Links
[1] https://hootsgo.org/?q=node/6334
[2] http://www.ib.bioninja.com.au/higher-level/topic-11-human-health-and/111-defence-against-infecti.html
[3] http://www.ib.bioninja.com.au/standard-level/topic-6-human-health-and/63-defence-against-infectio.html
[4] http://www.merckmanuals.com/home/infections/biology_of_infectious_disease/defenses_against_infection.html