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An antigen effectively stimulates an immune response primarily through the process of being captured and processed by specialized immune cells, such as macrophages. When a macrophage encounters an antigen, it engulfs and internalizes it in a process known as phagocytosis. After digesting the antigen, the macrophage processes it and presents fragments of the antigen (peptides) on its surface using molecules called Major Histocompatibility Complex (MHC) proteins.

This presentation is crucial, as it enables the activation of lymphocytes, specifically T cells. Once the processed antigen is displayed on the surface of the macrophage, T helper cells can recognize and bind to the antigen-MHC complex via their T cell receptors. This interaction is vital for T cell activation and triggering a broader adaptive immune response, including the activation of B cells that produce antibodies.

While circulating in the blood and coming into contact with macrophages, or direct interactions with T helper cells, or combining with antibodies are important aspects of the immune response, they do not encapsulate the essential mechanism by which antigens effectively stimulate the immune system. Processing and presentation by macrophages ensures that the immune response is tailored to the specific antigen, enhancing the body's ability to recognize and eliminate pathogens.