Subunit, conjugate, and synthetic vaccines are types of vaccines that share similar concepts. Rather than use an entire microbe, either live or inactivated, these vaccines instead use a particle that the immune system can recognize to mount a defense against the particle's associated infection^[1]. This approach has a number of advantages, such as a lower risk of adverse reaction to the vaccine and promoting an immune response against an ideal epitope.

Subunit Vaccine: Subunit vaccines use a purified antigen to elicit an immune response^[2]. The whole antigen may be used in this case, or it could be the epitope alone. Subunit vaccines are particularly useful for infections that have a toxin component (i.e. diphtheria and tetanus). In these cases, a toxoid, or inactivated toxin, can be used to mediate protection^[1].

Conjugate vaccine: Conjugate vaccines are made by linking polysaccharides to antigens^[1]. This provides something that T cells can recognize, which is important for B cell maturation^[3]. The result of this is a stronger response from B cells.

Synthetic vaccine: Synthetic vaccines use an artificially synthesized peptide. This could potentially have a variety of uses and advantages. In the case of a quickly mutating virus like HIV, a vaccine that targets a highly conserved portion of viral protein would have clear advantages, and this is an approach currently being used to create such a vaccine^[4].