The correct option is C Pathogen
Pathogens are the disease causing organisms. The surface molecules of the pathogen that are recognised 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 the non-specific immune response. So, the correct answer is option C.