Take up stations in tissues and act as scavengers, devouring pathogens and worn -out cells.
a. natural killer cells
b. amino acid cells
c. enteric cells
d. macrophages

D. Macrophages

These cells, unlike Neutrophils, tend to go and settle in a tissue and kill pathogens as well as worn out cells. Natural killer cells are used for something similar - they kill cells that have been infected by a pathogen and are displaying the pathogen's antigen on their surface (antigen presentation) - this is how the Killer-T-cells are able to recognise which cells need to be destroyed.