Hey can someone please explain to me what exactly antigens are I seem to be very confused? Some sources state it is anything that triggers and immune response whilst others say they are protein markers? Are self-antigens MHC markers found on our cells that identify them as belonging to us whilst non-self antigens are things like pathogens?
Thanks so much.
An antigen is something that can illicit an immune response. Often times they are protein markers on cell surfaces, but they can also simply be molecules of their own.
In response to your question about MHC markers, our own cells posess many different surface antigens. Howeverm as MHC markers are the most important in temrs of the immune system, we mainly focus on these. MHC proteins have two different classes: class I and class II. Class I MHC markers are expressed by all nucleated cells (things like red blood cells don't have them) and present protein fragments from within the cell. As such, these markers can be used to 'check' if the cell is functioning properly as aberrant cells (ie. virus-infected or cancer cells) would produce certain proteins that may be presented to immune cells on these MHC class I markers. MHC class II markers are expressing in certain immunce cells called antigen presenting cells (APCs). This class of MHC marker allows antigens engulfed by the cell to be presented (after processing) on the cell's surface. The main purpose of this is in allowing for activation of the active immune response.
In reguards to non-self antigens, there are many sources of these. Pathogens are one source, as they present and produce molecules that can trigger an immune response. But there are proteins and molecules - like those of allergens - which can also illicit an immune response that are not pathogenic.
Hope this helps.