13. Exponential, limiting
14. Keystone
15. Population (not completely sure on this one, may be something like biodiversity)
1) The antibodies destroy the antigen (pathogen) which is then swallowed and digested by macrophages.
2) White blood cells can produce antitoxins which destroy the toxins that are otherwise known as poison.
Processes like deforesstation can bring an enviromental change in which it is harmful but afforestion is another form which bring the enviroment in to a better situation
