The blood is a critical part of the human body’s defense against infection. It circulates nutrients and oxygen to tissues and removes waste products. It also contains white blood cells that recognize and destroy invading bacteria and other pathogens. However, sometimes bacteria or other microbes can enter the bloodstream and cause a serious condition called bacteremia or bloodstream infection. When this occurs, the body has several lines of defense to remove the bacteria from the blood and prevent worsening infection.
Physical Barriers
The first line of defense against bloodstream infections are physical barriers. The skin provides an impermeable barrier that prevents most microbes from entering underlying tissues and the bloodstream. Mucous membranes in the nose, mouth, and other areas also trap microbes and prevent them from reaching more vulnerable areas. When these physical barriers are breached, such as with a cut or scrape, infection becomes more likely. Good wound care and hygiene helps prevent breaks in these defenses.
Chemical Defenses
If microbes do manage to enter the tissues or bloodstream, the body has a number of chemical defenses to combat them. Antimicrobial proteins and peptides in the blood itself kill many types of bacteria. These include things like complement proteins, mannose-binding lectin, and bactericidal permeability-increasing protein (BPI).
Inflammatory chemicals also help recruit additional white blood cells to the area to combat an infection. These chemicals include cytokines like interleukin-1, interleukin-6, and tumor necrosis factor. Increased body temperature during fever also helps kill bacteria and stimulate the immune response.
Cellular Responses
Specialized white blood cells are a major line of defense against bloodstream infections. Neutrophils and monocytes are types of white blood cells that ingest and destroy invading bacteria through a process called phagocytosis. Neutrophils are usually the first responders to a bacterial infection. Monocytes arrive later and develop into macrophages that continue engulfing bacteria. Natural killer cells and dendritic cells also help coordinate the immune response to infection.
The Complement System
The complement system consists of small proteins in the blood that play a major role in removing bacteria. Complement proteins mark bacteria for destruction by coating their surface. This “opsonization” allows phagocytes like neutrophils to recognize the coated bacteria and consume them more efficiently.
Some complement proteins also combine to form “membrane attack complexes” which punch holes in bacterial cell membranes, causing the bacteria to rupture and die. This destroys the bacteria directly without needing phagocytes. Over 30 complement proteins work together in this cascade to eliminate bacteria from the blood.
The Liver and Spleen
The liver and spleen also filter blood to remove bacteria and other particles. Specialized macrophages in the liver (Kupffer cells) phagocytize bacteria from the blood as it passes through. The spleen contains macrophages and dendritic cells that similarly filter and destroy bacteria. The spleen also removes older red blood cells from circulation. Both of these organs play an important role in cleansing the blood of bacteria during infection.
Bone Marrow Response
The bone marrow produces white blood cells that fight bloodstream infections. When bacteria are detected in the blood, the bone marrow steps up production of neutrophils and monocytes to combat the infection. This is called leukocytosis and results in more phagocytic cells released into the circulation to clear bacteria. The bone marrow also produces more red blood cells (erythrocytes) and platelets in response to blood infection, since red blood cells and platelets can be damaged by bacteria.
Antibodies
Antibodies are proteins produced by plasma cells (derived from B lymphocytes) that specifically recognize bacteria or other pathogens. After a first exposure to bacteria, the immune system remembers it and produces antibodies targeting it on future encounters. These antibodies flag the bacteria for destruction by other immune cells and prevent the infection from spreading systemically. They also activate complement to help eliminate bacteria.
Medications
In severe or recurrent bloodstream infections, medications are often needed to help clear the bacteria from the blood. Antibiotics are the main treatment and work by directly killing bacteria or inhibiting their growth. The type of antibiotic used depends on the specific bacteria involved. Sometimes multiple antibiotics are used for difficult infections. Antibiotics work with the body’s natural immune defenses to combat the infection.
Dialysis
Dialysis is a technique that filters the blood externally to help remove wastes, fluids, and toxins. It works by drawing blood out of the body, filtering it through a special membrane, then returning the clean blood. The pores in the membrane are small enough to remove bacteria and other large particles from the blood. Dialysis can help cleanse the blood of bacteria in cases of serious kidney infections or sepsis when the kidneys are not functioning well.
Intravenous Immunoglobulins
Intravenous immunoglobulin (IVIg) provides antibodies obtained from healthy donors directly into the bloodstream. This can help combat serious bacterial infections when the body’s own immune system is overwhelmed or impaired. The wide variety of antibodies in IVIg preparations can opsonize many types of bacteria to facilitate their removal from the blood. IVIg also modulates and enhances other immune cells.
Leukapheresis
Leukapheresis is a procedure that removes white blood cells from the blood. The remaining blood components are returned back to the body. This activates the bone marrow to produce more white blood cells, which can help fight blood infections. It also reduces very high white blood cell counts that could be worsening sepsis. Leukapheresis provides a temporary boost in white blood cell production to combat infection.
Plasmapheresis
Plasmapheresis is similar to leukapheresis but removes plasma, the liquid part of blood, instead of cells. This also removes antibodies, complement proteins, cytokines and other immune substances from the blood. The plasma is discarded and replaced with either donor plasma or albumin solution. This replenishes depleted immune components to help fight infection while removing excessive inflammation-causing products.
Blood Transfusion
Transfusing fresh red blood cells, plasma, or platelets can help treat severe bloodstream infections. Red blood cell transfusions increase oxygen delivery to tissues when low red blood cell counts occur during sepsis. Plasma contains antibodies and clotting factors to fight infection and stop bleeding problems. Platelet transfusions help with clotting issues. Transfusions replace essential blood components depleted by infection.
Extracorporeal Blood Purification
Extracorporeal blood purification techniques remove harmful molecules from the blood to help treat sepsis. The blood is passed through specialized filters outside the body before being returned. These filters adsorb inflammatory mediators and bacterial toxins that can worsen sepsis. Types of these techniques include hemoadsorption, hemofiltration, and coupled plasma filtration absorption. They provide additional blood cleansing on top of the body’s natural defenses.
Conclusion
The body has an impressive arsenal of defenses to remove bacteria from the bloodstream. Physical barriers, chemical signals, specialized white blood cells, antibodies, and proteins work together to recognize and eliminate invading microbes. Critical organs like the liver, spleen, and bone marrow filter bacteria from the circulating blood and produce infection-fighting cells. When these natural systems are overwhelmed, medical therapies like antibiotics, dialysis, blood purification, and transfusions support the body in combating serious blood infections. Understanding all the ways bacteria are removed from blood provides insights on treating bacteremia and sepsis effectively.
| Defense Mechanism | Components | Actions |
|---|---|---|
| Physical barriers | Skin, mucous membranes | Block entry of bacteria into tissues and blood |
| Chemical defenses | Antimicrobial proteins, inflammatory mediators | Directly kill bacteria, recruit phagocytes |
| Cellular responses | Neutrophils, monocytes, dendritic cells, NK cells | Phagocytize and destroy bacteria |
| Complement system | Complement proteins (C3b, MAC, etc) | Opsonize bacteria, create pore-induced lysis |
| Liver and spleen | Specialized macrophages and dendritic cells | Filter blood and remove bacteria |
| Bone marrow | Neutrophils, monocytes | Produce more phagocytes during infection |
| Antibodies | Produced by plasma cells | Bind bacteria and activate complement |
| Medications | Antibiotics, IVIG, etc | Kill bacteria or boost immune function |
| Dialysis | Extracorporeal filtration | Filters bacteria from blood externally |
| Blood transfusions | Red cells, plasma, platelets | Replace depleted components |
| Blood purification | Hemofiltration, hemoadsorption, etc | Remove bacterial toxins and cytokines |