Changes

<p>Both T and B lymphocytes develop from a common stem cell ([[Haematopoiesis - Overview#Colony Forming Units|CFU-L's]]) during [[Leukopoiesis#Lymphopoiesis|lymphopoiesis]].</p>

<p>Both T and B lymphocytes develop from a common stem cell ([[Haematopoiesis - Overview#Colony Forming Units|CFU-L's]]) during [[Leukopoiesis#Lymphopoiesis|lymphopoiesis]].</p>

==Lymphocte Surveillance==

==Lymphocyte Surveillance==

<p>About two thirds of lymphocytes (all immunocompetent) are circulating in the blood and lymph systems. Most of these are T cells and long lived. In the lymphatic system they survey tissues. The other third do not circulate and are either short lived or immature, or can be specific cells destined for certain tissues i.e. cells that line connective tissue under the epithelium of the  respiratory, intestinal and urogential systems.</p>

<p>About two thirds of lymphocytes (all immunocompetent) are circulating in the blood and lymph systems. Most of these are T cells and long lived. In the lymphatic system they survey tissues. The other third do not circulate and are either short lived or immature, or can be specific cells destined for certain tissues i.e. cells that line connective tissue under the epithelium of the  respiratory, intestinal and urogential systems.</p>

<p>[[High Endothelial Venules|High endothelial venules (HEV)]] facilitate lymphocyte access to the [[Lymph Nodes - Anatomy & Physiology|lymph nodes]] from the bloodstream. Once inside the lymph node, the naive lymphocytes search for antigens. If there are no antigens present, the naive lymphocytes leave via the efferent lymphatic vessel and return back to the bloodstream. Each lymphocyte can search several [[:Category:Secondary Lymphoid Tissue|secondary lymphoid organs]] each day. This process is called '''surveillance'''.</P>

<p>[[High Endothelial Venules|High endothelial venules (HEV)]] facilitate lymphocyte access to the [[Lymph Nodes - Anatomy & Physiology|lymph nodes]] from the bloodstream. Once inside the lymph node, the naive lymphocytes search for antigens. If there are no antigens present, the naive lymphocytes leave via the efferent lymphatic vessel and return back to the bloodstream. Each lymphocyte can search several [[:Category:Secondary Lymphoid Tissue|secondary lymphoid organs]] each day. This process is called '''surveillance'''.</P>