Not sure about developing a cold but if you look at CMV (cytomegalovirus) and EBV (Epstein Barr virus), those are both prevalent in most people as latent infections. If you get another infection, are immune suppressed, or pregnant, then those viruses can flare up and cause their relevant symptoms.

Probably the most well known is glandular fever which is caused by EBV. You can have it, then recover, bit still have dormant/latent virus in your system that flares up again in the future. Same goes for TB

Viruses = sneaky. Hijacking a cell's own mechanisms to replicate and go on to infect more cells.

Basically what Marie said. You usually need enzymes to cut and edit DNA. You can cut some out to stop a function of a piece of DNA or even add some in. ZFNs, TALENs are slightly 'older' technologies and CRISPR is the new technology, which is being used for the purposes above. It may be enhanced/improved in the coming years and I wouldn't be surprised if there weren't other DNA editing technologies that appear in the future too.

I don't know what the cutoff is, but to compare animals, the most common way is to align their DNA sequence and then have a percentage of similarity. I do this as part of early research in drug discovery to have a look at how much alike some human molecules are to other species (eg monkeys, mice)

Yes, if an antibody on the surface of a B-cell binds to the target, then the B-cell will proliferate and attract other immune cells to it to respond to the disease. If it's an antibody that's been secreted, then there are immune cells monitoring the body, looking out for them, and the immune cells will then find them and respond appropriately.

In general (in a healthy individual), the immune is brilliant at knowing when to respond and when not to - T cells and B cells develop in lymph nodes/follicles in your body and go through both negative and positive selection steps - so they bind to foreign substances (ie diseases) but don't respond to your own body (ie they don't attack your own cells).

Antibodies are made by the immune system, specifically B-cells. They are used as targeting proteins so that your body can respond to diseases/pathogens. If they are expressed on the surface of the B-cell, then when they find and bind to something 'foreign', they can then cause the B-cell to proliferate (divide and multiply), meaning that your body can fight off the disease. They can also be secreted from other types of B-cells (plasma cells) which flat around to monitor your body, and then bind to foreign pathogens and act as a flag/beacon for other immune cells to come and investigate and attack (T-cells/NK cells).

In terms of what happens when they're not needed, the cell surface expressed antibodies will stay around as long as the individual cell lives (the cell probably recycles and makes more to put on the surface over it's lifespan) and the secreted antibodies will degrade over time or be processed and degraded by other immune cells. The plasma cells will tick over and are there to make more if the disease invades again.

Potentially, yes. But mitochondria have been part of human cells for a long time (assuming they were originally a helpful bacteria), so the immune system shouldn't see them as foreign. I've done a very quick bit of searching and couldn't find much on mitochondrial autoimmunity but maybe someone else has a bit more time to do an in-depth search.

The main way the immune cells recognise foreign things inside a cell is by detecting them outside the cell. Part of a cell's natural processes is to breakdown some components that are inside the cell and 'present' them on the outside of the cell. These are called antigens, or peptides.

If they are 'self' peptides, then the immune system has been trained to recognise these as safe and so it doesn't react to them.

If they are 'foreign' peptides, then the immune system recognises them, and subsequently targets that cell for destruction - either by releasing chemicals that perforate the cell membrane, or by recruiting other cells to do the job.

You can get some treatments, such as antibodies, which can have a chemical drug attached (ADC - antibody-drug conjugate). The antibody targets a specific cell type and then when it gets there, it releases the drug, which goes into the cell and targets a disease, or kills the cell itself.

There are other methods, some described in other answers that work in different ways but have similar aims of targeting something inside the cell.

When things go wrong with this: The immune system incorrectly recognises self peptide as foreign and attacks healthy cells (autoimmune disease) The immune system does not recognize foreign peptide correctly The disease stops the infected cell from presenting the foreign peptide on the cell surface - avoiding detection The disease hijacks the presentation process and gets the cell to present a self peptide that would normally not be presented suppressing the immune response that would normally happen.

If you're in the UK, see if you can watch "War in the blood" - goes behind the scenes of CAR T cell therapy in a couple of patients and one of the academic labs behind developing the therapy