Immune cells are typically activated by the presence of antigens. These antigens can be proteins, polysaccharides, lipids, or nucleic acids derived from pathogens. The interaction of antigens with antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells initiates the activation process. APCs process and present these antigens on their surface using Major Histocompatibility Complex (MHC) molecules.
