B2.1 How do our bodies resist infection?

B2.1 How do our bodies resist infection?

MICROORGANISMS are organisms that are too small to see with the naked eye. They include BACTERIA, VIRUSES and FUNGI. They can be beneficial to us (e.g. the bacteria that live in our intestines can produce certain vitamins) or they can cause us harm (e.g. bacteria that cause food poisoning).

PATHOGENS are microorganisms that cause infectious diseases. BACTERIA and VIRUSES are the main PATHOGENS.

Symptoms of an infectious disease are caused by damage done to cells by microorganisms or the poisons (TOXINS) they produce.

In the correct conditions (with warmth, moisture, nutrients) bacteria can multiply rapidly. The human body provides ideal conditions for microorganisms to grow. In the body, there is water, oxygen, food and heat, as well as different pH levels.


The form of growth is known as EXPONENTIAL GROWTH. It follows the formula:

When microorganisms enter the body, they release toxins. The toxins damage cells to cause the symptoms of the disease. The body’s first line of defence is its NATURAL BARRIERS which include:

  • Skin
  • Chemicals in tears
  • Chemicals in sweat
  • Stomach acid

The body’s first line of defence is called PASSIVE IMMUNITY, which means preventing the PATHOGEN from entering in the first place. If a pathogen manages to get into the body, the second line of defence takes over which is called ACTIVE IMMUNITY. The WHITE BLOOD CELLS have key functions in this. White blood cells are part of the body’s immune system and can:

  • Destroy pathogens by engulfing and digesting them
  • Produce antibodies to destroy pathogens
  • Produce antitoxins to neutralise the toxins released by the microbe

Each microorganism has its own markers made out of protein on its surface – these markers are called ANTIGENS. ANTIBODIES recognise microorganisms by the ANTIGENS that they carry on their surface therefore a different ANTIBODY is needed to recognise each different type of microorganism.

MEMORY CELLS are a type of white blood cell that can respond quickly when it meets a microorganism for the second time. They produce the right antibody very fast for the particular microorganism and destroy it before you feel unwell. This is described as being IMMUNE to a disease.