The development of several types of immunotherapies, such as vaccinations (both preventative and therapeutic), pathogens, monoclonal antibodies, recombinant proteins, cytokines, and cellular immunotherapies, is transforming how we approach many diseases, including cancer. The immune system is a complex universe of interconnected cells, soluble factors, interacting cells, and tissues. The immune system's complexity makes it difficult to see it as a whole, but researchers are now attempting to piece together all the puzzle pieces to gain a full picture. We are beginning to comprehend the various types of cell subsets, soluble factors, membrane molecules, and cell functions, as well as their involvement in health, ageing, and illness. Many gaps are being filled, and in some cases, previous assumptions have been challenged. For example, adaptive immune cells were previously thought to be unique memory cells until trained innate immunity was discovered, and several innate immune cells with features similar to those of cytokine-secreting T cells have been discovered.