|MadSci Network: Immunology|
You asked whether lymphocytes in the lymphatic system are included in the WBC count taken from blood. Strictly speaking, your teacher is correct: they are not, because they are not available for sampling in the blood stream. You are also correct in realizing, though, that the lymphatic system and the bloodstream are connected. Let's go through how it works.
The blood leaves the heart through the aorta, and passes through smaller and smaller vessels, the arteries to the arterioles to the capillaries. Capillaries are so small, they only allow the passage of one cell through at a time, and have thin walls. These thin walls and relatively slow transit allow for the exchange of oxygen to the tissues and the removal of waste products. Some of the plasma components "leak" out here, forming interstitial fluid, or "lymph." After the blood exits the capillaries, it moves on to venules, veins, and finally to the vena cava and the heart again. The blood then circuits through the pulmonary circulation to pick up oxygen and release carbon dioxide, and then back to the heart to get pumped throughout the body again.
At the level of venules, white blood cells can exit the bloodstream into the tissues. They do this when they sense a problem in the tissue - inflammation, infection, etc. There are also specialized venules in lymph nodes, called "high endothelial venules," which allow naive lymphocytes to exit the bloodstream into the lymph node to scan for antigen. Here are some slides depicting lymph node architecture: http://cpmcnet.columbia.edu/student/ssn/histology/lymph_slides.pdf#search='lymph%20node%20architecture' and http://medic.med.uth.tmc.edu/edprog/histolog/immuno/hist-09.htm. But how does the antigen get to the lymph node? Through the lymph, either free or carried by specialized antigen presenting cells. All of the interstitial fluid is drained from the tissues into the lymphatic vessels. If there is not proper drainage, edema can develop.
Lymphocytes that migrate into tissues can also migrate into the draining lymph node via the afferent lymphatic. Once the lymph and cells are "filtered" through the lymph node, they are released through the efferent lymphatics on to larger lymphatic vessels and back to the circulation by the thoracic duct. If lymphocytes recognize antigen in the lymph node, however, they will remain there for a period of time.
So, in all, the cells in the lymphatic system are not directly available for sampling from the blood, but the two systems are directly connected via the thoracic duct.
Hope this helps-
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