Immunotherapy is emerging as a novel and reliable therapeutic technique for treating diseases such as autoimmunity, HIV/AIDS, allergy and cancers. This approach works by modulating the patient's immune system, activating both the innate and humoral branches to combat life-threatening diseases. The foundation of immunotherapy began with the discovery and development of "serum therapy" by German physiologist Emil Von Behring who received the Nobel Prize in 1901 for his contributions to the treatment of diphtheria. Around the same time, Dr. William Coley expanded the field for cancer treatment by developing the first immune based cure for sarcomas using attenuated strains of bacteria injected directly into patient's tumours. As medical science advanced, a broader understanding of the immune system and its components led to the emergence of different immunotherapeutic techniques. These include adoptive cell transfer therapy, cytokine therapy, cancer vaccines, and antibody-drug conjugates. The chapter provides a comprehensive understanding of the history and the current techniques used in immunotherapy, detailing the principles behind their mechanisms and the types of diseases tackled by each immunotherapeutic technique. By examining the journey from early discoveries to modern advancements, the chapter highlights the transformative impact of immunotherapy on medical science and patient care.