Biological Systems MCAT Practice Exam

What structure differentiates between MHC I and MHC II molecules?

1. Location on all cells

2. Presence of antibodies

3. Type of immune response elicited

4. Protein-coding ability

The correct answer is: Location on all cells

The correct choice is based on the fundamental differences in the expression and location of MHC (Major Histocompatibility Complex) molecules in the body. MHC I molecules are present on almost all nucleated cells, including those of the liver, skin, and muscles. Their primary role is to present intracellular peptides, which can come from viral proteins or self-antigens, to CD8+ cytotoxic T cells. This process is crucial for the recognition and elimination of infected or abnormal cells. On the other hand, MHC II molecules are expressed predominantly on professional antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells. These molecules present extracellular peptides to CD4+ helper T cells, initiating a different type of immune response, often aimed at orchestrating the response by activating other immune cells. This distinction in cellular expression and function is pivotal in understanding how the immune system discriminates between different types of pathogens and responds accordingly. The other options do not capture this essential anatomical and functional difference, as they pertain to various roles and characteristics that are not directly relevant to the primary distinction between MHC I and MHC II.