Cytokine:

Cytokines are small proteins that share an ability to bind specific cell surface receptors causing cells to alter function.  The number of cytokines produced by various cells in the body in response to some challenge event continues to increase; Although there exits  some over-lapping characteristic between the various cytokines and the action they provoke. The small proteins are important players in both health and disease.

Here are a few YouTube videos to help in your understanding of cytokines:

http://www.youtube.com/watch?v=bePwyl2sHqk
http://www.youtube.com/watch?v=yRnuwTDR1og
http://www.youtube.com/watch?v=lZWOh3NEsag
http://www.youtube.com/watch?v=TTYNBM1EIWA
http://www.youtube.com/watch?v=yxz5BhZSNSE&feature=related