The human spleen as the center of the blood defense system.

Blood-borne infections can develop into sepsis and are therefore a major human health risk. The responsible pathogens thus need to be eliminated rapidly. Intravascular macrophages in the liver and bone marrow (BM) sinuses, and in the red pulp and marginal zone of the spleen, remove the majority of these microorganisms via innate immunity. However, specific antibodies are essential for their complete elimination. Splenic marginal zone B cells simultaneously produce many of the IgM and IgG2 antibodies that target blood-borne pathogens within a few days of infection. Subsequently, follicular B cells of the white pulp of the spleen produce specific IgG antibodies against the invading pathogens, known as adaptive immunity. Although the liver, BM, and spleen work together to activate the defense response to blood-borne pathogens through innate and adaptive immunity, the spleen acts as the center of the blood defense system (BDS). The structure of the spleen is introduced in this review in relation to its function in the BDS as part of the response of the immune system and of functioning organs to blood-borne pathogens. The concept of a BDS is also important for hematological disorders, such as lymphomas and; therefore, may be useful to hematologists and pathologists.