Yellow Blood Cells

Yellow Blood Cells are blood doses children receive as part of their development into Parasites. They are also referred to as ‘parasites shots’ by the children.

Origins
Yellow blood cell is another name for bone marrow. Bone marrow is the tissue responsible for producing red blood cells.

Yellow blood cells can generally be extracted from humans affected by radiation. It is not considered to be a cancer, it's a transmissible cancer - which means these cells from another organism can survive and proliferate within an individual.

Overview
During the prototype phase of the FRANXX prohram, Dr. FRANXX discovered that the units required fertile male and female pairs to function. Once this was resolved through his cloning research, he discovered that giving the children daily doses of yellow blood cells to increase their capacity and aptitude abilities.

These doses have significant effects on the children. As preadolescent children, they are emotionally strained to the point of showing no emotions and becoming composed. It is said that is the intention as the objective is to make future parasites obedient and reliant. The doses can also weaken the children’s bodies and make them vulnerable to fevers, possibly affecting their immune systems.

After officially becoming parasites, the children’s doses are given on occasional checkups rather daily. By then, the children develop an accelerated aging processes, which, coupled with the stress of piloting a FRANXX, makes them age at a faster rate. A heavier dosage is called the Elixir Injection, which is a powerful drug with immense yellow blood cells designed to increase one’s aptitude results but is known for having an incredibly low success rate.

Trivia

 * The children are considered to be ‘contaminated’ but the adults never give an explanation why.
 * In Episode 10, when Zorome gets lost in the city and stays with the Old Woman, a security team scans him and the indicator reads an unknown number of CFU (Colony Forming Units), which is a unit used to estimate the number of viable bacteria or fungal cells in a sample.
 * Zero Two bragged about how she inherited her skills from human and klaxosaur genomes. However, she requires high maintainance (heavy dosage of yellow blood cells) to function.