Phagocytosis and oxycytosis: two arms of human innate immunity.

Human innate immunity operates in two compartments: extravascular (the tissues) and intravascular (the bloodstream). Physical conditions (fluid dynamics) in the compartments are different and, as a result, bactericidal mechanisms and involved cells are different as well. In relatively static media (the tissues, lymph nodes), bacteria are killed by phagocytes; in dynamic media (the bloodstream), bacteria are killed by erythrocytes. In the tissues and lymph nodes, resident macrophages and transmigrated from blood leukocytes (neutrophils and monocytes) recognize, engulf, kill, and digest bacteria; the clearance of the bloodstream from bacteria is performed by oxycytosis: erythrocytes catch bacteria by electric charge attraction and kill them by the oxygen released from oxyhemoglobin. Killed by erythrocytes, bacteria are decomposed and digested in the liver and the spleen. Phagocytosis by leukocytes in the tissues and oxycytosis by erythrocytes in the bloodstream are the main bactericidal mechanisms of human innate immunity.