Antigen uptake, processing and presentation
Your cells are constantly sampling their environment, taking up all sorts of extracellular material. This material could be something that the cell needs to live, like nutrients, waste products to be broken down and re-used, or clearance of dead or apoptotic cells. Occasionally a cell will happen to take up material from potentially dangerous pathogens, and it is really important that these substances are reported to the adaptive immune system, in order that an appropriate immune response can be mounted. Most cells do not have the knowledge of exactly which substances are foreign and dangerous; it might simply sense the danger “in the air”. The safest bet is therefore to present everything that is taken up to the adaptive immune cells, which then make the decision whether to generate an immune response or not. To do this, the foreign substance must be presented in a form that the adaptive immune cells, the T and B cells, can recognise. B cells can recognise antigens directly, but a T cell needs the antigen to be chopped up and delivered in a very special package by “self” cells that it trusts, these are the APCs. APCs are experts in antigen presentation, as they have many different ways of taking up and processing antigens and also have that special presentation package that T cells prefer.
In the next few pages we are going to focus on antigen presentation to T cells, looking closer at how the antigen is taken up by the APC, chopped up, or processed into smaller bits and how this special antigen package looks when it is finally presented to the T cell.