Inflammation: The Body's Repair Response, and What Happens When It Persists

Inflammation is not simply redness or swelling. It involves blood vessels, immune cells and chemical signals. The response may protect tissue, cause damage, support repair or do several of these together.

Inflammation is a coordinated response Inflammation is a regulated response to infection, tissue injury or another harmful stimulus. It involves immune cells, blood vessels and inflammatory mediators. An inflammatory mediator is a chemical signal that changes blood vessels, attracts cells or alters pain, temperature and tissue function. The response can help contain or remove a cause, clear damaged material and begin repair. It can also injure healthy tissue when excessive, misplaced or prolonged. Inflammation is therefore neither automatically helpful nor automatically harmful. Its effect depends on the trigger, location, intensity, duration and the person's health. The familiar signs are not always present The classic local signs are redness, heat, swelling, pain and loss of function. They are easiest to recognise in skin, joints and other accessible tissues. Internal inflammation may not produce visible redness or swelling. Chronic inflammation can also occur without all five local signs. Vasodilation means widening of small blood vessels. It increases local blood flow and can contribute to redness and heat. Increased vascular permeability allows fluid and proteins to leave small blood vessels. This contributes to swelling. Pain can result from inflammatory mediators, pressure from swelling and tissue injury. Loss of function can reflect pain, swelling or structural damage. These signs are not specific to one cause. Infection, injury, crystal deposition and immune mediated disease can produce overlapping appearances. Acute inflammation has several possible outcomes Acute inflammation usually begins quickly after a harmful stimulus. Neutrophils often predominate early in many acute responses. A neutrophil is a white blood cell that can engulf microbes and release substances that help control infection. Those substances can also injure nearby tissue. Acute inflammation may resolve completely, form pus or an abscess, heal with scarring, or progress into a persistent process. It is often protective, but it is not always mild or self limiting. Severe acute inflammation can damage organs and contribute to critical illness. Acute and chronic are patterns rather than rigid time limits. Their features can overlap within the same disease or tissue. Resolution is active Resolution is the coordinated process that limits inflammation and restores tissue balance. It is not simply the response running out of fuel. Neutrophil recruitment reduces, inflammatory mediators change and dying cells are cleared. A macrophage helps remove microbes, damaged cells and cellular debris. Macrophages can also support tissue repair and influence whether healing restores tissue or produces a scar. Successful resolution may return tissue towards its earlier function. Severe injury can still leave lasting damage despite appropriate resolution. Suppressing inflammation and promoting resolution are related but not identical ideas. Treatment choices depend on the disease and available evidence. Chronic inflammation is not merely acute inflammation left on Chronic inflammation is a persistent or recurring inflammatory process involving ongoing immune activation, tissue injury and attempts at repair. It can follow unresolved acute inflammation. It can also develop gradually without a clear earlier acute episode. Possible drivers include persistent infection, immune mediated disease, prolonged exposure to harmful substances and some metabolic disorders. Macrophages and lymphocytes often have prominent roles. Neutrophils can still contribute in some chronic diseases. Ongoing repair can activate cells that deposit connective tissue. Excessive deposition produces fibrosis, which can distort tissue and reduce organ function. Fibrosis is not inevitable in every chronic inflammatory condition. Its extent and reversibility vary between diseases, organs and treatment stages. Inflammation and infection are different Infection occurs when an infectious agent establishes itself in the body. Inflammation is the host response to harmful stimuli. Infection commonly triggers inflammation, but inflammation also occurs without infection. Examples include trauma, gout, allergy and autoimmune disease. Inflammation can also continue after an infectious agent has been controlled. Conversely, serious infection may produce few obvious inflammatory signs in some people. This distinction affects treatment. An anti inflammatory medicine does not treat every infection, while an antibiotic does not treat non infectious inflammation. Clinicians therefore assess the cause before assuming that redness, swelling or a raised blood marker requires one specific treatment. Blood markers provide clues, not diagnoses C reactive protein, called CRP, and erythrocyte sedimentation rate, called ESR, can change during inflammation. These tests are non specific. Infection, immune mediated disease, injury and other conditions can alter them. A raised result does not identify the cause or location. A normal result does not exclude every inflammatory disease. Changes over time can help monitor some conditions, but the result must be combined with symptoms, examination and other evidence. Persistent joint inflammation as an example Synovitis means inflammation of the membrane lining a joint. It can cause swelling, warmth, pain, stiffness and reduced movement. Persistent synovitis has several possible causes. Rheumatoid arthritis is one possibility, but infection, crystal arthritis and other inflammatory conditions also require consideration. NICE recommends specialist referral for adults with suspected persistent synovitis of undetermined cause. Referral should be urgent when small hand or foot joints are affected, more than one joint is involved, or assessment has been delayed. Normal inflammatory markers, negative rheumatoid factor or negative anti CCP antibodies should not delay that referral. Sudden severe pain and swelling in one joint, especially with fever or feeling unwell, needs urgent same day assessment for possible septic arthritis. Taking part in the assessment Describe when the problem started, whether it fluctuates and which areas are affected. Mention fever, recent infection, injury, medicines and other symptoms. Ask what evidence suggests inflammation and what possible causes remain. Ask what tests can and cannot establish. Do not start, stop or increase anti inflammatory treatment to test the diagnosis yourself. Some treatments can mask infection or cause other harm. Seek an urgent GP appointment or contact NHS 111 for sudden severe single joint pain and swelling, skin colour change, fever or feeling generally unwell. Call 999 for confusion, very fast breathing, blue or blotchy skin, collapse or another possible sign of sepsis. This lesson explains general inflammatory biology. It cannot diagnose the cause of pain, swelling, fever or an abnormal blood result.

Inflammation is a regulated response with protective and damaging effects. Acute inflammation has several possible outcomes, resolution is active and chronic inflammation can involve simultaneous immune activity, tissue injury, repair and fibrosis.