Adjuvants can be molecules, compounds or macromolecular complexes that enhance the strength, quality, or duration of specific immune responses to an antigen, but they must cause minimal toxicity. Generally, adjuvants can be classified as immunomodulators or delivery systems, with some components possessing both properties. Adjuvants target cells known as antigen-presenting cells (APCs), which are crucial in both innate and acquired immunity. The beneficial effects of adjuvant formulations have been described for numerous clinical vaccines, but the important chemical and structural properties of individual oil and lipid components of adjuvants are often complex. Adjuvant molecules must be properly designed for maximum effect and stability. An optimally formulated adjuvant is one that is safe, stable (during storage), capable of stimulating an antigen-specific immune response, cost-effective to produce, and easy to use. In recent years, adjuvants have gained attention due to the development of purified, subunit, and synthetic vaccines, which are poor immunogens and require adjuvants to trigger an immune response. By using adjuvants, the immune response can be selectively modulated to major histocompatibility complex (MHC) class I or MHC class II, and to Th1 or Th2 type, which is crucial for protection against diseases caused by intracellular pathogens such as viruses, parasites, and bacteria. The aim of this thesis is to present various types of adjuvants used as additives in vaccines, explain their mechanisms of action, and define their roles.
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