How do the B-cells and T-cells recognize or distinguish a body cell from a foreign particle?
I suspect something to do with this causes Auto Immune Disorders.
Broad question. Summary:
The innate immune system processes everything. When it senses that something is dangerous it tells the adaptive immune system, that is T and B cells, that this thing I'm holding is dangerous (via coreceptors and cytokines).
T and B cells that are specific for this dangerous protein (or sometimes non-protein) are activated.
Ideally T and B cells that react to self proteins are deleted. These are from central tolerance mechanisms when the cells are developing (T and B cells are tested against self proteins; if they react they die) and in the periphery (once they're mature) by them only getting activated by the innate immune system and other mechanisms. There's also T regs which regulate the immune system by preventing any cells that react to self proteins from being activated.
Autoimmune disorders are caused by a break down of these mechanisms.
I often have to reiterate this question in my head to clarify to myself how it works, though I tend to take an approach that asks more: What is different between a part of the body and something foreign which triggers an immune response?
Here is the part that concerns B and T cells:
- The adaptive immune system (B and T cells) is based on random genetic reshuffling, producing an 'immune cell army' in which every 'soldier' (every cell) recognises a unique target. This 'army' is so vast in diversity that it covers virtually every molecule that occurs in the world - including those of the own body.
- This method requires the elimination of those randomly created cells which recognise the body's own substances. The mechanism of elimination is extremely beautiful and makes great use of the way genetics work: Because every cell contains ALL genetic information for your body (except that in almost all cell types the vast majority of it isn't being used most of the time), it is possible to have a few cells which will simply randomly produce bits that belong to a completely different part of the body. This way, everything that belongs into the body will be present where these specialised cells live - which is the bone marrow and the thymus. Thanks to them, it is possible to test every single B and T cell against everything that occurs in the body - and kill them if it turns out they recognise something.
- This is how you end up with an army that only reacts to things which are not part of the body.
- In other words, an individual cell doesn't distinguish foreign from body, it simply doesn't react to anything aside from its specific target - which should be foreign if the earlier selection mechanism worked.
Aside from this, the main method by which B and T cells determine whether to activate or not is context, which means two things:
- B and T cells require co-stimulation to activate - T cells from the so-called antigen-presenting cells (APCs), and B cells from T cells.
- In order to be activated, adaptive immune cells (T cells in particular) depend on the levels of certain inflammatory messengers (known as cytokines) in their environment. In other words, the presence of inflammation is a signal to immune cells that in this location where they currently are, abnormal processes are happening and there may be a need for them to take action. As the reasonable response, they will activate or enter a state ready for activation.
Both APCs and inflammation are mainly provided by parts of the innate immune system, adding another safety barrier with more recognition mechanisms to tell self from non-self.
Good answer Android. Recognition of self vs non-self is the central theme of immunology.
I wanted to clarify and kind of oversimplify a part of Android's response though… B Cells and T Cells get their antigen receptors made by DNA rearrangement as immature cells. This is a random process and leads to receptors capable of binding (theoretically) anything. As these cells are developing in controlled environments, anything they see is 'self'. If they react to these 'self' antigens, the cells are eliminated. Later in life, antigen receptor activation turns from a self-eliminating event into an activating event. If all goes well, this mature population has already been screened for anti-self cells, so anything that activates them is assumed to be foreign and therefore a threat.