Lymphoid organs are comprised of lymphoid tissue and are the site of lymphocyte activation and lymphocyte production. The structure of lymphoid organs includes primary lymphoid organs, secondary lymphoid organs and tertiary lymphoid organs. Primary lymphoid organs produce lymphocytes, the spleen and lymph nodes, through immature progenitor cells. Secondary lymphoid organs maintain mature lymphocytes, while initiating adaptive immune response. Tertiary lymphoid organs (TLOs) develop in peripheral tissues in adult organisms as a response to non-resolving inflammation. TLOs related diseases include chronic infection, allograft rejection, cancer and autoimmune diseases. They contain an active germinal center which is surrounded by a network of follicular dendritic cells (FDCs). The lymphatic system as a whole provides accessory return route for the blood. Furthermore, immune defense is another significant function of the lymphatic system. This book is a compilation of chapters that discuss the most vital advances in the field of Immunology with respect to lymphoid organs. It will provide comprehensive knowledge to the readers.