Immune evasion is a biological process by which pathogens, tumor cells, or other aberrant cells avoid detection, recognition, or elimination by the host immune system, thereby promoting persistence and proliferation. Mechanisms include antigenic variation, downregulation or loss of major histocompatibility complex (MHC) molecules, expression of immune checkpoint ligands (e.g., PD-L1), secretion of immunosuppressive cytokines, induction of regulatory T cells or myeloid-derived suppressor cells, and shielding of antigens within biofilms or physical barriers. This process critically influences infection chronicity, tumor progression, vaccine efficacy, and the outcome of host–pathogen or host–tumor coevolution.
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