Tattoo artists apply ink to the dermis, which is the middle layer of skin. The cells of the dermis are relatively stable, so the tattoo is permanent. The outer layer of skin, the epidermis, provides a protective layer through which the design is visible.
If the artist applies the ink too deeply, it increases the pain for the subject and causes the design to spread. If it is too shallow, in the epidermis, the design is lost as the body sheds dead skin cells. While the body sees the ink as a foreign substance and responds by sending white cells to attack it, the ink pigments are too large for the cells to break down. Laser tattoo removal breaks the pigment into smaller particles, which the white cells are able to remove.