Them = (cancerous or invaded-by-viruses) cells ?

Yes, perforin is expressed in killer T cells and works by attacking cancer or virus-infected cells.

You're reading some very complicated stuff, ori. I don't wonder that some of the concepts are made extremely difficult for you by trying to wade through the English syntax. You're doing a great job!

Thank you JPB.
You're so cool, both in English and science.

HA! I think you may have me confused with someone else when it comes to English, but thank you, you're very kind.

Yes, you are pretty cool in science, Doc. And English too!

Do you know why perforin isn't able to work in all situations? Why do some diseases, cancers, viruses kill? Is there simply not enough perforin?

heh, thanks.

I'm headed to bed, but the short (and very simplistic) answer is that perforin is just one of the chemicals expresses by killer T cells. Others include lymphotoxin and tumor necrosis factor, depending on what it runs into. Killer T cells work in numerous way to destroy foreign invaders, but they need to be activated first. That's done by Helper T cells. Some viruses, such as HIV, attack the Helper T cells preventing the Killer T cells from being activated. Re cancer -- there are a number of different things that can go wrong resulting in cancers. T cells are only active in some of those. In fact, some cancers (lymphomas) are a proliferation of too many T-cells.

In cases where it's not a lymphoma, T cell counts are tracked to see how well the immune system is holding up to the virus/tumor. As the T cell counts go down, the infection/tumor takes over and the prognosis worsens. Also, tagged T-cells are now being used in immuno-therapies whereby a patients own T-cells are tagged with cancer-destroying markers and injected back into the patient. This helps get tumor-specific therapies directly into the tumor.

I'm not a dr, just someone who's worked with infectious diseases and diagnostic immunology research for the past 30+ years.

No wonder that your remarks sounded so professional.