<P> Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes . It is exemplified by the lymph nodes, and the lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin, etc. that are associated with the mucosa - associated lymphoid tissue (MALT). </P> <P> In the gastrointestinal wall the appendix has mucosa resembling that of the colon, but here it is heavily infiltrated with lymphocytes . </P> <P> The tertiary lymphoid tissue typically contains far fewer lymphocytes, and assumes an immune role only when challenged with antigens that result in inflammation . It achieves this by importing the lymphocytes from blood and lymph .) </P> <P> The thymus is a primary lymphoid organ and the site of maturation for T cells, the lymphocytes of the adaptive immune system . The thymus increases in size from birth in response to postnatal antigen stimulation, then to puberty and regresses thereafter . The loss or lack of the thymus results in severe immunodeficiency and subsequent high susceptibility to infection . In most species, the thymus consists of lobules divided by septa which are made up of epithelium and is therefore an epithelial organ . T cells mature from thymocytes, proliferate and undergo selection process in the thymic cortex before entering the medulla to interact with epithelial cells . </P>

Lymph transports fats absorbed from the small intestine to the vena cava