Major histocompatibility complex
The major histocompatibility complex (MHC) is a group of approximately 150 connected genes situated on human chromosome 6. The two MHC genes, class I and class II, are relevant to infection due to their role in antigen presentation. The majority of individuals in the human population can synthesize six alternative class I molecules and the same number of different class II molecules. MHC class I and class II molecules vary between individuals which, due to their role in antigen presentation, means different people present antigens differently. This difference between immune systems, producing a variation in antigen presentation dependent on the individual, results in contrasting vulnerability to different pathogens.