medulla will have cords of lymphoid tissue, as well as passageways for lymph called medullary sinuses.

These sinuses eventually join one another and drain into the efferent lymphatic vessels.


Lymph nodes are important for the body's immune system. They are small, bean-shaped, encapsulated organs that are found throughout the body, most often in the neck, groin, and underarms. Lymph nodes are made up of clusters of lymphocytes and other white blood cells that help fight infection.


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