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Definition: Protein (immunoglobulins) in cell surfaces and dissolved in blood or tissue fluid, capable of combining with the antigens that stimulated their production. Formation of antibody-antigen complexes immobilize invading organisms, which can then be removed by phagocytosis of the complexes (opsonization).
Source: Hickman, C.P., et al. 2011. Integrated Principles of Zoology. Fifteenth Edition. McGraw-Hill Higher Education.