Defensins are part of our innate immune response. They are an old family of antimicrobial peptides. COPE encyclopedia provides thorough information on immune system-related proteins:

http://www.copewithcytokines.de/cope.cgi?key=Defensins

As filip said they are part of innate immunity. All in all they are the first defense line during infection and control proliferation of pathogens. Defensin expression is a complex mechanism. There is a basal level which is enhanced e.g. by PAMPs and PRRs. Some are secreted and activated extracellular and some are stored in granules. Most of them form pores on microbial membranes which leads to rupture and microbial cell death

Sir Filip and Swidergall thank you very much.

But,when oxygen burst happen and when defensins get activated?

Defensins are still present in immune cells- but expression can be boosted by contact with PAMPs of pathogens or different interleukins. Respiratory burst is a series of events triggered by phagocytosis or exposure to certain inflammatory mediators. Within phagocytes, AMPs (defensins) enhance respiratory burst in leukocytes and increase ROS release.

Dear sir..................Marc Swidergall

Thank you very much.Now i got the context and the topic.