This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax lynng@alsirat.com with numerous contributions by others.
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From Gregory Froehlich, MD (from a posting to sci.med):
First, hepatitis B *antigen* is used to make Hep B vaccine. The
antigen is grown in yeast culture; formerly, it was purified from the
blood of people who were chronic hepatitis carriers. Antibodies are
used in the gamma globulin shots used for hepatitis A or for passive
immunization against hepatitis B if you're exposed.
The local blood bank does not specifically test for exposure to
hepatitis A (the kind you'd get from contaminated water). If a person
has an active hep A infection, it will be picked up by elevated liver
enzymes; if the person had such an infection in the past, it's over and
done with--hep A doesn't give you a chronic, subclinical infection.
Antibodies to hepatitis A should not preclude blood donation.
They check for chronic hep B carriers by testing for hep B surface
antigen. They test for recent hep B infection by testing for hep B
core antibody. This antibody does not carry disease, but rather
indicates that the person was recently infected and might or might not
still be infectious. They do not test for surface *antibody*, which
would indicate either (a) former hep B infection which was cleared, or
(b) immunization against hep B--in either case, not infectious. I've
got hep B surface antibody, because I was immunized; I can still donate
blood.
Blood banks also test for hepatitis C antibody; people with this
antibody can still be infectious.
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