These are substances that are not supposed to be in your body (like viruses, bacteria, or cell changes that are very abnormal). Your immune system's job is to protect you from such substances. When the immune system discovers them, it springs into action. The immune system makes antibodies (special proteins) to attack the antigens.
For more information, see Breastcancer.org's Understanding Your Immune System section.
Audio contributed by: Shirley Jones