Immunology

[genovese]

Could someone please remind me whether foreign proteins once inside cells are safe from immunological attack. When cells are destroyed in autoimmune diseases is it just the surface proteins on the cells that are under attack or can intracellular proteins also be attacked?

[biohazard]

Intracellular proteins can be "indirectly" attacked by the immune system. Virtually all nucleated cells express MHC class I proteins, which present antigen fragments from the cytoplasm that are recognized by cytotoxic CD8+ T cells. If they are judged to be of non-host origin, the cell expressing the foreign antibody fragments is signalled to die via the apoptotic cell death pathway.

This is an important way e.g. when battling against many viral infections (where the pathogen prevails & reproduces intracellularly), and it is believed to play a major part also in destroying potential malignant cells before they can turn into a cancer.

If by "safe" you mean that the proteins cannot be targets of a direct immunological response, i.e. be a target of antibody or complement etc., then I think that is the case: when inside cells, they are safe from direct actions. So when it comes to autoimmunity, the protein must be either on the cell surface or presented on the MHC complex.

Sometimes, however, autoimmunity can be systemic, if e.g. T cell regulation fails on some level, which leads to an immune response that is not dependant on any exact antigen. Another example is when autoimmunity takes place due to an inappropriate cytokine production, which can cause cell death and tissue damage. Again, this does not necessarily require a specific target antigen. Someone correct me if I'm wrong

Hope this helped!

[genovese]

Thank you biohazard for that info.