Introduction to antigen and antigen presenting cells
Who delivers this “message of threat”? What form does the message take? Cells can’t talk! No, but they can feel! Like all cells, our adaptive immune cells, the T and B lymphocytes, have been educated to respond to things that they touch, a bit like reading Braille. Take a moment to imagine that you are a cell that wants to let the adaptive immune cells know that you need help eliminating pathogenic bacteria. How would you do that? Since the adaptive cells need to feel the pathogen, why not bring it to the adaptive immune cell to feel? This is what happens in your body! The actual “message of threat” is a part of the pathogen, referred to as an antigen and the cells that deliver the antigen are therefore called antigen presenting cells (APCs). Many cell types can be antigen presenting cells and we’ll come to that later on.
When the antigen is presented by the APC, the adaptive immune cells first evaluate if this is an antigen that it recognises. Each and every adaptive immune cell only recognises one part of one foreign substance, so most T and B lymphocytes (also called T and B cells) will take no notice of the antigen and the APC. However, a small handful of them will recognise one part of the antigen, become activated and start fighting alongside the innate immune cells.
A foreign substance can be non-pathogenic in origin too, like a piece of a birch pollen protein. The difference is that an adaptive immune cell which recognises this pollen protein chose not to do anything, after all why fight a tree? However, in some people the adaptive immune cells do respond, fighting even the harmless birch. This response leads to an immune response, the allergic immune response, and in this case the antigen is referred to as an allergen.
Many factors influence how the adaptive immune cells choose to respond when they recognise an antigen, one of those things being the urgency by which the message, the antigen, is delivered by the APC. An APC that is delivering an antigen from the “war-zone” is likely to be very distressed and activated, relaying this with the cytokines and molecules expressed on its surface. This is something that we also will look into a bit later in this module.
Let’s start our journey into the world of antigen presenting cells, the cellular linkers of innate and adaptive immunity, by looking at how they deliver the antigen.