Most mediators act through cell surface receptors. Exceptions are those that have a direct enzymatic effect (lysosomal enzymes) or direct toxic effect (reactive oxygen species).Stimulus response coupling leads to biological response.Mediators can stimulate target cells to release secondary mediators.Mediators are short lived. They are rapidly inactivated (helps prevent harmful effects).
Chemical mediators of immunity
PLASMA DERIVED MEDIATORS
Most mediators act through cell surface receptors.
Exceptions are those that have a direct enzymatic
effect (lysosomal enzymes) or direct toxic effect
(reactive oxygen species).
Stimulus response coupling leads to biological
Mediators can stimulate target cells to release
Mediators are short lived. They are rapidly inactivated
(helps prevent harmful effects).
Series of inactive proteins circulating in the plasma
that are converted to proteolytic enzymes that activate
Interrelated systems that are triggered by activation of
Hageman factor (Factor XII of the coagulation
cascade). Activated by endotoxin, activated platelets or
contact of plasma with damaged tissue (collagen,
3 interrelated systems are active within this
1. Kinin system
2. Complement system
3. Clotting system
Factor XII of intrinsic coagulation cascade
Negatively charged surfaces
Proteases from inflammatory cells
Activation of fibrinolytic system
Provides an amplification system
Activated by Hageman factor (XIIa)
Release of vasoactive nonapeptide bradykinin
Generated from the plasma
Increased vascular permeability
Contraction of smooth muscle
Stimulates release of histamine
Activates the arachidonic acid cascade
Coagulation (clotting) system
Forms a fibrinous meshwork at an injured or inflamed
Prevents the spread of infection
Keeps microorganisms and foreign bodies at the site of
greatest inflammatory cell activity
Forms a clot that stops bleeding
Provides a framework for repair and healing
Main substance is an insoluble protein called fibrin
results in production of thrombin, factor Xa and
formation of fibrinopeptides
thrombin cleaves fibrinogen to form fibrin and
enhances leukocyte adhesion
fibrinopeptides are chemotactic and increase vascular
factor Xa increases vascular permeability and
The blood clotting system or coagulation pathway, like the
complement system, is a proteolytic cascade.
Each enzyme of the pathway is present in the plasma as a
zymogen, in other words in an inactive form, which on activation
undergoes proteolytic cleavage to release the active factor from
the precursor molecule.
The coagulation pathway functions as a series of positive and
negative feedback loops which control the activation process.
The ultimate goal of the pathway is to produce thrombin, which
can then convert soluble fibrinogen into fibrin, which forms a
The generation of thrombin can be divided into three phases,
the intrinsic and extrinsic pathways that provide alternative
routes for the generation of factor X, and the final common
pathway which results in thrombin formation
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