Answer :

Leukocytes(white blood cells) are blood cells that assimilate bacteria and foreign invaders

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.