Re: can you mix spider cells with a human's ?

The short answer is "Probably not." The long answer is a bit more
complicated, but your answers are pretty close.

There are two main obstacles to overcome if you want to put spider cells
into a human's body. First, the cells have to avoid being detected and
destroyed by the immune system. Second, the cells must somehow stay in
place and grow without becoming a cancer (another possibility you suggested).

It is possible to transplant cells from one organism into another, though,
if you give drugs that suppress the immune response. Researchers have put
mouse cells in rats, and even transplanted in bird cells, after giving the
rats drugs to keep them from rejecting the cell transplants. Also,
researchers have been making chimeras (combined animals) for decades now by
combining the embryos of quail and chicks early on, before they develop
their immune system.

The problem with transplantation is that immunosuppression by drugs is
dangerous. Your hunch that your body could become unable to fight off
illness is right on the money. In order to get the human body to accept
foreign cells, it's often necessary to knock down the immune system. This
is one cause of death in heart transplant patients, for example, and also
can cause a syndrome called Graft-Versus-Host Disease in people who get
bone marrow transplants.

We could try to get around this by genetically engineering spider cells
that express signals that make the immune system accept them, but this
would be difficult. And, even assuming the body didn't reject the spider
cells, they'd still need to maintain their population stably. Most cells
rely on a number of local cues to keep them in place and keep them from
overgrowing, and the spider cells might not respond to these properly.

That said, we shouldn't give up entirely on spider science. One company,
Nexia Biotechnologies, is using genetic engineering to make the milk glands
of goats secrete spider silk, which they harvest and sell under the name
BioSteel. As spider silk is extremely strong for its weight, they hope that
this will be useful for building on a small scale.

A page on Graft-Versus-Host Disease: http://www.nlm.nih.gov/medlineplus/ency/article/001309.htm

A page on hybrids and chimeras: http://www.ratbehavior.org/RatMouseHybrid.htm

Company page on BioSteel: http://nexiabiotech.com/en/01_tech/01-bst.php