Answer: The correct answer for the blank is-
Leukocytes.
Leukocytes ( also called white blood cells) are a type of blood cells that play significant role in the defense mechanism of our body (the immune system).
These cells can be of various types ( such as eosinophils, basophils, neutrophils, lymphocytes).
They assimilate/ ingest any pahtogenic agent ( foreign agent such as bacteria and virus, which cause disease) that enters body and prevent the body from getting infection.
Thus, leukocytes is the right answer.