Clinical correlate - Thoracic duct: The thoracic duct may be ruptured during surgery in the neck or thoracic area. It will present as a fistula leaking chylous, milky white fluid. Chylothorax, or accumulation of chyle in the pleural space, can be seen after penetrating chest injuries or following thoracic surgery, lymphomas, cirrhosis, filariasis, sarcoidosis, tuberculosis, and amyloidosis, and rarely congenital causes. It clinically presents as a pleural effusion.
Lymphocyte Homing: Think of it as “coming home”. T cells and B cells reach the lymph nodes after activation by pathogens. The chemokines CCL19 and CCL21 are essential in the transmigration of lymphocytes across high endothelial venules (HEVs). HEVs are highly specialized postcapillary venules within the lymph nodes and Peyer’s patches. CCL21 is constitutively expressed within the lumen of HEVs, whereas CCL19 is produced by other cells within the lymph node. CCL19 and 21 are ligands for chemokine receptor CCR7. CCR7 is expressed by HEVs, T cells, B cells, and dendritic cells.
Splenomegaly causes: The most common causes of splenomegaly include:
Liver disease (cirrhosis, hepatitis)
Acute or chronic infection (bacterial endocarditis, infectious mononucleosis, HIV, malaria, tuberculosis, histiocytosis)
Hematologic malignancy (lymphomas, leukemias, myeloproliferative disorders)
Congestion (splenic vein thrombosis, portal hypertension, congestive heart failure)
Inflammation (sarcoidosis, lupus, rheumatoid arthritis, systemic lupus)
Splenic sequestration (pediatric sickle cell, hemolytic anemias, thalassemias)
Other (metastases, abscess, trauma, hemangiomas, rare drug reactions (RhoGam), cysts, Gaucher Disease, Niemann-Pick disease)
Differences between chyle and lymph: Chyle is a milky opalescent fluid containing chylomicrons and very low density lipoproteins (formed from dietary long-chain triglycerides) and cells. Chyle is rich in protein, fat, and white blood cells. Prolonged high-volume loss of chyle leads to nutritional failure and immunosuppression. Lymph is similar to plasma and interstitial fluid, consisting of proteins and immune cells. Lymph from the GI tract is chyle as it is high in fats absorbed from foods.
M cells: Poliovirus, Reoviruses (think Rotavirus), EPEC E.coli, CXCR4 tropic strains of HIV, Shigella flexneri, Salmonella typhimurium, Yersinia pseudotuberculosis and Prions bind to M cells and penetrate the intestinal epithelium in this way.
MALToma: It is a lymphoma, also called marginal zone B cell lymphoma, arising from the MALT tissues of the stomach (most common site). It is of B cell origin and has been associated with chronic gastritis caused by H pylori. They are CD5+ and Cyclin D1+. In the early stages, treatment of H.pylori infection leads to complete resolution of the lymphoma. Other associations for MALTomas are - Chlamydia psittaci and ocular adnexal MALT lymphoma, Hepatitis C virus and salivary gland MALT lymphoma, and C. jejuni and immunoproliferative small intestinal disease.