Antibodies are produced by the immune system to respond to foreign molecules like bacteria, viruses and abnormal or "non-self" cells.

When produced, antibodies combine with and inactivate the foreign invaders, thus protecting the body.

References

National Institute of Health Website. Last Accessed on 4/10/06. Primary Immunodeficiency Booklet http://www.nichd.nih.gov/publications/pubs/PrimaryImmunoBooklet.htm