Check this site. Although it's trying to sell an immune modulator, it seems to offer a brief explanation about how it works.
http://www.rivexpharma.com/therapeutic_imunovir.html
http://www.rivexpharma.com/therapeutic_imunovir.html
Second, since this is not my area of expertise, I searched Google under the key words "Mesopotamia irrigation" to get these possible sources:
http://www.learner.org/interactives/collapse/mesopotamia.html
http://www.geology.ucdavis.edu/~cowen/~GEL115/115CH17oldirrigation.html
http://www.waterencyclopedia.com/Hy-La/Irrigation-Systems-Ancient.html
http://i-cias.com/e.o/mesopotamia.htm
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
I hope this helps. Thanks for asking.
To explain it in simple terms, think of the immune system as a security system in your body. Just like a security system can be adjusted to be more sensitive or less sensitive, immune modulation works in a similar way. It helps to fine-tune the immune response, making it appropriate and efficient in different situations.
Now, if you're interested in delving deeper into the science behind immune modulation, you may want to explore specific research papers or articles. But for a basic understanding, the explanation above should give you a good starting point.