Subunit, conjugate, and synthetic vaccines are three of several different types of vaccines. Each are derived differently, but they all are designed to stimulate an immune response to better protect oneself from various pathogens.

A subunit vaccine is a type of vaccine that is made from the pathogenic subunit that best triggers an immune response, such as a protein or a polysaccharide. Once scientists identify the subunit (a potentially lengthy process), they use different technologies to isolate the subunit. For example, scientists may grow the microbe and chemically break it apart to obtain the important subunit, or they may manufacture the antigen molecules using recombinant DNA technology. The chances of adverse reactions to the vaccine are lowered, because the entire pathogen is not used in eliciting an immune response. Some examples of subunit vaccines include vaccines against Hepatitis B, diphtheria, and tetanus.

Some pathogens contain a polysaccharide coating that may mask their antigens. In this case, a conjugate vaccine may be used, as a conjugate vaccine contains this polysaccharide, though crucially it is chemically linked to a protein. With this link, the polysaccharide can be recognized by B cells, and the MHC molecule can be displayed with the peptide to an already differentiated T cell. An example of a conjugated vaccine is the Haemophilus influenzae B (Hib) vaccine.

In a synthetic vaccine, the specific peptide sequences triggering a protective immune response are identified, and completely synthetic versions of these components are produced. Synthetic vaccines are thought to have a decreased risk of mutations or reversion, and the speed of production could be increased with these vaccines. However, the production of such vaccines has its difficulties, as the antigenic epitope is often not as simple as a sequence of amino acids. An example of a synthetically produced vaccine is a vaccine for the hepatitis virus (using recombinant proteins).

References: 

-Resource 1: Immunization Basics PowerPoint

-Resource 4: Principles of Vaccination Document from the CDC

-http://www.vaccines.gov/more_info/types/#subunit

-http://www.who.int/biologicals/vaccines/synthetic_peptide_vaccines/en/

-Abbas & Lichtman: Basic Immunology, 5th Edition

Subunit, Conjugate, and Synthetic Vaccine examples (among others)