Intravascular immunity describes the immune response in the bloodstream, and its role is to fight and prevent the spread of

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

s. Components of intravascular immunity include the

cellular immune response Cell-mediated immunity or cellular immunity is an immune response that does not involve antibodies. Rather, cell-mediated immunity is the activation of phagocytes, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in ...

and the macromolecules secreted by these cells. It can result in responses such as

inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

and immunothrombosis. Dysregulated intravascular immune response or pathogen evasion can create conditions like

thrombosis Thrombosis (from Ancient Greek "clotting") is the formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system. When a blood vessel (a vein or an artery) is injured, the body uses platelets (thro ...

sepsis Sepsis, formerly known as septicemia (septicaemia in British English) or blood poisoning, is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. This initial stage is follo ...

, or

disseminated intravascular coagulation Disseminated intravascular coagulation (DIC) is a condition in which blood clots form throughout the body, blocking small blood vessels. Symptoms may include chest pain, shortness of breath, leg pain, problems speaking, or problems moving parts o ...

.{{Cite journal, last=Jenne, first=Craig N., last2=Kubes, first2=Paul, date=2015-05-19, title=Platelets in inflammation and infection, journal=Platelets, volume=26, issue=4, pages=286–292, doi=10.3109/09537104.2015.1010441, pmid=25806786, issn=0953-7104

Cellular Defenses

In a healthy individual, immune cells patrol blood vessels to detect and respond to danger through molecules frequently found on pathogens called

PAMPs Pathogen-associated molecular patterns (PAMPs) are small molecular motifs conserved within a class of microbes. They are recognized by toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) in both plants and animals. A vast arra ...

, and molecules that are released by damaged cells,

DAMPs Damage-associated molecular patterns (DAMPs) are molecules within cells that are a component of the innate immune response released from damaged or dying cells due to trauma or an infection by a pathogen. They are also known as danger-associated m ...

. Immune cells involved in intravascular surveillance are

neutrophil Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying in ...

monocytes Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also infl ...

, invariant natural killer T cells, kupffer cells, platelets, and mast cells. These cells express particular receptors such as

toll-like receptor Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. They are Bitopic protein, single-pass membrane-spanning Receptor (biochemistry), receptors usually expressed on sentinel cells such as macrophage ...

s and proteins like CD36 that allow them to recognize and respond to danger signals. Endothelial cells lining the vasculature are also a part of the intravasculature's cellular defense system. They express molecules such as,

CD14 CD14 (cluster of differentiation 14) is a human protein made mostly by macrophages as part of the innate immune system. It helps to detect bacteria in the body by binding lipopolysaccharide (LPS), a pathogen-associated molecular pattern (PAMP). C ...

, TLR2, TLR4,

TLR9 Toll-like receptor 9 is a protein that in humans is encoded by the ''TLR9'' gene. TLR9 has also been designated as CD289 (cluster of differentiation 289). It is a member of the toll-like receptor (TLR) family. TLR9 is an important receptor expresse ...

, MD2, and MyD88, to detect bacteria in the blood. Leukocytes move through blood vessels using protein-protein interactions between cells and are also assisted by blood flow. Circulating immune cells behave differently in the presence and absence of an infection. For example, in the absence of an invader, monocytes migrate randomly throughout the microvasculature, cerebral vessels, and mesentery vessels. However, in the presence of an invader, monocytes emigrate to the infected area. Similarly, neutrophils use a rolling mechanism to counteract the blood flow and localize to the infected area. In a healthy state, neutrophils have been observed to exhibit a similar but brief crawling mechanism. The function and precise mechanism is not yet known.

Immune Responses

Inflammation

For more details on this topic, see

Inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

. Inflammation is an immune response in the body tissue due to stimulation of immune cells by pathogens, DAMPs, or stress. The vasculature provides a means of transportation for alerting and recruiting immune cells.

Immunothrombosis

Thrombosis is the formation of blood coagulation and platelet aggregation and may result in lack of blood flow through the circulatory system. The depletion of oxygen may cause irreversible damage to organs. However, in other circumstances, the physiological process can be beneficial for the body. This process is known as immunothrombosis. The process isolates infections using blood clots formed by activated platelets,

leukocytes White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...

, and coagulation factors assist leukocytes in adhering and migrating to infected areas. Activated platelets produce

fibrin Fibrin (also called Factor Ia) is a fibrous, non-globular protein involved in the clotting of blood. It is formed by the action of the protease thrombin on fibrinogen, which causes it to polymerize. The polymerized fibrin, together with platele ...

in the blood vessel which seal leaky vessels and are important in blood coagulation. Fibrin provides a matrix to trap pathogens and recruit immune cells. Characteristics such as elongation and thickness of fibrin and protofibril, the precursor of fibrin, are determined by many factors including environmental conditions, physiological conditions, and branching of the fibrin fibers. This in turn influences the clot structure such as permeability, stiffness, and how easily the clot can be retracted. The shift from immunothrombosis to a more pathogenic thrombosis is due to dysregulated immunothrombosis. Branches of immunology