Natural products are considered an essential source of chemical compounds with relevant biological activities. Important antibiotics, anti-virals, and anticancer agents within healthcare facilities have been isolated from them. In recent decades, the study of alkaloids and the modulation of the immune response has aroused interest. The survey of SAR alkaloids with immunomodulatory effects is of great importance since structural modifications impact the effectiveness of the immunomodulatory activity, thus promoting the rational design of more effective and specific drugs. This review exhibits the main alkaloids with an immunomodulatory effect and their relationship with SAR studies, particularly in activating leukocytes and secretion of molecules with critical inflammatory activities such as cytokines, chemokines, and nitric oxide. These studies optimize the properties of alkaloids and allow a deeper understanding of the mechanisms of action at a molecular level, which is fundamental for the innovation of immunological therapies.