The immunostimulatory nature of mRNA lipid nanoparticles (LNPs) is a critical factor in their clinical success and safety. mRNA-LNPs have revolutionized medicine, particularly with the development of vaccines against COVID-19. However, their immunostimulatory properties can lead to adverse effects, necessitating a thorough understanding of their mechanisms. This review discusses the structural modifications of synthetic mRNA, the role of ionizable lipids in LNP functionality, and the challenges in advancing mRNA-LNP technology. mRNA, first discovered in the 1960s, has been modified to enhance its stability, translational efficiency, and reduce immunogenicity. Key modifications include the 5' cap, poly(A) tail, and untranslated regions (UTRs). These modifications are crucial for mRNA stability and function. However, unmodified mRNA can trigger immune responses, leading to inflammation and adverse effects. To mitigate this, modified nucleosides such as pseudouridines are used to reduce immunogenicity. Ionizable lipids are essential for LNP functionality, enabling efficient mRNA delivery and endosomal escape. These lipids have a structure–activity relationship that influences their biodegradability, immunogenicity, and delivery efficiency. The head group, linker, and tail of ionizable lipids play critical roles in determining their properties. For example, the pKa of ionizable lipids affects their ionization state and interaction with the endosomal membrane. Cholesterol, phospholipids, and PEG lipids also contribute to LNP stability and biodistribution. The immunostimulatory effects of LNPs are a double-edged sword, beneficial for vaccination but potentially harmful in other applications. Understanding these effects is crucial for the successful translation of mRNA-based therapies. Challenges include optimizing lipid formulations to balance immunogenicity and therapeutic efficacy, as well as ensuring safe and efficient delivery. Clinical trials have shown that mRNA-LNPs can be effective, but further research is needed to address safety concerns and improve delivery efficiency. The role of LNPs in vaccine immunogenicity is significant, highlighting the need for continued research to enhance their therapeutic potential.The immunostimulatory nature of mRNA lipid nanoparticles (LNPs) is a critical factor in their clinical success and safety. mRNA-LNPs have revolutionized medicine, particularly with the development of vaccines against COVID-19. However, their immunostimulatory properties can lead to adverse effects, necessitating a thorough understanding of their mechanisms. This review discusses the structural modifications of synthetic mRNA, the role of ionizable lipids in LNP functionality, and the challenges in advancing mRNA-LNP technology. mRNA, first discovered in the 1960s, has been modified to enhance its stability, translational efficiency, and reduce immunogenicity. Key modifications include the 5' cap, poly(A) tail, and untranslated regions (UTRs). These modifications are crucial for mRNA stability and function. However, unmodified mRNA can trigger immune responses, leading to inflammation and adverse effects. To mitigate this, modified nucleosides such as pseudouridines are used to reduce immunogenicity. Ionizable lipids are essential for LNP functionality, enabling efficient mRNA delivery and endosomal escape. These lipids have a structure–activity relationship that influences their biodegradability, immunogenicity, and delivery efficiency. The head group, linker, and tail of ionizable lipids play critical roles in determining their properties. For example, the pKa of ionizable lipids affects their ionization state and interaction with the endosomal membrane. Cholesterol, phospholipids, and PEG lipids also contribute to LNP stability and biodistribution. The immunostimulatory effects of LNPs are a double-edged sword, beneficial for vaccination but potentially harmful in other applications. Understanding these effects is crucial for the successful translation of mRNA-based therapies. Challenges include optimizing lipid formulations to balance immunogenicity and therapeutic efficacy, as well as ensuring safe and efficient delivery. Clinical trials have shown that mRNA-LNPs can be effective, but further research is needed to address safety concerns and improve delivery efficiency. The role of LNPs in vaccine immunogenicity is significant, highlighting the need for continued research to enhance their therapeutic potential.