2. You are designing a vaccine to protect against infection by an invasive encapsulated bacterial species that establishes infection by living and replicating outside of the body's cells. Encapsulated means that the bacterium has a thick polysaccharide coating around the cell, making it resistant to phagocyte engulfment as well as complement-mediated lysis via the alternative pathway of complement activation. From previous research, you have learned that the most protective immune response to this type of bacteria is the production of antibodies that are specific to (and bind to) the polysaccharide molecules that form the bacterial capsule.
a. What are the immune response mechanisms triggered by these protective antibodies and how do they result in clearance of the bacterial infection?
b. You first decide to try vaccinating with the polysaccharide antigen along with an adjuvant, to evoke specific antibody production. This evoked a short-term immune response in adults, but did not lead to memory B cell formation. Describe the characteristics of the immune response that was elicited in this case.
c. Your next strategy is to conjugate the polysaccharide molecule to a protein component, then inject with the chemically linked compound and an adjuvant. This resulted in high levels of high affinity IgG specific for the bacterial capsule polysaccharide and generated long-term memory B cells. Why was this strategy effective? Describe the key elements of the adaptive immune response that occurred upon this vaccination strategy that gave rise to immunological protection.