Inflammation

Inflammation is a bodily response to cell damage.

The four cardinal signs of inflammation, as described by the Roman physician and science writer Celsus, are:

• Rubor - redness
• Tumor - swelling
• Calor - heat
• Dolor - pain

The fifth sign, which is somethimes present, is loss of function, or functio laesa - this was originally described and added to the four signs described by Celsus by another Roman physician and science writer, Galen, but was popularized in the 1800s by Rudolph Virchow, the "Father of Modern Pathology".

Functions:

• To destroy and remove pathogens and debris.
• To confine pathogens; prevent spread of infection.
• To repair or replace damaged tissue (sets stage for wound repair).

Vasodilation and Increased Permeability of Blood Vessels

Pattern recognition receptors (PRRs): Receptors that recognized conserved molecular sequences associated with a number of different pathogens (PAMPs - pathogen associated molecular patterns). PRRs are found on the surface and in the cytoplasm of macrophages, dendritic cells, mucosal epithelial cells, endothelial cells, and lymphocytes. Some secreted molecules recognize PAMPs and act as PRRs as well.

For a moderate list of the PRRs click here.

Secreted PRRs:

Circulating acute phase proteins, like C-reactive protein, mannose-binding lectin, and complement proteins C3b and C4 bind to PAMPs on the surface of a number of different pathogens. This causes opsonization and phagocytosis of the pathogen and activation of the complement cascade.

Cytoplasmic PRRs:

Intracellular receptors recognize nucleic acid sequences, cell wall components of gram-positive and gram-negative bacteria, and a number of other pathogen associated molecules. Interaction with their ligands activates cytokine production and HLA upregulation.

Phagocytosis Receptors:

Macrophages have cell-surface receptors that recognize certain PAMPs, including, those containing mannose. When a pathogen displays a cell surface polysaccharide containing mannose it is engulfed into a phagosome.

Toll-Like Receptors (TLRs):

Surface membrane receptors that recognize a number of different PAMPs. There are at least 10 different TLRs.

 

Binding of the pathogen to the TLR initiates a signaling pathway leading to the activation of the transcription factor NF-κB. NF-κB turns on cytokine genes, such as those for tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), and chemokines, which attract white blood cells to the site. All of these effector molecules lead to inflammation at the site.

Mast cells, injured tissue cells, neutrophils, lymphocytes, and basophils all release inflammatory mediators as well.

The release of histamine, kinins, and prostaglandins causes vasodilation and increased permeability of blood vessels.

Histamine causes vasodilation, increases vascular permeability, and is chemotactic for eosinophils.

Kinins cause clotting, vasodilation, increased vascular permeability, and pain.

• Factor XII (Hageman Factor) is activated by endotoxin, uric acid, calcium pyrophosphate, and basement membrane proteins (collagen).
• XIIa activates Factor XI to initiate clotting and cleaves prekallikrein to kallikrein.
• Kallikrein converts plasminogen to plasmin, HMW kininogen to bradykinin, and cleaves C5 to release C5a and C5b.
• C5a stimulates inflammation

Arachidonic acid metabolites

Cyclooxygenase products

• Prostaglandins:
• PGE₂ increases vascular permeability, sensitizes to pain, and is pyrogenic.
• PGI’s cause vasodilation.
• Thromboxanes cause vasoconstriction.

Lipooxygenase products

• Leukotrienes are produced by mast cells, basophils, macrophages, and eosinophils.
• LTB4 (SRS-A) is chemotactic, causes vasoconstriction, and increases endothelial stickiness.
• LTC and LTD cause bronchoconstriction, allergy, increase vascular permeability

Blood clots can form around an abscess to prevent dissemination of the infection.

Epithelial mucosal cells increase their release of b- defensins (broad spectrum antimicrobial proteins) when the epithelial barrier has been breached and the underlying connective tissue is inflammed.

Phagocyte Migration and Phagocytosis

Phagocytes have the ability to stick to the lining of the blood vessels (margination).

They also have the ability to squeeze through blood vessels (emigration).

PMNs show up first and release ROI (reactive oxygen intermediates), like superoxide anions, hydroxyl ions, hydrogen peroxide, and the enzyme myeloperoxidase, which converts hydrogen peroxide to hypochlorous acid (HOCl, which dissociates to H⁺ and OCl^-, the hypocholorite anion, basically bleach). PMNs also release defensins and are phagocytic.

Pus is the accumulation of damaged tissue and dead microbes, granulocytes, and some macrophages. Generally macrophages show up late to clean up the cellular debris (debride the wound) and set the stage for wound healing.

Tissue Repair

A tissue is repaired when the stroma (supporting tissue) or parenchyma (functioning tissue) produces new cells.

Stromal repair by fibroblasts produces scar tissue.

Fever

Fever is an abnormally high body temperature produced in response to a bacterial or viral infection.

Bacterial endotoxins and interleukin-1 can induce fever.

A chill indicates a rising body temperature; crisis (sweating) indicates that the body’s temperature is falling.