Immunity and Vaccines

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Vaccines can Protect Individuals and Populations Against Disease

While your B-cells are busy dividing to build up their numbers to deal with a pathogen (i.e. the primary response), you suffer from the disease. Vaccination can help avoid this.

Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease. This means you become immune without getting any of the symptoms.

Vaccines protect individuals that have them and, because they reduce the occurrence of the disease, those not vaccinated are also less likely to catch the disease (because there are fewer people to catch it from). This is called herd immunity.

Vaccines always contain pathogens - these may be free or attached to a dead or attenuated (weakened) pathogen.

Vaccines may be injected or taken orally. Th disadvantages of taking a vaccine orally are that it could be broken down by enzymes in the gut or the molecules of the vaccine may be too large to be absorbed into the blood.

Sometimes, booster vaccines are given later on (e.g. after several years) to make sure that memory cells are produced.

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Antigenic Variation Helps Some Pathogens Evade the Immune System

Antigens on the surface of pathogens activate the primary response.

When you're infected a second time with the same pathogen (which has the same antigens on its surface) they activate the secondary response and you don't get ill.

However, some pathogens can change their surface antigens. This antigen variability is called antigenic variation. (Different antigens are formed due to changes in the genes of a pathogen).

This means that when you're infected for a second time, the memory cells produced from the first infection will not recognise the different antigens. So the immune system has to start from scratch and carry out a primary response against these new antigens.

This primary response takes time to get rid of the infection, which is why you get ill again.

Antigenic variation also makes it difficult to develop vaccines against some pathogens for the same reason. Examples of pathogens that show antigenic variation include HIV and the influenza virus.

Here's how antigenic variation affects the production of vaccines to prevent people catching influenza:

The influenza (flu) vaccine changes every year. That's because the antigens on the surface of the influenza virus change regularly, forming new strains of the virus.

Memory cells produced from vaccination with one strain of the flu will not recognise other strains with different antigens. The strains are immunologically distinct.

Every year there are different strains of the influenza virus circulating in the population, so a different vaccine has to be made.

New vaccines are developed and one is chosen every year that is the most effective against the recently circulating influenza viruses.

Governments and health authorities then implement a programme of vaccination using the most suitable vaccine.

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Immunity can be Active or Passive

ACTIVE IMMUNITY

This is the type of immunity you get when your immune system makes its own antibodies after being stimulated by an antigen. There are two different types of active immunity:

Natural - this is when you become immune after catching a disease.

Artificial - this is when you become immune after you've been given a vaccination containing a harmless dose of antigen.

PASSIVE IMMUNITY

This is the type of immunity you get from being given antibodies made by a different organism - your immune system doesn't produce any antibodies of its own. Again, there are two types:

Natural - this is when a baby becomes immune due to the antibodies it receives from it's mother, through the placenta and in breast milk.

Artificial - this is when you become immune after being injected with antibodies from someone else. E.g. if you contract tetanus you can be injected with antibodies against the tetanus toxin, collected from blood donations.