This is a pathogen. In this case, it is Pneumonia, which is an infectious disease.
The first line of defense against pathogens is your skin. It stops most foreign bodies from entering the body.
But our Pneumonia cell has found an open wound in the skin, letting it into the body. Unfortunately, the wound was too small for anything else to enter and alert the immune system to begin inflammation. It has however gotten stuck in a viscous substance underneath the skin.
Suddenly a Naïve T Cell spots the Pneumonia cell.
Carefully, it picks up some antigens dropped by the intruder.
The now activated T cell returns to the lymph to proliferate into effector and memory T cells. The Pneumonia virus doesn't have much more time.