Although subunit vaccines promote T-independent responses in adults, they are ineffective in children under 18 months old and the elderly. Both of these populations are vulnerable to infection by encapsulated bacteria, therefore a viable alternative vaccine is required. 

The Negative Characteristics of Subunit Vaccines

Subunit vaccines are composed of capsular polysaccharides which promote the development of complement-fixing antibodies that bind to the capsule and break down the bacteria. 



Tulane University

Bacterial vaccines are made from whole bacteria, their secreted toxins, or capsular polysaccharides.

- Live-attenuated bacterial vaccine

- Vaccines that generate toxin-specific neutralizing antibodies

- Subunit/Conjugate vaccines

Types of Bacterial Vaccines

Parham, P. (2009). Vaccination to Prevent Infectious Disease. In 1145902806 862124677 J. Foltin (Ed.), The Immune System (3rd ed., pp. 439-440). New York, New York: Garland Science, Taylor &amp; Francis Group.

The Immune System

Many bacteria are pathogenic because they produce toxins that cause disease by altering the normal functions of cells. In order to affect the human body, the bacterial toxin must bind to an extracellular receptor of a specific cell. Vaccines stimulate the generation of antibodies that bind to this receptor-binding polypeptide to neutralize a toxin. 



Toxin-Specific Neutralizing Antibodies

Subunit Vaccines

Similar to live-attenuated viral vaccines, these bacterial vaccines are more effective at eliciting protective immunity because the attenuated bacteria can continue to replicate in order to mimic a real bacterial infection. Although there is still ongoing research to produce new live-attenuated bacterial vaccines, the number remains relatively small. 

Ex:

- Bacille Calmette-Guérin (BCG) Vaccine

- Salmonella typhi Vaccine

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