Complex sentence: Reactive arthritis is a complication strongly associated with a particular genetic make-up, that is, persons who have the human lymphocyte antigen B27 (HLA-B27) are most susceptible.
Split into simpler sentences: Reactive arthritis is a complication strongly associated with a particular genetic make-up. That is, persons who have the human lymphocyte antigen B27 (HLA-B27) are most susceptible.