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Antigen: In immunology, an antigen is a molecule or molecular structure, such as may be present at the outside of a pathogen, that can be bound by an antigen-specific antibody or B cell antigen receptor. The presence of antigens in the body normally triggers an immune response.
Antigen-presenting cell: An antigen-presenting cell or accessory cell is a cell that displays antigen complexed with major histocompatibility complexes (MHCs) on their surfaces; this process is known as antigen presentation. T cells may recognize these complexes using their T cell receptors (TCRs).
Antigenic shift: Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. The term is often applied specifically to influenza, as that is the best-known example, but the process is also known to occur with other viruses, such as visna virus in sheep.
Antigen presentation: Antigen presentation is a vital immune process that is essential for T cell immune response triggering. Because T cells recognise only fragmented antigens displayed on cell surfaces, antigen processing must occur before the antigen fragment, now bound to the major histocompatibility complex , is transported to the surface of the cell, a process known as presentation, where it can be recognized by a T-cell receptor.
Antigenic drift: Antigenic drift is a kind of genetic variation in viruses, arising by the accumulation of mutations in the virus genes that code for virus-surface proteins that host antibodies recognize. This results in a new strain of virus particles that is not effectively inhibited by the antibodies that prevented infection by previous strains.