# Tailored Nanoparticles as Vaccine Components
## 1 Vaccination
Such MHC class I epitope peptides can originate from any protein that is synthesized in the cell, including nuclear or cytoplasmic proteins, and so CTL are especially suitable for detecting pathogen-infected or tumor cells [4]. Recognition by a CTL leads to the death of the presenting cell by induced apoptosis, which limits the spread of the virus or the propagation of the tumor. CTL responses against tumor cells are in principle similar to those against virus-infected cells, but inefficient initiation of the response (in the absence of inflammation) and the immunosuppressive microenvironment of the tumor (which employs suppressive signaling or mechanical barriers) imposes additional barriers. After clearing the pathogens or tumor cells, T and B cells undergo programmed cell death, leaving the immune system with a small number of antigen-specific memory cells that can be rapidly reactivated and expanded if the same pathogen is encountered again. The purpose of a vaccine is, therefore, to present the antigens to the adaptive immune system such that it will generate memory B and T cells that warrant a rapid immune response [3,20,23]. For this, the vaccine formulation must contain the right antigen; for example, if the purpose of the vaccine is to generate antibodies to the surface proteins of a virus, then these surface proteins, or their most exposed parts that are most likely to be the epitope of antibodies, are the antigenic epitopes to be used in the vaccine. These antigens  
Figure 1: The course of an antiviral or antibacterial immune response is simplified. The pathogen enters the body and is recognized by the innate immune cells through its specific molecular patterns, the cytokines then triggering an inflammation. This inflammation attracts more immune cells, with the pathogen being taken up by dendritic cells and moved to the lymph nodes. The dendritic cells then present antigens that activate effector T cells and memory T cells. The effector T cells either kill the infected cells or stimulate effector B cells to make antibodies, with the effector cells dying after the pathogen is eliminated. In contrast, memory T and B cells remain in reserve to be used in the future encounter of the same pathogen. (or indeed their genetic information, as in a nucleic acid vaccine) must be contained in a vaccine in a physical form that is effectively taken up, processed, and presented by the DCs [24; 25]. The vaccine should also contain an adjuvant that causes mild inflammation in order to activate the DCs in the first place; such adjuvants are mineral salts, saponins, emulsions, polymers, cytokines, liposomes, or nanoparticles (see below) [4; 26]. Sometimes, the antigen itself can function as an adjuvant [3; 27]. The vaccine may be formulated to ensure a slow and sustained release of the antigen since a strong adaptive response might take days to weeks to develop [27]. Many successful vaccines have been designed, especially against pathogens that cause acute infections such as tetanus, polio, smallpox, and measles [3; 25]; in contrast, there are no effective vaccines yet for diseases that cause chronic infections such as HIV [25]. Another focus of development is therapeutic vaccines for cancer or autoimmune diseases and allergies [25]. In the past, the development of vaccines that contained the actual pathogen often took many years; for therapy or to rapidly deal with newly emerging pathogens, faster development cycles and universally usable technical platforms are highly desirable, and with synthetic vaccine formulations, these aims can be achieved, as the many vaccines against SARS-CoV-2 demonstrate [1]; in this endeavor, artificial nanoparticles play a central role.