(18) http://commons.wikimedia.org/wiki/File:Throat_with_Tonsils_0011J.jpeg.
GNU Free Documentation.
(19) http://en.wikipedia.org/wiki/File:Gray1217.png. Public Domain.
(20) http://upload.wikimedia.org/wikipedia/commons/4/4f/3DSciencelymphatic
system.jpg. Public Domain.
(21) http://en.wikipedia.org/wiki/File:WEEDS056.jpg. Public Domain.
(22) http://www.co.la-crosse.wi.us/Health/Environmental/Images/Handwashing/
handwashing%20steps.jpg. Public Domain.
(23) http://en.wikipedia.org/wiki/File:
EM_of_pap_virus,_2C_basal_tissue_grafted_to_mouse.jpg. CC-BY-SA.
(24) http://commons.wikimedia.org/wiki/Image:HIV_Daughter_Particles.jpg.
Public Domain.
(25) CK-12 Foundation.http://en.wikipedia.org/wiki/Image:Glucose_meters.jpg.
Public Domain.
(26) http://commons.wikimedia.org/wiki/File:SEM_Lymphocyte.jpg. Public Domain.
(27) http://commons.wikimedia.org/wiki/File:Cytotoxic_T_cell.jpg. Public
Domain.
(28) http://commons.wikimedia.org/wiki/File:Ambrosia_5977.jpg. GNU Free
Documentation.
