Pathogens are the disease causing organisms. The surface molecules of the pathogen that are recognized by the immune system to initiate immune response are termed as antigens. B cells are responsible for the production of antigen-specific antibodies while T cells serve in cell-mediated immunity. Fibrinogen is involved in blood clotting process. Phagocytes are the component of non-specific immune response and kill the pathogens that invade the body. Phagocyte cell engulfs the pathogen like bacteria or other small cells and digests them e.g., macrophage cells. Thus, the correct answer is D.