At the school playground, Amelia plays with her friends.

Let's take a closer look at what is going on!

Our skin forms a barrier to help keep pathogens out, an important part of the first line of defense. This type of defense is non-specific, meaning that it works to protect against all pathogens/non-self cells. As Amelia goes about he daily life, her skin barrier is hard at work keeping unwanted pathogens out.

Our nose and mucus membranes are also an important part of the first line of defense, helping to protect and keep out pathogens. They can trap pathogens and route them to the throat to be swallowed and transferred to the stomach, where there are acids that can kill certain pathogens.

Oh no!! Amelia fell down the slide and skinned her knee.

Because the skin barrier is damaged, pathogens can enter the body much more easily, meaning the first line of defense has failed.