The planned terrorist tries to hide and bypass through security. Security represents the mechanical part of the first line of defense.
The terrorist was spotted passing through the first line of defense. Now the terrorist is undergoing an attack through spray gas. He is dodging the chemical part of the first line of defense- security.
Next, the second line of the airports defense begins. Much like the phagocytes in the immune system, the airport has red lasers to chase off the criminals and intruders.
After escaping the airport security and the lasers from the second line of defense, the terrorist makes it aboard the plane. This is when the third and final line of defense begins.The antigens of this line set out and find a marker on the intruder, as he is the only person with a mask on.
Finally the pilot of the plane learns that he is the only one left t o stop the terrorist. Much like the antibodies in the third line of defense in the immune system the pilot kills off and stops the terrorist to finalize it and prevent any further damage.