How does the immune system distinguish between self and non-self entities?

• A. By the presence of antibodies
• B. Through cell membrane permeability
• C. By using major histocompatibility complex (MHC) molecules
• D. Through the action of cytokines

Answer: (C)

The immune system distinguishes between self and non-self entities primarily through the use of major histocompatibility complex (MHC) molecules. MHC molecules are surface proteins that present peptide fragments derived from both self and non-self proteins to T cells, which are a crucial component of the adaptive immune response.

When T cells encounter MHC molecules displaying peptides, they assess whether the presented peptides are derived from the body’s own proteins (self) or from pathogens or foreign substances (non-self). This recognition process is vital for the immune system's ability to mount an appropriate response. T cells that recognize self-peptides as non-threatening are allowed to mature, while those that recognize self-peptides as potentially harmful are typically eliminated during their development in the thymus. This process helps to prevent autoimmune responses where the immune system attacks the body's own cells.

The other options involve elements of the immune response but do not directly explain the distinction between self and non-self. Antibodies are produced as part of an immune response against non-self antigens but do not play a role in initial recognition. Cell membrane permeability relates to the ability of cells to allow substances to enter or exit, which is not directly tied to the self/non-self distinction. Cytokines are signaling molecules