Immunology | Immune system| Immunity

         Lymph nodes are the sites where immune responses are mounted to antigens in lymph.They are encapsulated bean-shaped structures containing a reticular packed with lymphocytes,macrophages,and dendritic cells.Clustered at junctions of the lymphatic vessels,lymph nodes are the first organized lymphoid structure to encounter antigens that enter the tissue spaces.

Morphologically, node can be divided into three roughly concentric regions:the cortex,the paracortex and the medulla,each of which supports a distinct microenvironment. The outermost layer,the cortex,contains lymphocytes(mostly B cells),macropahges,and follicular dendritic cells arranged in primary follicles.After antigenic challenge,the primary follicles enlarge into secondary follicles ,each containing a germinal center.In children with B-cell deficiencies,the cortex lacks primary follicles and germinal centers.Beneath the cortex is the paracortex,which is populated largely by T lymphocytes and also contains interdigitating dendritic cells thought to have migrated from tissues to the node.These interdigitating dendritic cells express high levels of class II MHC molecules,which are necessary for presenting antigen. Lymph nodes taken from neonatally thymectomized mice have unusually few cells in the paracortical region; the paracortex is therefore sometimes referred to as thymus-independent area in contrast to the cortex,which is a thymus-independent area.The innermost layer of a lymph node,the medulla,is more sparsely populated with lymphoid-lineage cells;of those present,many are plasma cells actively secreting antibody molecules.

FUNCTIONS OF LYMPH NODES :
         This work suggests an explanation for the curious fact that patients receiving a liver transplant sometimes inherit the donor’s allergies and immune repertoire, so in keeping with the idea that donor immune information is being transplanted. It also suggests that the liver as an immune organ is an evolutionary remnant from the time before lymph nodes developed in higher birds and mammals. Cold-blooded vertebrates have functioning T and B cells but no lymph nodes. The main achievement of the development of lymph nodes in mammals is a drastic improvement for the production of better antibodies. T cells on the other hand have not changed their function much during evolution and the work by the Zurich group finally provides solid evidence for the versatility and promiscuity of this cell type.