If a pathogen gets past the ❌ defences and into our bodies, then the ❌ system starts to work. This system attacks ❌ by using several different types of ❌ blood cell, made in the bone marrow. Some white blood cells ❌ microorganisms and digest them, while another type of white blood cell produces ❌ that recognise and destroy particular microorganisms - a more targeted response.
Pathogens have proteins, called ❌, in their cell membranes that our immune systems recognise as ❌. ❌ antibodies are made for each antigen by the white blood cells. Antibodies attach to the antigens on the surface of the ❌. This inactivates the microbes and makes them ❌ together, allowing them to be easily engulfed by other white blood cells.

Bacteria and ❌ may develop resistance to antimicrobials. ❌ microorganisms are not easily killed by antimicrobial substances. The normal population of microorganisms contains ❌ that are resistant , however when we use the antimicrobial all those ❌ will be destroyed, leaving the resistant ones. These then ❌, resulting in a more resistant ❌. the more the antimicrobial is applied, the more resistant the remaining population is. These extremely resistant microbes are known as '❌'.

An important part of the ❌ control system is the ❌ of changes that have put the body system off balance. This reversal helps the body back to the ❌ state it should be in. It is called ❌ feedback.
The body's response to temperature changes is a good example of negative feedback. If the body temperature rises above 37 degrees celsius, the ❌ detect this and send a message to the processing centre in the brain. The processing centre then directs the ❌, in this case the blood vessels, to ❌ so that heat can be lost through the skin and the body temperature drops. Once the temperature has dropped back to normal, the receptor would detect this and pass the message to the ❌ centre, which would direct the effectors to return to their normal ❌.