The blood has a major role within the body’s immune system and is responsible for a number of important defence functions. It’s composed of white blood cells or leukocytes. These are also located in the tissue fluid, lymph and body cavities like the alveoli. There are numerous types of leukocytes but they fall into four main categories:

• phagocytes: for phagocytosis
• granulocytes: for inflammation
• T lymphocytes: for cell-mediated immunity
• B lymphocytes: for antibody-mediated immunity

Phagocytes and granulocytes belong in the non-specific immune system. It’s called ‘non-specific’ because the leukocytes use non-specific methods to kill foreign bodies, such as phagocytosis and inflammation.

T lymphocytes and B lymphocytes belong in the specific immune system.

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Vaccines

Vaccines are used to artificially active the immune response and can be created in a number of ways.

• They can contain dead pathogens that activate the body’s immune response but have no negative effect on the body. Examples include salk polio, whooping cough and influenza.
• Some contain the toxin secreted by the pathogen, like tetanus and diphtheria vaccinations.
• Certain vaccines use a weakened or modified form of the pathogen so that it’s still alive but unable to cause harm to the body, like for sabin polio.
• Today, vaccines can also be genetically engineered to produce a purified antigen, for example the Hepatitis B vaccine.

The end result is that memory cells are produced so if the individual were to actually contract the disease the more powerful secondary immune response would kick in, killing the pathogen before it could do any damage. This is known as active immunity.