Mediators of inflammation
Inflammatory mediators can be classified based on their origin:
1. Plasma-derived mediators
These are produced mainly by the liver and circulate in an inactive form, becoming activated during
inflammation. They include:

Complement system (e.g. C3a, C5a – chemotactic and pro-inflammatory)

Kinin system (e.g. bradykinin – increases vascular permeability and pain)

Coagulation and fibrinolysis systems (e.g. thrombin, fibrin degradation products – involved
in clot formation and resolution, and also inflammation)
2. Cell-derived mediators
Produced by inflammatory and resident cells (e.g. macrophages, mast cells, endothelial cells).
They can be:


Preformed mediators (stored in granules):
o
Histamine (vasodilation, increased permeability)
o
Serotonin (mainly in platelets – vasoconstriction or vasodilation depending on
context)
o
Lysosomal enzymes (degrade microbial and host tissues)
Newly synthesized mediators (produced after stimulation). They require transcription, and
so time. :
o
Prostaglandins (vasodilation, fever, pain)
o
Leukotrienes (increased vascular permeability, chemotaxis)
o
Platelet-activating factor (PAF) (activates platelets, vasodilation,
bronchoconstriction)
o
Reactive oxygen species (ROS)
o
Nitric oxide (NO) (vasodilation, microbial killing)
o
Cytokines (e.g. TNF, IL-1 – regulate inflammation and immune responses)
Complement system
The complement system is composed of over 20 plasma proteins, mainly serine proteases, which
circulate in an inactive (zymogen) form. Once triggered, they are activated in a cascade-like
enzymatic sequence, leading to opsonization, inflammation, and direct lysis of pathogens.
The central event in all three pathways is the activation of C3, forming C3 convertase, which
amplifies the response and leads to C5 activation and membrane attack complex (MAC) formation.
Classical Pathway
Trigger: Antigen–antibody complexes (especially IgG or IgM)
Key steps:

C1 complex (C1q, C1r, C1s) binds to the Fc region of antibodies bound to antigens.

This activates C1r and C1s, which then cleave:
o
C4 → C4a + C4b
o
C2 → C2a + C2b

C4b and C2a combine on the surface to form the classical C3 convertase (C4b2a).

C3 convertase cleaves C3 → C3a + C3b.

C3b can then participate in opsonization or join the complex to form C5 convertase.
Functions:

Links adaptive immunity (via antibodies) to innate immunity.

Promotes opsonization, inflammation, and MAC formation.
Alternative Pathway
Trigger: Direct contact with microbial surfaces (e.g., endotoxins, fungal cell walls); no antibodies
required
Key steps:

Spontaneous hydrolysis of C3 produces C3(H₂ O) in plasma.

C3(H₂ O) binds factor B, which is then cleaved by factor D into Ba and Bb.

C3(H₂ O)Bb acts as a fluid-phase C3 convertase, generating more C3b.

C3b binds to microbial surfaces and associates with factor B, then cleaved again by factor
D to form C3bBb, the alternative C3 convertase.

This is stabilized by properdin and produces even more C3b (positive feedback).

When another C3b binds, it forms the alternative C5 convertase (C3bBbC3b).
Functions:

Provides a rapid, antibody-independent response.

Especially important in the early innate response against pathogens.
Lectin Pathway
Trigger: Binding of mannose-binding lectin (MBL) or ficolins to specific sugars (like mannose or
fucose) on microbial surfaces.
Key steps:

MBL or ficolins bind to microbial carbohydrates.

This activates MBL-associated serine proteases (MASP-1 and MASP-2).

MASPs cleave:

o
C4 → C4a + C4b
o
C2 → C2a + C2b
C4b and C2a form the C3 convertase (C4b2a) – same as in the classical pathway.

The rest of the pathway proceeds identically, including C3 and C5 activation, and eventually
MAC formation.
Functions:

Allows innate immune recognition of carbohydrate patterns on pathogens.

Does not require antibodies, making it important in the early phase of infection.
MAC formation
After the formation of C5 convertase (via any complement pathway), C5 is cleaved into C5a and
C5b. C5a acts as a potent anaphylatoxin and chemoattractant, while C5b initiates the assembly of
the Membrane Attack Complex (MAC). C5b binds sequentially to C6 and C7, forming the C5b67
complex, which inserts into the pathogen’s lipid bilayer. Next, C8 binds to the complex and further
anchors it into the membrane, beginning to disrupt membrane integrity. Finally, multiple C9
molecules polymerize around the complex, forming a cylindrical pore in the membrane. This polyC9 structure completes the MAC, creating a transmembrane channel that allows the uncontrolled
influx of ions and water, leading to osmotic lysis and cell death of the targeted microbe.
Effector functions of the complement system
The complement system plays a crucial role in innate immunity through multiple effector
mechanisms:

Opsonization and phagocytosis:
C3b, one of the key fragments generated during complement activation, binds to microbial
surfaces and acts as an opsonin, enhancing phagocytosis by neutrophils and macrophages
through complement receptors. It effectively "tags" the pathogen for destruction, functioning
almost like a new antigenic marker for immune cells.

Inflammation:
The small fragments C3a and C5a, known as anaphylatoxins, are potent pro-inflammatory
mediators. They recruit and activate leukocytes, especially neutrophils, to the site of
infection. C5a is particularly strong and also increases vascular permeability and adhesion
molecule expression on endothelial cells.

Cytolysis (cell lysis):
The terminal pathway leads to the formation of the Membrane Attack Complex (MAC),
composed of C5b-C9, which inserts into the microbial membrane, forming a pore that
results in osmotic lysis of the pathogen.

Anaphylatoxins:
Both C3a and C5a also act as anaphylatoxins, promoting degranulation of mast cells and
release of histamine, contributing to vasodilation, edema, and increased vascular
permeability during inflammation.
Kinin system
Kinins are vasoactive polypeptides generated in plasma and inflammatory exudates through the
action of proteolytic enzymes known as kininogenases.
The precursors of kinins are alpha-globulins called kininogens. There are two main types of
kininogens: High Molecular Weight Kininogen (HMWK) and Low Molecular Weight Kininogen
(LMWK).
Kininogenases, also known as kallikreins, are enzymes that cleave kininogens to release kinins,
such as bradykinin and kallidin. These kininogenases can be:
o
Present in plasma (plasma kallikrein)
o
Released or activated by damaged tissue, neutrophils, or other components of the
inflammatory response
Enzymes like C1-activated esterase (C1-INH) and elastase can also modulate kinin release.
Although small molecules, kinins exert powerful local effects:

Pain induction (via sensory nerve stimulation)

Vasodilation of arterioles

Increased vascular permeability

Chemotaxis of immune cells

Stimulation of eicosanoid production (e.g., prostaglandins, leukotrienes)

Activation of leukocytes (degranulation, ROS production)

Activation of fibroblasts → promotes connective tissue formation

Contraction of smooth muscle cells
If produced or released in large amounts, kinins can cause systemic effects:
o
Hypotension (due to systemic vasodilation)
o
Shock-like symptoms (vasodilatory collapse, sometimes called “anaphylactoid
reaction”)
The kinin system is closely linked with the coagulation and fibrinolytic systems:
o
Hageman factor (Factor XII) activates both kallikrein and the intrinsic coagulation
pathway.
o
Bradykinin can enhance inflammatory and coagulative responses, creating a
feedback loop in inflammation, vascular injury, and tissue repair.
Coagulation and fibrinolytic system
Hemostasis is the process that stops bleeding following a vascular injury. It has two main
components:
1. Primary hemostasis: involves the action of platelets
2. Secondary hemostasis: involves the coagulation cascade. The coagulation process also
plays a role in inflammation.
Hemostasis Is like build a wall. Plateles act as the bricks (primary plug), and coagulation process
act as mortar (secondary and robust plug).
Platelets and primary hemostasis
Platelets are produced in the bone marrow from common myeloid progenitors (CMP) and
megakaryocytes. Megakaryocytes are large cells that give rise to platelets, which are cell
fragments without a nucleus. Platelets have a disc shape and contain actin and microtubules,
allowing them to change shape. They also contain granules.

Dense granules contain ADP, ATP, and serotonin, which promote platelet aggregation.

Alpha granules contain several important molecules:
o
von Willebrand factor, fibrinogen, fibronectin, and thrombospondin, which help form
platelet aggregates
o
platelet factor IV (CXCL4) and beta-thromboglobulin, which inhibit heparin and
support coagulation, inflammation, and tissue repair
o
P-selectin, which mediates leukocyte adhesion
o
growth factors like PDGF, involved in wound healing and atherosclerosis
Platelets change shape after activation, forming multiple protrusions to cover a larger area at the
site of injury.
Primary hemostasis is divided into three steps:
1. Platelet adhesion to components of the subendothelium.
At the site of vascular injury, components of the extracellular matrix (such as collagen) are
exposed, indicating vessel disruption. The subendothelium binds von Willebrand factor
(vWF), which normally circulates in plasma. Platelets adhere via the glycoprotein receptor
GpIb, which binds to vWF anchored on the damaged site.
2. Shape change and primary platelet aggregation.
Once adhered, platelets undergo shape change, forming protrusions to increase surface
area and enhance interaction. They begin to aggregate through fibrinogen bridges that link
platelets via the GpIIb/IIIa complex, an integrin of the beta3 family. This initial aggregation is
weak and unstable. During this phase, platelets progressively become more active.
3. Platelet degranulation, full activation, and secondary aggregation.
Activated platelets release various molecules that amplify their own activation and that of
nearby platelets. Key mediators include ADP, platelet-activating factor (PAF), and
thromboxane A2. PAF is produced by platelets, endothelial cells, neutrophils,
monocytes/macrophages, mast cells, and basophils. In addition to activating platelets, PAF
contributes to inflammation by increasing vascular permeability, activating leukocytes, and
promoting chemotaxis. Thus, hemostasis and inflammation are bidirectionally linked.
After full activation, platelets degranulate, releasing their granular content. Secondary aggregation
is more robust and stable. A key player is the integrin αIIbβ3 (also known as GpIIb/IIIa), which
forms bridges with fibronectin, vWF, and fibrinogen. Additionally, thrombospondin mediates
bridging via CD36. Neutrophils and other leukocytes can bind to platelets through P-selectin and its
ligand PSGL-1.
Platelet pathology
Platelet pathologies (PP) can be divided into two categories:

Thrombocytopenia, where the platelet count is low

Thrombocytopathy, where the platelet count is normal but platelets are dysfunctional
In both cases, primary hemostasis is impaired, and spontaneous bleeding can occur.
Thrombocytopenia
Thrombocytopenia is defined as a platelet count below 150,000/µL. Spontaneous bleeding typically
occurs when the count falls below 5,000/µL.
Clinical manifestations include:

Petechiae (smaller than 3 mm)

Purpura (3–10 mm)

Ecchymoses (larger than 1 cm)

Large hemorrhages, especially in the central nervous system (such as the brain, which is
very rich in microcirculation)
The causes of thrombocytopenia can be divided into:
1. Reduced platelet production
a. Due to generalized bone marrow dysfunction, such as marrow infiltration (e.g., tumors) or
aplastic anemia
b. Due to selective suppression, for example from chronic alcohol use, certain drugs, or
infections such as measles and HIV
2. Splenic sequestration in cases of hypersplenism, which can occur in portal hypertension,
lymphomas, or leukemia
3. Increased platelet destruction
a. Immunologic:
o
In immune thrombocytopenic purpura (ITP), autoantibodies are produced against
platelets
o
These antibodies can trigger complement activation (MAC) or lead to destruction by
macrophages in the spleen or liver
o
Drug-dependent antibodies (e.g., induced by heparin, antibiotics, diuretics),
infections, or systemic lupus erythematosus (SLE) can also damage platelets
b. Non-immunologic:
o
Caused by mechanical destruction (e.g., artificial heart valves)
o
Vasculitis (inflammation of blood vessel walls)
o
Disseminated intravascular coagulation (DIC)
Coagulation alterations
Some coagulation disorders are hereditary, and they are classified based on the pattern of genetic
transmission:
• X-linked (linked to the X chromosome):
– Hemophilia A, caused by a deficiency of factor VIII
– Hemophilia B, caused by a deficiency of factor IX
→ Both conditions affect the intrinsic pathway of the coagulation cascade.
• Autosomal recessive:
– Deficiency of factors II (prothrombin), VII, X, XI, and XII
– Afibrinogenemia, which is the complete absence of fibrinogen, an essential protein for clot
formation.
Other coagulation disorders are acquired and may result from various pathological conditions:
1. Impaired production of coagulation factors, such as:
– Vitamin K deficiency, which affects the γ-carboxylation and activation of factors II, VII,
IX, and X
– Liver failure: the liver is the main site of synthesis for most coagulation factors
2. Disseminated Intravascular Coagulation (DIC):
– A complex condition involving widespread activation of coagulation, leading to
consumption of clotting factors and bleeding
3. Inactivation of coagulation factors, due to:
– Administration of heparin (an anticoagulant)
– Presence of autoantibodies against specific coagulation factors (as seen in acquired
hemophilia)
4. Hyperactivation of fibrinolysis, for example:
– Certain tumors may release plasminogen activators (PA), leading to excessive
breakdown of fibrin and impaired clot stability
Thrombosis
Thrombosis is a pathological phenomenon that occurs within blood vessels. A thrombus is a solid
mass that forms inside the vascular system, composed of a fibrin network and circulating blood
cells such as platelets and red blood cells. Unlike a normal blood clot, which typically forms outside
of vessels or in response to injury as part of the healing process, a thrombus develops abnormally
within the circulation. It is characterized by an irregular surface, is rigid in consistency, firmly
attached to the vessel wall, and tends to be friable, meaning it can easily break into fragments.
These fragments can sometimes detach and travel through the bloodstream, potentially leading to
embolism.
Thrombosis can occur in both veins and arteries. When it affects the venous system, it is referred
to as phlebothrombosis. The causes of venous thrombosis are diverse and often linked to reduced
blood flow or increased coagulability. The main risk factors include:

Cardiac insufficiency, which slows venous return and circulation

Severe trauma and extensive burns, which trigger systemic inflammation and promote clot
formation

Post-operative states, especially after major or prolonged surgeries

The post-partum period, due to hormonal shifts, vascular injury, and temporary immobility

Disseminated cancer, as some tumors release substances that activate the coagulation
system

Use of oral contraceptives, particularly those containing estrogen, which can increase the
risk of thrombosis by affecting coagulation factors
Arterial thrombosis is usually associated with damage to the arterial wall and increased platelet
activation. The main causes include:

Atherosclerosis, which is a chronic inflammatory process involving lipid accumulation and
fibrosis within the arterial wall

Rheumatic fever, caused by a group A streptococcal infection, which can lead to chronic
valvular damage and increase the risk of thrombosis

Aneurysms of the aorta or other major arteries, where altered blood flow and vascular wall
abnormalities promote thrombus formation

Temporal arteritis, an inflammatory disease of autoimmune origin affecting large and
medium-sized arteries, especially in the elderly. Also known as Horton’s disease

Smoking, which damages the endothelium and enhances platelet aggregation

Radiation exposure, which can lead to vascular injury and long-term endothelial dysfunction
Endothelial injury
Damage to or activation of the endothelium plays a central role in triggering thrombosis. It leads to:

Endothelial dysfunction and exposure of pro-thrombotic molecules such as von Willebrand
factor (vWF) and tissue factor (TF)

Activation of platelets, especially in cardiac and arterial thrombi, which are typically rich in
platelets

Decreased expression of natural anticoagulants such as thrombomodulin and tissue factor
pathway inhibitor (TFPI)

Increased expression of plasminogen activator inhibitors (PAIs), along with decreased
production of tissue plasminogen activator (t-PA), reducing fibrinolysis
Abnormal blood flow
Both turbulence and stasis can promote thrombosis:

Turbulence is commonly involved in arterial and cardiac thrombosis

Stasis is the main contributor in venous thrombosis
These conditions:

Promote endothelial activation and increase the expression of procoagulant factors

Allow platelets and leukocytes to come into closer contact with the vessel wall

Slow the washout of activated coagulation factors

Limit the inflow of natural anticoagulant substances
Hypercoagulability of the blood
This refers to an abnormal tendency of the blood to form clots, particularly in the venous
circulation. It may be:

Genetic, such as mutations in factor V (e.g. Factor V Leiden) or factor II, or inherited
deficiencies of natural anticoagulants like protein C, protein S, or antithrombin

Acquired, as seen in conditions like prolonged bed rest, immobilization, myocardial
infarction, atrial fibrillation, cancer, or the use of certain medications (e.g. oral
contraceptives)
The fate of a thrombus can vary depending on its size, location, and the body’s response. The
main possible outcomes include:

Resolution
The thrombus is completely dissolved by the fibrinolytic system, restoring normal blood
flow.

Embolization
A fragment of the thrombus may detach and travel through the circulation, becoming an
embolus. In venous thrombosis, this often leads to pulmonary embolism, while in arterial
thrombosis it can cause infarctions in organs such as the brain or kidneys.

Organization and incorporation into the vessel wall
Over time, the thrombus may be invaded by fibroblasts and endothelial cells, becoming part
of the vessel wall through a scarring process.

Organization and recanalization
Small channels may form within the thrombus, allowing partial restoration of blood flow
through the obstructed vessel. This process is known as recanalization and represents a
form of natural bypass.
Disseminated Intravascular Coagulation (DIC)
Disseminated Intravascular Coagulation (DIC) is a condition characterized by systemic
activation of the coagulation cascade, leading to the formation of small thrombi throughout the
microcirculation. This widespread clotting triggers fibrinolysis and causes a consumption of
coagulation factors and platelets, increasing the risk of bleeding.
Two key initiating mechanisms:
1. Release and exposure of tissue factor (TF) into the circulation
2. Exposure of von Willebrand factor (vWF) and subendothelial collagen due to vascular
injury
Main causes of DIC:
1. Tissue injury and TF release
o
Obstetric complications: placental abruption, retained dead fetus
o
Sepsis, especially from Gram-negative bacteria
o
Certain cancers: leukemias and mucin-producing adenocarcinomas
2. Severe endothelial injury
o
Sepsis and systemic inflammatory response syndrome (SIRS)
o
Infectious diseases (e.g., malaria)
o
Autoimmune diseases with immune complex deposition (e.g., systemic lupus
erythematosus)
o
Severe trauma or extensive burns (especially head injuries, as brain tissue is rich in
thromboplastin and lipids)
3. Stasis and abnormal vascular structures
o
Giant hemangiomas (Kasabach-Merritt syndrome)
o
Aortic aneurysms
o
Pulmonary embolism
4. Other rare causes
o
Snake bites (venoms with procoagulant activity)
o
Severe cardiac failure or shock
Pathogenesis of DIC: two-phase model
1. Coagulation activation and thrombus formation
o
The coagulation cascade is excessively activated, leading to the formation of
numerous microthrombi
o
These obstruct small vessels, causing tissue ischemia and organ dysfunction
o
Clinical signs may include neurological symptoms, skin manifestations (purpura,
cyanosis), renal failure, acute respiratory distress syndrome (ARDS), and
gastrointestinal ischemia
2. Consumption of coagulation factors and fibrinolytic activation
o
Coagulation factors and platelets become depleted
o
The fibrinolytic system is simultaneously activated, leading to hemorrhagic
manifestations such as intracerebral bleeding, gingival bleeding, hematuria,
epistaxis, and melena