Major Histocompatibility Complex is one of the most complex molecular systems found in the body, responsible for immune response and self-identity. There are several synonyms used to describe this critical system, with some of the most common being Human Leukocyte Antigen (HLA), Major Histocompatibility Antigen (MHA), and Human Major Histocompatibility Complex (MHG). These terms have the same meaning but are used interchangeably in different scientific and medical settings. The Major Histocompatibility Complex plays a crucial role in organ transplant rejection, tissue compatibility, and autoimmune disease. Understanding its function and key synonyms is vital for researchers and clinicians seeking to uncover new treatments for immune disorders.