Antigen-presenting cells (APCs) play a crucial role in the immune response by processing and displaying antigen fragments on their surface using major histocompatibility complex (MHC) molecules. This presentation is essential for activating T lymphocytes, which recognize these antigens and initiate a targeted immune response. Without APCs, T cells cannot identify pathogens effectively, compromising adaptive immunity. This function distinguishes APCs from other cell types involved in non-immune tasks such as oxygen transport or hormone secretion.

Reference: Janeway's Immunobiology, Kenneth Murphy, 9th Edition.