Antigen-presenting cells (APCs) are a crucial part of the immune system. They capture, process, and present antigens—substances that can trigger an immune response—on their surface. This presentation helps activate T cells, which are essential for recognizing and responding to pathogens like viruses and bacteria. APCs include various cell types, such as dendritic cells, macrophages, and B cells. By displaying antigens, they play a vital role in initiating and regulating the body's immune response, ensuring that the immune system can effectively identify and eliminate foreign invaders.