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I'm learning about antibodies. As I understand it, antibodies detect stranger cells/bacterial/viruses by the molecules present in their membranes. In cancer cells, the cancer cell have produce some unknown protein, so why antibody cannot detect that molecule and kill them?

Is it that the cancer cell is surrounded by a normal cell?

Or is the protein it produces is not obvious in the cell membrane?

Or that the protein can bypass antibody? if so, why?

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This article on the University of Kentucky website is pretty good, it seems the cancer cells hide because they do not express CD80, a co-stimulating molecule. Co-stimulating molecules are often necessary for an effective immune response.

This is a big part of the article and I think they do a good job of explaining it so I'll just quote:

"People once thought that cancer occurred because of a weakness in the body's immune system," says Yannelli, "but we have known for years that overall that is not the case. The immune system in cancer patients actually works quite well. Otherwise, they could not survive the disease for long periods of time nor could they fight off other viruses, which they routinely do. The problem is not that the immune system doesn't work; it is that the immune system doesn't recognize the tumor."

According to Yannelli, for the body's defense system to work properly, key antigens must be presented in a specific way to lymphocytes, the cellular mediators of the immune system, which then kill the cancerous cells. Normally, antigens are tipped off by co-stimulating molecules. When the co-stimulating molecules are not present, the immune system does not "see" the cancer cell, and therefore does not act to destroy it.

Scientists have discovered that lymphocytes have very narrow job descriptions, each one designed to attack only a specific molecule. The researchers have also discovered a wide range of co-stimulating molecules. A critical co-simulating molecule is CD80. Because cancer cells do not express CD80, they are invisible to the lymphocytes responsible for destroying them.

This paper has more on the function and mechanism of CD80.

Thanks to @Nico for pointing out that not all cancers are the same so the generalization discussed in this answer may not be true of all cancers.

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We also need to specify that "cancer" is not ONE illness but a series of very different pathologies with very different etiology and development. What is true for one cancer may not be true for another one, so it is always difficult to generalise. – nico Jul 31 '13 at 12:09
@Nico Thanks, added that note. – rg255 Jul 31 '13 at 12:09

In general the immune system sees the world in very simplistic terms: Self vs Non-Self. Ideally, threats to the health of the animal are all non-self and can be attacked strongly. Whereas self cells and proteins are tolerated - effectively invisible to the immune system. Cancer is a particularly insidious disease because it arises, not from the outside world, but from subtle changes within the host's own cells. Therefore, cancer cells are starting as 100% self, but can adopt changes that make them more different over time. A good review of this interaction between cancer cells and the immune system can be found here. A second aspect of your question that is important is how antibodies are made and what they are capable of 'seeing'. I wrote a short essay about B cells, the cells that produce antibodies, and how they are activated on my blog. That might be a good resource to check for you - I also link to one or two other articles there. Briefly, B cells can only 'see' cellular antigens that are either expressed on the cell surface or secreted. Although some cancer cells alter these types of molecules, many more are part of the internal workings of the cell and therefore invisible to antibodies. For this reason, many oncologists focus on T cell responses, which are capable of detecting non-native internal antigens.

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