An antigen is a molecule that can bind to the highly variable receptors of the immune system. There are T cell receptors, B cell receptors and antibody (the secreted form of B cell receptors).
T cell receptors recognise short peptide antigens that are presented to the T cell by another cell using MHC.
B cell receptors, and antibodies can bind antigens of all kinds. So, for example, they will be able to bind sugars that coat the surface of bacteria.
Antigen can be soluble single antigen. It can be bound to the coat of organisms such as viruses and bacteria, it can be bound to the surface of cells with the help of complement, it can be specially presented on the surface of a cell.
All cells present antigens from inside the cell using MHC class I – to be recognised by a CD8 type of T cell. This is to check that there aren’t any infections that got inside our cells. Only some cells can take in infectious agents from outside the cell and then present them on MHC, in this instance on MHC class II (presenting to CD4 type of T cell). These are called professional antigen presenting cells and include dendritic cells, macrophages, B cells.
Because a B cell can internalise antigen from outside and process it to be recognised by a CD4 T cell there is a special synergistic relationship between B cells and T cells in the activation of the immune system.
What is an Antigen?
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