Answer:
Cell membrane.
Step-by-step explanation:
Cytokines may be defined as the small molecules that plays an important role in the cell signalling as well as in the immune system as well. They are made of the peptides.
Pattern recognition receptor are important molecules that plays an important role in the innate immunity. These receptors are present on the surface of the cells like macrophages and monocytes. These receptors are made of the proteins and cannot pass the lipid bilayer. Hence, these receptors are located on the surface of the cell membrane.
Thus, the answer is cell membrane.