Vaccines are a key way to protect both individuals, communities, and whole populations from diseases. Of course, at the time of writing, this is a very topical subject due to the roll out of the Covid-19 vaccine. In this article we will be looking at how vaccines work to induce immunity, and how pathogens can mutate to avoid being attacked by the immune system.
Vaccines
Most vaccines contain an antigen, always in a harmless form. It could be a dead or attenuated (weakened, unable to replicate or cause disease) form of the pathogen, or just the antigen itself. Sometimes a vaccine can contain a mix of antigens, for example the MMR vaccine contains antigens for the measles, mumps, and rubella viruses. Some of the Covid-19 vaccines are the result of years of new research into mRNA vaccines. In an mRNA vaccine, the vaccine contains messenger RNA (mRNA) which is translated into the protein antigen by the cells of the body. The immune response is then triggered in response to the antigen. Eventually, vaccination results in immunisation (active artificial immunity that was mentioned in the last article) because the body has produced memory cells. Symptoms of the disease were avoided and the individual is now protected against the disease. If you want to know more about how the Covid-19 vaccine works, try reading this article.
Vaccines are normally injected, but some can be taken orally or by inhalation of an aerosol. However, taking a vaccine orally means that it must pass through the digestive system. It could be broken down by digestive enzymes or be too large to be absorbed across the gut epithelium. Booster vaccines are often given after a long period of time to reinforce the immune response and make sure that the memory cells are present in the body.
Although a vaccine only protects the individual who receives it, if large numbers of a population receive it then herd immunity can develop in a population. Herd immunity is where the occurrence of a disease is hugely reduced because it cannot spread amongst a vaccinated community. So even individuals in that population that have not received the vaccine are indirectly protected because they are much less likely to catch the disease from somebody else. This can help prevent epidemics and pandemics, which is something we all want!
Antigenic variation
Unfortunately, pathogens can be sneaky. They are able to change their antigens in order to evade the immune system. This is called antigenic variation. It is an evolutionary advantage for them to do this to ensure their survival in the host.
The influenza virus is a classic example of a virus which frequently mutates and changes to form new strains. Because of this, new flu vaccines must be developed and chosen every year to target the new antigens. Each strain is immunologically distinct (has unique antigens), and memory cells produced to counter one strain will not be effective against a new strain because the antibodies (on B memory cells) and receptors (on T memory cells) will no longer be complementary to the antigen.
The human immunodeficiency virus (HIV) is another virus which can evade the immune system by antigenic variation. In addition, it also infects and kills T helper cells so it compromises the immune system. We will take a detailed look at how HIV infects T helper cells in another article.
There are other ways that pathogens can evade the immune system. For example, the bacteria that cause pulmonary tuberculosis (TB) produce substances that prevent the lysosome fusing with the phagocytic vacuole in phagocytosis. Therefore they are not broken down by the lysozyme enzyme and will survive to replicate.
Summary
- Vaccines cause immunisation against a pathogen through exposure to the antigen.
- Herd immunity can develop in a largely vaccinated community.
- Antigenic variation is one way in which pathogens can evade the immune system as the memory cells no longer recognise the antigen.
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