Question

Describe the process of lymphocyte trafficking and activation in secondary lymphoid tissues.

Answer 1

Lymphocytes are important immune cells which circulate in the lymphatic system and protects the body from pathogens and foreign antigens. The lymphocytes helps in defending the body against a host of pathogens, microbes and foreign substances. The lymphocytes are able to travel from blood to the lymph and again enter the lymphatic system and hence helps in eliciting the immune response. The lymphocyte trafficking is the recirculation of the lymphocytes between the lymphatic system and blood and hence it enables them to travel across various parts of the body and look out for antigenic substances. The trafficking of the lymphocytes to various parts are controlled by adhesion molecules as well as chemokines. The lymphocytes are produced in the primary lymphoid organs and once they are mature but still, they travel to the secondary lymphoid organs where they are activated. They tend to enter the secondary lymphoid organs through a process called as homing. The lymphocyte homing is the process by which the lymphocytes which are circulating in the blood adhere to specialized endothelial cells which are found in the lymphoid organs. These adhesion molecules which are called homing receptors are found on lymphocytes and through this they adhere to the endothelial cells. When they detect an antigen, these lymphocytes get activated and differentiate into formation of effector cells. However, when they don’t detect any antigen, they tend to leave these secondary lymphoid organs through the efferent lymphatic vessels. The lymph with the lymphocytes is collected within the lymph nodes and it activates the nodes. The lymph after this enters the thoracic duct and enters the circulation again and moves to another secondary lymphoid organ. They continue to recirculate until they find an antigen which activates the lymphocyte. The T-cells travel from blood into lymph node through the high endothelial venules which consists of adhesion molecules and chemokines that enables the homing of the lymphocytes. If they don’t encounter the antigen, the native T-cell leaves lymph node and travel to next lymph node and continue to do so in search of antigen. In case they don’t encounter any antigen, it enters the thoracic duct and returns to blood circulation. However, when it meets an antigen, after the naïve T-cell recognize the antigen, it is activated by the dendritic cells. The activated T-cell now goes back into the blood circulation and moves into the site of the inflammation or infection.