Secondary lymphoid organs: Secondary lymphoid organs are also known as peripheral lymphoid organs. These are the organs or regions where the mature and immuno competent lymphocytes colonize and function in response to antigenic stimuli.
The mature lymphocytes from primary lymphoid organs migrate to secondary lymphoid regions through blood circulation to give protection from invading foreign agents. The secondary lymphoid organs are the regions, where the immune competent lymphocytes interact with foreign agents and execute immune response.
Production of antibodies from activated B cells, activation of cytotoxic T cells in response to antigens takes place in the secondary lymphoid organs.
The secondary lymphoid organs may be encapsulated to have a specific shape or may be in the form of diffused tissues. Spleen, lymph nodes, adenoids, tonsils are the encapsulated secondary lymphoid organs. Mucosa associated lymphoid tissue distributed throughout the body such as alimentary canal, respiratory and urinogenital tracts etc. are the diffused secondary lymphoid regions.
Efficiency of antigen trapping is same for both capsulated organs and diffused tissue. The secondary lymphoid organs are usually small and poorly developed at the time of birth but unlike primary lymphoid organs they show progressive growth with age. Out of all, lymph nodes and spleen are highly organized structures.