Evolution would be the answer you may be looking for.
Macrophages as they are versatile cells which reside in the tissue
Ecologists make models to gain insight into complex phenomena such as the effects of global warming on ecosystems.
hope this helps
The immune system and the nervous system maintain extensive communication, including 'hardwiring' of sympathetic and parasympathetic nerves to lymphoid organs. Neurotransmitters such as acetylcholine, norepinephrine, vasoactive intestinal peptide, substance P and histamine modulate immune activity.