The Veterinary Microscopic Anatomy Review series is a 21-volume set of interactive books to allow a Q&A type review of microscopic anatomy structure and function. The histological sections are taken from multiple domestic species, including the canine, feline, bovine and equine. In this unit of the series, the primary lymphatic organs (chiefly the thymus and cloacal bursa), lymphatic tissues (tonsils, Peyer’s patches), and secondary lymphatic organs (lymph nodes, spleen) are examined. In addition to B & T lymphocytes, other cells present and involved in the lymphatic structures are observed. The circulation of lymph or blood through these tissues/organs and the functions of the cells, tissues and organs in the immune system are reviewed.