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To understand how vaccines work, we must first learn how our immune system responds to invading pathogens.
When a new pathogen enters the body, it meets with the body’s first-line defense, the innate division of the immune system. The innate response is immediate, but non-specific. If this fails to contain the infection, the adaptive immune system comes into play. The adaptive response is more effective, but it may take many days to activate, during which time the person is being sick. The adaptive response produces the so-called antibodies, which specifically bind to a component on the surface of the pathogen, labeling it for destruction. This component is called an antigen and is the one that has triggered the production of antibodies against it.
Remarkably, the adaptive response also leaves the body with a “memory” of the pathogen, so it can react faster the next time the same pathogen attacks. In fact, pathogen-specific antibodies are produced so fast upon reexposure to the pathogen, that no signs of illness are visible. This is called immunity, and it explains why most people get diseases such as chickenpox only once, even though they may be exposed multiple times in their lifetime.
A vaccine is basically an altered form of a pathogen, or part of it that acts like an antigen. It is introduced to the body to trigger production of antibodies, mimicking the first infection, but without causing the illness. The immune system now has the antibodies, and is ready for a fast response whenever it is exposed to the real pathogen.
When enough people in a community are vaccinated, the whole community, including the individuals that were not vaccinated, is protected against the disease. This phenomenon is known as herd immunity. Herd immunity is possible because a pathogen cannot spread without a sufficient number of vulnerable hosts.
There are several types of vaccines:
- Live, attenuated vaccines are live pathogens that have been weakened so they don’t cause disease in people with healthy immune systems. Being the closest thing to a natural infection, they are most effective and can provide life-long immunity with a single dose. However, weakened pathogens can still be strong enough to cause illness in people with compromised immune systems, and therefore cannot be used for this group of people.
- Inactivated vaccines are pathogens that have been completely inactivated by heat or chemicals. They are safer than attenuated vaccines but less effective, and multiple doses may be required to achieve and maintain immunity.
- Subunit vaccines use only part of a pathogen, usually a peptide. These vaccines are very safe as they cannot cause disease, but to make such a vaccine, scientists must first identify the part of the pathogen that can elicit a good immune response, and this can be a difficult task.
- Toxoid vaccines: Some bacteria cause illness by releasing toxins. These toxins are inactivated and used as vaccines. Inactivated toxins do not cause disease, but can induce production of antibodies against the natural toxins.
- Conjugate vaccines: Some bacteria have a protective coat that helps them evade the immune system. This is because the coat is a weak antigen, it does not provoke a strong production of antibodies. Vaccines based on weak antigens will not protect the person effectively. To overcome this problem, the weak antigen is combined with a strong antigen from another source as a carrier, in a conjugate vaccine, to boost the immune response.

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