# Tailored Nanoparticles as Vaccine Components
## 1 Vaccination  
In the current pandemic caused by the novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (**Abbreviations**: a**APC**, artificial APC; **ABC**, amphiphilic block copolymer; **APC**, antigen-presenting cell; **AuNP**, gold nanoparticle; **BCR**, B cell receptor; **CAR**, chimeric antigen receptor; **CTL**, cytotoxic T lymphocyte; **DCs**, dendritic cells; **HA**, hemagglutinin; **HBV**, hepatitis B virus; **HIV**, human immunodeficiency virus; **HPV**, human papillomavirus; **ISCOM**, immunostaining complex; **LPN**, liposome-polyation nanoparticle; **MHC**, major histocompatibility complex; **MSN**, mesoporous silica nanoparticles; **NFA**, nuclear factor of activated T cells; **NP**, nanoparticle; **SARS-CoV-2**, severe acute respiratory syndrome coronavirus 2; **SiNP**, silica nanoparticles; **SVP**, subviral particle; **TCR**, T cell receptor; **Treg**, regulatory (inhibitory) T cell; **VLP**, virus-like particles; **VSP**, variant-specific surface protein; **ZIKV**, ZIKA virus), the need for the development of effective vaccines has again come to the forefront of public attention [1]. Of course, vaccines against infectious diseases or cancer have been a predominant medical concern throughout history and will remain so in the future [2]. This review focuses on the contribution of nanoparticle technology to vaccine development in recent years, providing introductions into both fields for the non-specialized reader. For deeper information, the reader is referred to the considerable body of detailed recent reviews [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. The mammalian immune system includes stationary cells such as macrophages and dendritic cells (DCs) in the skin and the mucosal tissues of the airways and intestines that act as guards wherever pathogens (mainly bacteria, viruses, or fungi) can enter the body [3; 20]. Should this happen, the macrophages trigger an inflammation, which constitutes a so-called 'innate' (rapid but low-specificity) immune response (Figure 1). Such an inflammatory reaction usually requires whole (entire) pathogens, whose typical surface patterns of cell wall molecules are recognized by the innate receptors of macrophages. These then endocytose the pathogens or attack them with reactive oxygen species and other substances, simultaneously secreting cytokines to attract more immune cells. At the same time, DCs also become activated through innate receptors, take up the pathogens by receptor-mediated or fluid-phase endocytosis, and travel to lymph nodes to present processed protein antigens, i.e., fragments of the pathogen proteins, to the T cells of the adaptive immune system. Activated T helper cells then activate macrophages (to kill endocytosed pathogens) or B cells (to secrete antibodies), and activated cytotoxic T lymphocytes (CTL) kill virus-infected or tumor cells. The generation of activated effector B and T cells is called the adaptive immune response [20]. Antibodies are soluble proteins that are produced by activated B cells (the humoral adaptive immune response), and they bind to epitopes (molecular regions) on antigens, which can be proteins, nucleic acids, or lipids of the pathogen [20]. Therefore, antibodies are especially suitable for detecting pathogens in the blood, the extracellular space, and the interior (mucosal) surfaces of the body, such as the lungs or the intestines [20,21]. The binding of antibodies to their epitopes on the surface of pathogens leads to their inactivation, cell death, or phagocytosis by immune cells [20]. Antibodies can only survey the external surface of a pathogen or an infected cell, but not its interior [22]. Activated cytotoxic T lymphocytes (CTL), in contrast, recognize peptide fragments of intracellular proteins presented to them by MHC (major histocompatibility complex) class I proteins (the cellular adaptive immune response).