Module 3 Flashcards
A series of reactions occur at the site of injury, what does the series of reactions constitute? What are the two purposes of these reactions?
Inflammatory Reaction
Purpose:
- To destroy or limit the spread of the injurious agent.
- To allow for repair or replacement of the damaged tissue.
How is inflammation best described?
The characteristic response of living tissues to an injury.
What are 5 important factors of inflammation?
- Inflammation is a dynamic process that starts with the injury and culminates with healing or repair.
- It is primarily a protective response, but it may be potentially harmful.
- The inflammatory reaction is non-specific: i.e., a stereotyped process regardless of the
nature of the injury. - The intensity, duration and outcome of the inflammatory reaction are modified by a
variety of host factors and factors related to the injurious insult or etiologic agent. - In order for inflammation to occur, the injury must be non-lethal.
What are the two types of inflammation? Describe each:
Acute
- The initial, rapid response to infections and tissue damage
- It typically develops within minutes or hours and is of short duration, lasting for several hours or a few days.
- Its main characteristics are the exudation of fluid and plasma proteins (edema) and the emigration of leukocytes, predominantly neutrophils (also called polymorphonuclear leukocytes or PMNs).
- When the injurious agent is removed, the reaction subsides and residual injury is repaired.
Chronic
- If the initial response fails to clear the stimulus, the reaction progresses to a protracted type of inflammation
- Chronic inflammation may follow acute inflammation or arise de novo.
- It is of longer duration and is associated with more tissue destruction, the presence of lymphocytes and macrophages, the proliferation of blood vessels, and fibrosis (tissue repair).
What are 5 causes of inflammatory reactions?
- Infection
- i.e., invasion and multiplication within tissues by organisms.
- These, of course, include various bacteria, fungi, viruses and protozoa, which in many instances, cause damage by release of toxins which directly or indirectly destroy host cells. - Trauma
- this includes penetrating injury (e.g. stab wound, a wood sliver), blunt trauma, thermal injury (excessive heat or cold), chemical injury (acid or alkali).
- Recall that necrosis is accompanied by an inflammatory response (triggered by molecules released by necrotic cells). - Immunologically-mediated (humoral or cellular).
- As a result of the loss of blood supply (ischemia).
- Foreign bodies - (splinters, dirt, sutures) may elicit inflammation by themselves or because they cause traumatic tissue injury or carry microbes.
What do acute inflammatory reactions include?
1) Vascular changes
2) Cellular Events
3) Mediation by Chemical Substances
Acute Inflammation
What do the vascular reactions consist of? What are they designed to do?
Changes in the flow of blood, and permeability of vessels
- both designed to maximize the movement of plasma proteins and leukocytes out of the circulation and into the site of infection or injury
Acute Inflammation
Explain the 3 steps following immediately after injury:
1) Immediately following an injury, there is frequently a transient vasoconstriction of arterioles.
- This is mediated by nerves to the smooth muscle within the arteriolar walls (i.e. it is a neurogenic response).
- This process is brief and usually lasts up to five minutes.
II. The transient vasoconstriction is followed by the hemodynamically more important vasodilatation, first of arterioles and then the remaining microcirculation (capillaries, venules).
o Increased blood flow = redness (rubor), heat (calor).
- Vasodilation is induced by inflammatory mediators such as histamine.
- This results in increased blood flow to the injured area and thus, on gross examination, the area appears red or erythematous.
- With very mild injuries, the vascular changes may not proceed any further than this stage. With more severe injury, the vasodilation is soon followed by slowing or stasis of the blood flow.
III) Vasodilatation is often accompanied by increased vascular permeability.
- This refers to the outpouring of fluids and proteins from the blood vessels; exudation of fluids = edema/swelling (tumor).
- This affects firstly and predominantly venules; however, capillaries and arterioles are also involved. Increased vascular permeability is induced by histamine, kinins, and other mediators that produce gaps between endothelial cells, by direct or leukocyte-induced endothelial injury, and by increased passage of fluids through the endothelium.
- The process of the escape of plasma and plasma proteins along with white blood cells from the vessel is known as exudation.
- This inflammatory exudate accounts for an increase in the volume of interstitial fluid (edema) and tissue swelling at the local site of injury.
Acute Inflammation
What occurs during increased vascular permeability?
- Increased vascular permeability allows plasma proteins and leukocytes, the mediators of host defense, to enter sites of infection or tissue damage.
- Recall Starling’s forces and control of normal fluid balance by an opposing set of forces: Fluid moves out by osmotic pressure of interstitial fluid and intravascular hydrostatic pressure; and fluid moves in by osmotic pressure of plasma proteins and tissue hydrostatic pressure.
- These forces are usually balanced such that net movement is outward and quite small and is outward in direction. this excess interstitial fluid drains into the lymphatics and under normal conditions no edema occurs. In inflammation the arteriolar vasodilatation is followed by a rise in pressure within the capillaries and venules. This rise in hydrostatic pressure in the microcirculation and the leaky endothelium due to permeability changes results in the passive transport of a large volume of fluid along with small molecules and cells into the interstitium (space between tissues) - an exudate.
- Reduction of intravascular osmotic pressure and increased interstitial osmotic pressure causes further impairment of return of fluid to blood vessels (venules) producing marked inflammatory edema. Thus, an inflammatory exudate is characterized by having a high specific gravity ( > 1.020), high protein levels ( > 2-4 gms/dl), and numerous cells or cell fragments.
- In contrast, a transudate is due to a rise in hydrostatic pressure, reduced plasma proteins (oncotic pressure), lymphatic obstruction or Na+ retention. It consists of fluid similar to water with a low specific gravity (<1.0120) with little to no protein or cells (or cell fragments).
- Transudates are NOT associated with inflammation but with clinical situations such as: heart failure, venous obstruction, malnutrition, among others (also see later Module on Disturbed Blood Flow).
What are 6 mechanisms by which the endothelium becomes leaky during inflammation?
- Endothelial cell contraction leading to wide intercellular gaps.
- Immediate transient response
- Occurs only in small venules, not in capillaries or arterioles o Mediated by histamine and other chemical mediators.
2. Junctional disruption o Delayed prolonged response o Involves structural reorganization of the cell’s cytoskeleton and disruption of intercellular junctions - Cytokine mediated (likely TNF, IL-1).
- Direct endothelial injury
- Immediate sustained response in severe injuries (e.g., thermal burns, infections,
cuts, abrasions); most clinically significant injuries induce this immediate-
sustained response.
- Sometimes a delayed prolonged response (e.g., sunburns) - Leukocyte-dependent endothelial injury
- During the cellular phase, leukocytes adhere to endothelium and may become
activated in the process, releasing ROS and proteolytic enzymes that damage
the endothelium
- Only happens in vascular beds where leukocytes adhere for longer periods of
time (e.g., Lung, glomeruli)
o Delayed prolonged response. - Increased transcytosis
- VEGF and other chemical mediators increase permeability via a vesiculovacuolar
organelle - New blood vessel formation
- New vessels are “leaky” until they form intercellular junctions
- New vessels also have increased density of receptors for histamine, VEGF etc.
What is local hemoconcentration?
What does this result in?
The viscosity of blood is increased as a result of fluid loss from the vessel. This leads to packing or sludging of red blood cells, therefore slowing blood flow.
- As intravascular blood flows more slowly we start to see the CELLULAR EVENTS.
The cellular elements in a blood vessel normally travel in a stream in the centre of the vessel.
- As a result of loss of fluid because of increased vascular permeability and of slowing of blood flow in the vessel, the circulating cellular elements, including white blood cells, become displaced to the periphery of vessels where they come in to contact with the endothelial cells.
- This is referred to as margination.
What is the most important feature of inflammation?
accumulation of leukocytes in the affected tissue
What do leukocytes do?
- engulf, degrade bacteria, immune complexes and cell debris
- release lysosomal enzymes
- release chemical mediators
- release toxic radicals.
What is the main cellular phase of acute inflammation? What is it mediated by?
Migration of leukocytes (exudation) through vessel walls during inflammation into adjacent tissues is the main cellular phase of acute inflammation and occurs in a multi-step process that is mediated by adhesion molecules and cytokines.
What is the multistep process of the cellular phase of acute inflammation?
- Adhesion of leukocytes to endothelium at the site of inflammation
- Trasmigration of leukocytes through vessel wall
- Movement of cells towards the offending agent
Acute inflammation - cellular events
Margination
When blood is viscous, as a result of stagnation of blood flow,
WBCs are pushed to the periphery of vessels because of sludging of RBCs (rouleaux formation).
- WBCs are pushed to the periphery because they are smaller particles.
Acute inflammation - cellular events
Rolling
WBCs tumble and transiently adhere to the endothelium via selectin molecules, i.e. pavementing.
- The rolling and adhesion of cells in inflammation is the result of interaction of cell adhesion molecules (CAMs) in both endothelial cells and leukocytes.
- These molecules are either expressed, induced or enhanced by chemical mediators.
Acute inflammation - cellular events
Adhesion
WBCs firmly stick to endothelial surfaces.
- Firm adhesion of leukocytes to endothelium is mediated by a family of leukocyte surface proteins called integrins.
- Molecules of the immunoglobulin superfamily expressed on the endothelial cells bind to integrins expressed on WBCs.
- ICAM-1 on endothelial cells binds integrins CD11/CD18 (LFA-1 and Mac1) on leukocytes VCAM-1 on endothelial cells binds integrin VLA-4.
- Cytokines such as TNF and IL-1 induce expression of ICAM-1, VCAM-1 on endothelial cells.
- Integrins are normally expressed on leukocytes but do not adhere to their appropriate ligands unless activated by chemical
mediators. - The leukocytes stop rolling, and engagement of integrins by their ligands delivers signals leading to cytoskeletal changes that arrest the leukocytes and firmly attach them to the endothelium.
Acute inflammation - cellular events
Transmigration/Diapedisis
Transmigration / Diapedesis: After being arrested on the endothelial surface, leukocytes migrate through the vessel wall by squeezing between the widened endothelial cell junctions and then penetrate the basement membrane. This process is active and requires energy.
- This extravasation of leukocytes, called transmigration, occurs mainly in postcapillary venules, the site at which there is maximal retraction of endothelial cells.
- This movement of leukocytes across the basement membrane to the extravascular space is called diapedesis. Mediated via PECAM-1/CD31 expressed on both the leukocytes and the endothelial cells.
- Basement membrane is focally degraded by collagenases secreted by leukocytes. Red blood cells (RBCs) often leave the vascular lumen behind a white blood cell.
- This is a passive phenomenon and results from hydrostatic pressures forcing red blood cells out of the permeable vessel.
Acute inflammation - cellular events
What mediates the attachment of leukocytes to endothelial cells?
Complementary adhesion molecules on the two cell types whose expression is enhanced by cytokines.
- Cytokines are secreted by cells in response to microbes and other injurious agents, thus ensuring that leukocytes are recruited to the tissues where these stimuli are present.
The two major families of molecules involved in leukocyte adhesion and migration are the selectins and integrins
- The adhesion molecules most important at this stage (L-Selectin) are present on the leukocytes’ surface [i.e., inflammatory cells capable of phagocytosis such as PMNs (polymorphonuclear neutrophils), macrophages, basophils, etc.] and bind to CD34 on the endothelial cells.
- In the endothelial cells, two main subclasses of adhesion molecules have been described: P-Selectin and E-selectin (endothelial cell - leukocyte adhesion molecules or ELAM-1).
- They interact with and bind to the phagocyte adhesion molecules (Sialyl-Lewis X modified protein on the leukocytes).
What is the first type of leukocyte to appear in the inflamed area? Why?
PMNs
- This is partly due to the fact that they are faster and more numerous. - Thus, the cell type in the inflammatory response varies with the age of the lesion and type of stimulus.
- In most acute inflammations, PMNs (neutrophils) predominate (early - dependent on C5a generation, in the first 6 - 24 hours) to be later replaced by monocytes or macrophages (days later).
- Exceptions: viral infections (lymphocytes first).
- This is also due to the fact that the PMN is short-lived, the monocyte migration is sustained longer and chemotactic factors for PMN and monocytes are activated at different periods.
- Once outside the blood vessel, monocytes are referred to as macrophages or histiocytes.
- Once in the tissues, these cells stimulate and control subsequent inflammatory response and interact with the immune system.
What is chemotaxis?
The unidirectional migration of leukocytes towards an attractant or locomotion oriented along a chemical gradient.
- Chemotactic agents bind to GPCR receptors on leukocyte surfaces, activate intracellular signaling pathways, and induce polymerization of actin at the leading edge of the cell.
- This is a calcium-dependent process.
- The leukocyte then moves by extending filopodia that pull the cell in the right direction.
- All WBCs respond to such stimuli but PMNs and monocytes are most reactive.
Describe exogenous and endogenous chemotactic factors:
Exogenous: bacterial and viral proteins/products
Endogenous: Components of the complement system; complement fractions (C5a);
products of the lipoxygenase pathway of Arachidonic acid metabolism, particularly leukotriene B4 (LTB4); and cytokines (especially chemokines like IL-8)
Describe activation:
- What happens?
- What do the responses include?
Leukocyte surface receptors recognize the offending agents (e.g., microbes, necrotic tissue, Ag-Ab complexes, etc.,) which in turn triggers intracellular signalling pathways within the leukocytes that result in functional responses - “activation”
- These responses include production of arachidonic acid metabolites, degranulation, activation of oxidative burst, and secretion of cytokines to amplify the inflammatory response.
What does it mean when you find a large number of neutrophils at a site of injury?
- Acute reactions?
- After 2-3 days
- Normal life span of macrophages? pH?
It is a major histological feature of acute inflammation
In most acute reactions, neutrophils appear first and are followed by macrophages
- This is due to their greater mobility and also because there are many more neutrophils than monocytes in the circulation
After two or three days, macrophages begin to outnumber neutrophils in most inflammatory reaction sites (an exception to this would be in those instances in which the injurious stimulus continues to operate)
The normal life span of macrophages is much greater (months to years) versus the life span of an average neutrophil (2-4 days)
- Also, macrophages are relatively resistant to the lower pH (acidity) which is found at inflammatory sites.
What is phagocytosis?
The process of ingestion of particulate matter, i.e., bacteria, foreign debris, by the cell
- It literally means “cell eating”
- It may be regarded as the culmination of an inflammatory reaction
What cells have the greatest phagocytosis properties? What other cells are also phagocytosis?
Neutrophils and macrophages
- Eosinophils
What cells are not phagocytosis?
Lymphocytes and plasma cells
Phagocytosis is the process of whereby some of the ____ at an _______ _____ is ______ (primarily done by ______); the remained is removed by ________.
1) Debris
2) Inflammatory
3) Site
4) Removed
5) Macrophages
6) Lymphatics
What are the three steps of phagocytosis?
- Recognition and attachment (may be mediated by opsonins).
- Engulfment (pseudopods flow around organisms/particle with complete enclosure followed by fusion with lysosomal granules resulting in a phagolysosome; requires the presence of calcium and magnesium).
- Killing and/or degradation (Lysosomes then fuse to the sack and release their enzymes directly into this sack or vacuole).
Describe the first stage of phagocytosis:
Most micro-organisms are not recognized until they are coated by naturally occurring serum factors called opsonins. These are substances which coat bacteria and render them more susceptible to phagocytosis. They are usually immunoglobulins (e.g., IgG) or complement fractions (e.g., C3b). The opsonized particles attach to two receptors or PMNs or macrophages:
- a receptor for the Fc fragment of IgG (to react with the IgG opsonin) and
- a receptor to C3b (to react with C3b opsonin)
Describe the second stage of phagocytosis:
Pseudopods flow around organisms with complete enclosure followed by fusion with lysosomal granules resulting in a phagolysosome
- While this takes place, leakage of enzymes and metabolic products (H2O2) from the leukocyte into the extracellular space occurs, thus increasing the tissue damage
- This process of engulfment requires the presence of calcium and magnesium and is associated with calcium transport across the plasma membranes
- The Ca acts as a second messenger to initiate the cell events in the microfilaments and microtubules, culminating with engulfment
- The biochemical events involved in phagocytosis are somewhat similar to those of chemotaxis; it is associated with receptor-ligand binding, phospholipase with activation and there is an eventual increase in Ca in the cytosol
