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The main function of lymph nodes is to allow the interaction of antigen, antigen-presenting cells
and lymphoid cells in the generation of an immune response.
Different types of stimulus generate different patterns of response in lymph nodes, identification of which may be helpful in diagnosing the cause.
The most common reason for patients to develop enlargement of lymph nodes is as a reaction to antigenic stimuli (reactive lymphadenopathy).

There are five main patterns of reactive response:
Follicular hyperplasia: increase in B-cell germinal centres.
Paracortical hyperplasia: increase in T-cell paracortical region.
Sinus hyperplasia: increase in histiocytic cells in medullary sinuses.
Granulomatous inflammation: formation of histiocytic granu-lomas in nodes.
Acute lymphadenitis: acute inflammation and suppuration in lymph nodes.

Follicular hyperplasia shows increase in size and number of germinal centres
Follicular hyperplasia is a common response to most types of antigen exposure.
associated with nodes draining sites of chronic inflammation, and in rheumatoid disease.
Acute lymphadenitis occurs with acute bacterial infections.

Paracortical hyperplasia shows increase in size of T-cell areas
Expansion of the T-cell paracortical zone is associated with follicular hyperplasia
as part of a reaction to chronic inflammation.
Relatively pure expansion of the paracortex is seen in drug hypersensitivity
reactions (e.g. lymphadenopathy with phenytoin treatment) and viral infections.

In sinus hyperplasia histiocytic cells accumulate in medullary sinuses and may store phagocytosed material
e.g. In many non-specific reactions to chronic inflammation, and in nodes draining tumours.
A particular type of lymph node enlargement is seen in nodes draining
inflamed skin. In these cases the medullary region is expanded by
histiocytic cells that often contain melanin pigment and lipid
(dermatopathic lymphadenopathy).
Patients with HIV infection may develop lymphadenopathy at several occaions.

Granulomatous inflammation in nodes may be seen in several diseases Granulomatous inflammation in lymph nodes can present with generalized or localized lymphadenopathy; lymph node biopsy is performed to establish the cause.
The main causes are:

Tuberculosis. Granulomas typically undergo caseous necrosis. TB must be confirmed by culture of biopsy
tissue in all cases of undiagnosed lymphadenopathy.

Sarcoidosis may affect regional nodes or be part of a generalized lymphadenopathy.

Cat scratch disease is caused by a Gram-negative bacterium transmitted by cat scratches.
It leads to a self-limited febrile illness with localized lymphadenopathy

Crohn's disease. Granulomas are a frequent finding in the enlarged nodes draining the bowel in Crohn's

Toxoplasmosis. Numerous small aggregates of histiocytic cells forming 'mini granulomas' are seen within
enlarged nodes.
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Revised: 02-11-2014.