Ans.
White blood cells (WBCs) or leukocytes are immune cells that are associated with protection of body against foreign invaders (pathogens) and infectious diseases. These cells include neutrophils, eosinophils, basophils, B cells, T cell, and NK cells.
These cells migrate to the site of injury to prevent infection and entry of pathogens from broken skin. The B cells generate specific antibodies for particular pathogens and help in killing pathogens with the help of other immune cells, such as T cells.
Thus, the correct word to be filled in the blank is 'pathogen.'