Veterinary Immunology and Serology

LYMPHOCYTE CIRCULATION

• The predominant lymphocytes in blood are T cells.
• They leave the blood by two routes.
• Those which have not encountered antigen previously bind to venules in the paracortex of lymph nodes.
• These are called high endothelial venules (HEV) because they possess tall rounded endothelial cells unlike the flattened ones found in other blood vessels.
• They are joined by discontinuous junction which facilitate passage of lymphocytes between them.
• HEVs are not normally found in sheep lymph nodes.
• In contrast to naïve T cells, memory T cells leave the blood via conventional blood vessels in tissues and are them carried to lymph nodes by afferent lymph.
• Ninety per cent of the lymphocytes leaving a node are derived form cells entering through HEVs, whereas 10% enter by way of afferent lymph.
• The lymph leaves lymphnode by way of efferent lymphatics which join together into large lymph vessels.
• The largest lymph vessel is thoracic duct. It collects the lymph from lower body and intestine and empties in to the anterior vena cava.
• If the thoracic duct is cannulated and the lymph removed, blood lymphocytes (mainly T cells) drop significantly.
• T cells also disappear from the paracortex of lymph nodes. This implies that thoracic duct lymphocytes normally circulate back to lymph node through the blood.
• In pigs, the circulating lymphocytes enter the lymph node through HEVs. But they leave the lymph node not through the lymphatics but migrate back in to the blood stream through the HEVs of the paracortex. Hence, very few lymphocytes are found in pig lymph.