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Lolly,
Excellent observation! Always a keen eye! In a nutshell, you have it! Inartfully, I was trying to refer to the HER-2 receptor, as blocked by Herceptin, as an antigen. Technically, the antigen term is used incorrectly by me, but it is how I think of the HER-2 receptor after it is blocked by herceptin. You are correct that herceptin is a monoclonal antibody. A few definitions may help --
Antigen: Any foreign or "non-self" substance that, when introduced into the body, causes the immune system to create an antibody.
Antibody: A protein produced by certain white blood cells in response to a foreign substance (antigen). Each antibody can bind only to a specific antigen. The purpose of this binding is to help destroy the antigen. Antibodies can work in several ways, depending on the nature of the antigen. Some antibodies disable antigens directly. Others make the antigen more vulnerable to destruction by white blood cells.
With this background, Herceptin is clearly a man-made monoclonal antibody. It is necessary because we do not have natural antibodies that recognize and destroy HER-2 positive cancer cells because the body does not view those cells as foreign. Herceptin attaches to the HER-2 receptor. If you view the overexpressed HER-2 receptors on the cell surface as "antigens," the definitions make sense. In this way, an antibody (i.e., Herceptin), attaches to an antigen (i.e., the HER-2 receptor) and forms a antibody/antigen complex. The existence of the antibody/antigen complex on the cancer cell surface makes the HER-2 cancer cell more vunerable to destruction by the immune system's natural killer (NK) cells.
The funny thing is that the HER-2 receptor is a normal part of our human cell -- it is simply found in abundance in a defective HER-2 positive cancer cell. However, if you think of overabundant HER-2 receptors as "foreign" or "defective" in a sense that they do not belong in a normal cell -- the medical definitions make perfect sense.
Lolly, thanks for clarifying my original write-up. I guess I was trying to be more understandable but failed. I hope that I have clarified, and not muddied, my prior description
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