A girl and her brother were waiting for the bus to school. The brother was sick and was coughing.
The pathogen he coughed up traveled through the air to the girl's body.
When she inhaled the pathogen, her nose hair and mucus trapped them and prevented them from entering her lungs. Her skin prevented it from going into her body from other areas. This defense from her body is called non-specific defense, because it does not target specific pathogen.
...However, the germs entered the body through the mouth. This is when the body called its second line of defense, or specific defense.