Immunity and Vaccination

Once your body has fought off a pathogen, it does a pretty clever thing and makes something called memory cells which ‘remember’ the pathogen if it reinfects you in the future. If this happens antibodies can be generated quickly and in high numbers so that your immune system kills the pathogen before you can develop symptoms - this is called the secondary immune response.

 
 

Primary and Secondary Immune Response

The primary immune response occurs when you are infected with a pathogen for the first time. It is composed of the non-specific and the specific immune response. This process is slow because it takes time for the correct B cell to be activated (clonal selection) and divide into lots of plasma cells (clonal expansion) in order to produce antibodies with a complementary shape to the antigen. The infected person will experience symptoms while the T and B cells mount an immune response. As part of the primary response, both T and B cells produce memory cells.

If you are re-infected with the same pathogen in the future, the T and B memory cells will recognise the antigen and start dividing. T memory cells will divide into the correct type of T killer cell to kill any cells that are infected with the pathogen. B memory cells will divide into plasma cells to produce a large number of antibodies which is complementary to the antigen molecules on the pathogen. This is the secondary immune response and is much quicker than the primary response. It happens so quickly that the pathogen is suppressed before you are able to experience symptoms - you are immune to the pathogen.

 
 

Active vs Passive Immunity

Active immunity occurs when your body makes its own antibodies after being stimulated by an antigen. This process can be natural (e.g. if you catch a cold, your body will make its own antibodies against the common cold virus) or it can be artificial (e.g. if you are vaccinated with a harmless form of an antigen).

Passive immunity occurs when you are given ready-made antibodies that have been produced by another organism. This process can be natural (e.g. babies receive antibodies from their mother when they breastfeed) or artificial (e.g. antibody injections can be given for some diseases - e.g. this is being tested as a treatment for Ebola).

 
 

Active immunity lasts longer since memory cells will remain in the body for years but it takes time to develop. Passive immunity provides immediate protection against pathogens but is short-lived (antibodies that are transferred from mother to child during breastfeeding only remain in the body for about 18 months).

Vaccines

Vaccines contain a dead or weakened form of a pathogen which is injected into the bloodstream. Sometimes vaccines contain just the antigen proteins without the rest of the pathogen. Other vaccines might contain a variety of different antigens to protect against different strains of pathogens which vary in the shape of the proteins on their cell surface membranes (antigenic variation). The injection of weakened antigens stimulates a primary immune response and the production of memory T cells and memory B cells, resulting in immunity without becoming ill from the disease.

Evasion mechanisms

Our immune systems have evolved to increase our defence against pathogens. As this has happened, pathogens have also evolved to become better at infecting us. This is known as an evolutionary race. For example, the HIV virus has evolved certain evasion mechanisms to escape detection by our immune system:

  1. HIV has a high rate of mutation in the genes which code for its antigens. This means that different strains of HIV will show slight differences in the shape of their antigens (antigenic variation). Memory cells that have formed from one HIV strain therefore may not detect the presence of antigens from other HIV strains.

  2. HIV replicates inside T lymphocytes and destroy the cells when they burst out of them. This disrupts the person’s immune response because their T cell levels will be too low to produce an effective immune response against the virus.

  3. HIV disrupts antigen presentation in host cells which prevents the activation of T helper cells.

The bacterium responsible for TB, Mycobacterium tuberculosis also has ways in which it evades our immune system:

  1. When M. tuberculosis gets inside our lungs, it is engulfed by phagocytes as part of the non-specific immune response. Phagocytes usually kill pathogens by digesting them but the M. tuberculosis bacteria is able to stop this by preventing the phagosome from fusing with the lysosome. The bacteria is able to survive and replicate inside the phagocyte.

  2. M. tuberculosis disrupts antigen presentation in host cells which prevents the activation of T helper cells.

Did you know...

The first ever vaccine was developed for smallpox. It involved injecting patients with a similar virus which produces cowpox in cows and was so successful that it helped us to eradicate smallpox in 1980. The name ‘vaccine’ comes from the word vacca which means cow in Latin. The name has stuck, even though modern vaccines do not use cow viruses.