Blood & the Immune System
Blood carries red cells (oxygen via hemoglobin), white cells (defense), platelets (clotting), and plasma; the immune system mounts innate (immediate, nonspecific) and adaptive (targeted, with memory) defenses.
T cells coordinate the adaptive response and kill infected cells directly, and the lymphatic system filters pathogens through lymph nodes while returning tissue fluid to the blood. Type O negative blood is the universal red-cell donor and AB positive the universal recipient.