Our bodies continuously encounter microbes that pose potential or actual threats to our survival. Our immune system has to be able to detect all possible threats to our bodies, whatever shape or form they may take. So how does the immune system recognize a threat?
Humans and microbes share the same basic building blocks: proteins, carbohydrates, and fats. The most varied of these building blocks are the proteins. Proteins are themselves chains of interconnected building blocks, amino acids. The sequence of amino acids determines the properties of the protein, and the order in which the amino acids are put together is determined by our genes. Thus, the number of genes we have limits the number of different proteins our bodies are made of.
The microbes we encounter will have a distinct set of proteins to the human body. 20 different amino acids are used in both the human body and in microbes. These amino acids can be assembled in any order to form chains, or peptides, that consist of anything from a few dozen to several thousand amino acids. There is therefore an endless number of possible foreign proteins that the immune system needs to be able to detect and react to
Part of the solution to this challenge lies in the structure and function of the HLA molecules. These molecules have a surface cavity with room to bind a peptide of 8-9 amino acids. The peptides may come from the body’s own selection of proteins, or from a microbe.
Excepting red blood cells, HLA molecules are found on the surface of all cells in the body. Their main function is to exhibit peptides from the proteins found within the cell. The T cells of the immune system constantly check the peptides displayed by the HLA molecules. As long as only peptides from the body’s own proteins are displayed, the T cells remain unreactive. But if a T cell encounters a microbial peptide in an HLA molecule, it might trigger the activation of a T cell response.
T cells stimulated in this way will initiate an immune response that aims to eliminate, or render harmless, the microbial invader.
Blogpost by Anne Spurkland, published 27th November 2014
Translation edited by Alisa Dewan, 26th February 2015
Originally published in Norwegian 09/08/12