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Introduction to pathology
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Inflammation classically describes four key signs - each of which have a latin derivation.
Calor or heat, dolor or pain, rubor or redness, and tumor or swelling.
Sometimes these four signs combine to cause a fifth sign, which is functio laesa or temporary loss of function due to pain or swelling.
Okay - so inflammation usually starts with some stimuli, like a pathogen.
Now, even though pathogens are a common cause of infection which can lead to inflammation, inflammation can be caused by other things as well like toxins and trauma.
Ultimately, the goal of inflammation is to respond to the stimuli and restore balance.
Oftentimes that includes eliminating the cause of tissue injury, clearing out necrotic or dead cells, and starting tissue repair.
Broadly speaking, inflammation can be triggered by external and internal factors.
External factors can be non-microbial or microbial. Non-microbial factors include allergens, irritants, and toxic compounds.
Now, the two main microbial factors that trigger inflammation are virulence factors and pathogen associated molecular patterns or PAMPs.
Virulence factors are molecules that help pathogens colonize tissues and cause infection.
PAMPs are small molecules with conserved patterns that are shared across many different pathogens, including bacterial wall components like peptidoglycan, lipopolysaccharide or LPS, and lipoteichoic acid, and fungal wall components like mannan.
For intracellular pathogens, like viruses, PAMPs might include the viral RNA or DNA.
Our immune system recognizes virulence factors and PAMPs as foreign substances, and can trigger an inflammatory response against them.
Now, in terms of internal factors, it turns out that there’s an endogenous equivalent to PAMPs, called damage associated molecular patterns or DAMPs.
DAMPs are intracellular proteins that get released when a cell’s plasma membrane is injured or when a cell dies.
So DAMPs are a signal that there’s serious cell damage and they trigger inflammation.
Now, PAMPs and DAMPs are recognized by Pattern Recognition Receptors or PRRs, which are cell surface receptors on various leukocytes that help to activate those cells and spark the inflammatory response, which can be thought of as the innate immune system.
Key features are that this response is non-specific - meaning that PRRs don’t distinguish one specific pathogen from another, although they can distinguish between broad categories like viruses from bacteria.
Inflammation is a natural response that our body has to injury or infection. It helps protect us from potential harm and promotes healing. Inflammation involves blood vessels dilating and becoming more permeable, and attracting more immune cells and fluid into local tissue. The classical signs of inflammation are heat, pain, redness, swelling and they can lead to a loss of function. The inflammatory response ends with wound repair and resolution, either restoring the initial tissue integrity, or leaving a fibrous scar.
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