Lymph nodes dot the network of lymphatic vessels and provide meeting grounds for the immune system cells that defend against invaders.
Pockets of lymphoid tissue are in many other locations throughout the body, such as the bone marrow and thymus. Tonsils, adenoids, Peyer's patches, and the appendix are also lymphoid tissues.
Gut-associated lymphoid tissue (GALT) is found througout the intestine in such places as the lamina propria of the submucosa and in between epithelial tissue cells. Also there is nodule type tissues similar to a lymph node in function called Peyer's patches. Peyer's patches are most evident in the ileum.
The two major classes of lymphocytes are B cells, which grow to maturity in the bone marrow, and T cells, which mature in the thymus, high in the chest behind the breastbone.
In addition to the GALT discussed above, lymph nodes that receive lymph draining from the gut (mesenteric nodes) and Kupffer cells (phagocytic cells in the liver) play important roles in protecting the body against invasion.
Sources:
http://www.vivo.colostate.edu/hbooks/pathphys/digestion/basics/gi_immune.html
http://www.niaid.nih.gov/final/immun/immun.htm
http://www.rkm.com.au/imagelibrary/index.html#medical