The acute phase response is the body’s built-in, whole-body emergency mode, switched on within hours of an infection, injury, burn or operation. It reaches far beyond the sore or infected spot, changing your blood, your temperature, your appetite for iron and even how your body burns fat and muscle. It is designed to be temporary, and much of what doctors read as “inflammation” on a blood test is really this response showing up in numbers.
How the Alarm Gets Raised
Immune cells carry sensors that recognise parts of bacteria and viruses, and also chemicals that leak from damaged or dying cells.1The Blood Project. Acute Phase Response The sensors pick up general patterns shared by many germs, such as pieces of bacterial cell walls or viral genetic material, so the first reaction is broad, and other parts of the immune system tailor the fight to the specific germ over the following days. Because the sensors respond to damage as well as germs, the same response can follow a chest infection, a broken bone, a heart attack or surgery. A raised marker after an operation is therefore expected, not proof of infection. Doctors use timing and symptoms to tell normal recovery from a new infection.
Once triggered, these cells release cytokines, small protein messengers that act as immune alarm signals.2The Blood Project. Acute Phase Response Three of them do most of the work: IL-6, IL-1 and TNF.3IntechOpen. Perspective Chapter: Cytokine-mediated Acute Phase Response and Protein Synthesis At the site of a problem, nearby immune cells release them first, and the area turns red, warm and swollen. When the problem is big enough, enough signal spills into the bloodstream to reach distant organs. A small scrape mostly stays local and rarely changes your blood tests, while pneumonia, a major burn or an operation sends the alarm body-wide.
The signals travel in the blood to the liver, which links a local problem to a body-wide change in protein production.4IntechOpen. Perspective Chapter: Cytokine-mediated Acute Phase Response and Protein Synthesis Immune cells living inside the liver add their own signals, shaping exactly which proteins get made.5PubMed. Acute-phase protein synthesis: a key feature of innate immune functions of the liver IL-6 and IL-1 work together, so the liver’s output rises far more with both than with either alone.6PubMed Central. Synergistic gene expression during the acute phase response is characterized by transcription factor assisted loading Most cytokines aren’t measured in routine tests, so what appears on a report are the downstream proteins, which are easier and cheaper to measure.
The Emergency Proteins Your Liver Makes
Acute phase proteins are the proteins whose blood levels shift sharply once the alarm signals reach the liver. Among those that rise, C-reactive protein (CRP) is the best known and the one most often tested.7PubMed. Properties of four acute phase proteins: C-reactive protein, serum amyloid A protein, alpha 1-acid glycoprotein, and fibrinogen CRP sticks to the surface of bacteria and to debris from dead cells, tagging them so immune cells can swallow them.8PubMed. Properties of four acute phase proteins: C-reactive protein, serum amyloid A protein, alpha 1-acid glycoprotein, and fibrinogen Its main job seems to be cleaning unwanted material out of the blood, whether that is germs or your own damaged cells.9PubMed. Acute phase proteins with special reference to C-reactive protein and related proteins (pentaxins) and serum amyloid A protein
Serum amyloid A can rise as much as 1,000-fold during inflammation.10PubMed. Serum amyloid A, the major vertebrate acute-phase reactant It is found in every vertebrate studied, and fish, birds and mammals all mount versions of this response, which suggests it is a very old survival tool. Other rising proteins trap microbes, neutralise harmful enzymes, mop up free haemoglobin and help switch on the complement defence system.11PubMed Central. Acute-phase proteins: As diagnostic tool
The raised levels on a report come from the liver making more of these proteins on instruction from the alarm signals. They aren’t leaking out of the damaged tissue, so they are a readout of how strongly the alarm is firing. A badly damaged liver can limit how much it is able to make, so levels need interpreting in context.
Other proteins fall instead, including albumin, transferrin and transthyretin.12PubMed. Reference distributions for the negative acute-phase serum proteins, albumin, transferrin and transthyretin: a practical, simple and clinically relevant approach in a large cohort Building blocks are limited, so the liver shifts them from routine jobs to defence. Fibrinogen, a clotting protein, rises and is costly to make, using about 2.6 grams of muscle protein for each gram.13PubMed Central. STAT3 Activation in Skeletal Muscle Links Muscle Wasting and the Acute Phase Response in Cancer Cachexia Albumin is often used as a nutrition marker, but during infection or after surgery it falls regardless of diet, so a low value during an illness can reflect inflammation rather than poor eating.
Fever, Low Iron and Other Whole-Body Changes
A chemical messenger called prostaglandin acts on a heat-control area of the brain to raise body temperature.14PubMed Central. Ventromedial preoptic prostaglandin E2 activates fever-producing autonomic pathways. The alarm signals seem to keep a fever going more than they start it.15PubMed. Fever induction pathways: evidence from responses to systemic or local cytokine formation The same signals reach the brain and prompt the stress-hormone chain that ends in cortisol release.16PubMed. The mechanism of action of cytokines to control the release of hypothalamic and pituitary hormones in infection
A raised temperature seems to make life harder for some germs and may help immune cells work, so fever counts as part of the defence rather than a malfunction. Paracetamol or ibuprofen lower it by reducing prostaglandin production, the same messenger that raised it. Using them is mainly for comfort and is reasonable if you feel miserable. A very high fever, a fever that lasts for days, or a fever with a stiff neck, confusion or trouble breathing needs medical attention.
IL-6 tells the liver to make more hepcidin, a hormone that locks iron away.17PubMed Central. Hepcidin and Iron in Health and Disease Iron then gets held inside storage cells instead of being released into the blood.18PubMed Central. Hepcidin and Iron in Health and Disease Free iron in the blood feeds many bacteria and helps them multiply fast, so blood iron drops early in infections.19PubMed Central. Hepcidin and Iron in Health and Disease In healthy young women, a vaccine was enough to double hepcidin and lower blood iron within 24 hours.20Haematologica. The opposing effects of acute inflammation and iron deficiency anemia on serum hepcidin and iron absorption in young women If inflammation drags on, though, this iron lockdown starves red blood cell production and can cause anaemia.21PubMed Central. Hepcidin and Anemia: A Tight Relationship
That matters if you are offered iron pills during an infection. Low iron from inflammation is different from iron deficiency, and hepcidin blocks absorption, so pills may do little while you are inflamed. Ask before starting supplements based on a test taken while ill.
The response also breaks down muscle protein and releases fat from fat stores.22The New England Journal of Medicine. Acute-Phase Proteins and Other Systemic Responses to Inflammation The liver makes more triglycerides, yet blood cholesterol falls in both LDL and HDL.23PubMed. Changes in the concentration and composition of plasma lipoproteins during the acute phase response A cholesterol test taken while you are acutely ill may therefore read misleadingly low. In a severe infection, bone marrow switches to emergency mode and churns out neutrophils, the front-line white cells.24PubMed. Emergency granulopoiesis Together, fever, muscle breakdown and hormone shifts explain the aches and fatigue of being sick.
Mention recent illness, vaccination or surgery when bloods are taken. Doctors often repeat such tests a few weeks after recovery.
How the Response Switches Off
Removing the cause, by killing the germ or healing the injury, is the first requirement for the response to stop.25PubMed Central. Resolution of inflammation: state of the art, definitions and terms Production of alarm signals then halts and the existing ones are broken down.26PubMed Central. Resolution of inflammation: state of the art, definitions and terms As long as germs or damaged tissue remain, the sensors keep firing and the alarm keeps being renewed. That is why a hidden abscess or an infected wound can keep markers high. A marker that stays high despite treatment can be a clue that the source hasn’t been cleared, and doctors may look for it with scans or other tests.
Neutrophils die on schedule, and clean-up cells swallow them.27PubMed Central. Resolution of Acute Inflammation and the Role of Resolvins in Immunity, Thrombosis and Vascular Biology This swallowing itself calms the clean-up cells and lowers alarm signals.28PubMed Central. Resolution of Acute Inflammation and the Role of Resolvins in Immunity, Thrombosis and Vascular Biology They then switch from fighting to repairing, and healing begins.29PubMed Central. Resolution of Inflammation: What Controls Its Onset? The brakes start working early, limiting how many white cells pour in to match the size of the injury.30PubMed Central. Resolution of Acute Inflammation and the Role of Resolvins in Immunity, Thrombosis and Vascular Biology
After surgery, IL-6 peaks within hours and CRP about a day later. Markers then fall over days as you recover. A second rise after they have fallen can signal a new problem worth checking.
Once the threat has gone, you may keep feeling tired for days or weeks. The body is clearing debris and rebuilding tissue, and emergency protein levels take time to come down. Proteins that were cut back, such as albumin, recover gradually as the liver returns to its normal output. See a doctor if a fever lasts several days without improving, if you get worse after initially improving, or if you have ongoing weight loss or exhaustion without a clear cause.
What CRP and ESR on Your Blood Test Mean
ESR (erythrocyte sedimentation rate, “sed rate”) measures how fast red cells settle in a tube, which mostly reflects fibrinogen levels.31PubMed. Erythrocyte sedimentation rate and C-reactive protein: how best to use them in clinical practice Fibrinogen lasts much longer in the blood than CRP.32PubMed. Erythrocyte sedimentation rate and C-reactive protein: how best to use them in clinical practice CRP is therefore better for spotting a sudden infection and seeing whether treatment works, while ESR suits tracking long-term conditions.33PubMed. Erythrocyte sedimentation rate and C-reactive protein: how best to use them in clinical practice Both tests also change through the day, with ESR and fibrinogen higher mid-afternoon in one study of ill patients.34PubMed. Circadian variation of human acute phase response
The two don’t always agree. In people with rheumatoid arthritis, about 15% of those with no swollen joints still had a high ESR with normal CRP, while under 5% had the reverse.35PubMed. Which Measure of Inflammation to Use? A Comparison of Erythrocyte Sedimentation Rate and C-Reactive Protein Measurements from Randomized Clinical Trials of Golimumab in Rheumatoid Arthritis So an ESR can stay high when CRP has already settled. CRP isn’t affected by age, which is one reason some specialists prefer it.36PubMed. Which Measure of Inflammation to Use? A Comparison of Erythrocyte Sedimentation Rate and C-Reactive Protein Measurements from Randomized Clinical Trials of Golimumab in Rheumatoid Arthritis Neither test names the cause, so doctors read them alongside your symptoms and exam.37PubMed. Erythrocyte sedimentation rate and C-reactive protein: how best to use them in clinical practice
A raised CRP says inflammation is present, not where or why. Infection, injury, surgery and flares of autoimmune disease all raise it. Very high values more often point to bacterial infection, though other causes can do the same, so doctors confirm with other tests.
A normal CRP makes significant active inflammation unlikely at that moment, but it doesn’t rule out disease. Some conditions, such as lupus, often cause little CRP rise even when active. CRP can also be normal very early in an illness, since it takes several hours to climb after the trigger.
ESR depends on more than inflammation. Anaemia can raise it, because with fewer red cells they settle faster. Pregnancy, older age and some kidney problems can raise it too. Some conditions that thicken the blood can lower it, so a low reading can sometimes mislead.
Useful questions for your doctor: Is this level expected given my recent illness or procedure? Should it be repeated, and when? Does it change my treatment?
When the Response Doesn’t Switch Off
Acute inflammation is a short burst that ends with healing. Chronic inflammation is low-level and lasting, often seen in autoimmune disease or cancer. The same proteins are involved, but the cost builds up over time.
In cancer and other chronic diseases, an ongoing acute phase response is closely tied to cachexia, severe weight and muscle loss.38PubMed Central. The impact of inflammation and acute phase activation in cancer cachexia Making emergency proteins for months drains the body’s protein, much of it taken from muscle.39PubMed Central. STAT3 Activation in Skeletal Muscle Links Muscle Wasting and the Acute Phase Response in Cancer Cachexia Cachexia lowers tolerance for treatment, limits walking and shortens survival.40PubMed Central. The impact of inflammation and acute phase activation in cancer cachexia
Cachexia isn’t just weight loss from eating less. The body breaks down muscle even when someone eats reasonably well, which is why eating more doesn’t fully reverse it. People often notice loose clothes, weaker grip, trouble climbing stairs and a poor appetite, and these changes can creep up gradually. Fat and muscle are both lost, but muscle loss is what most affects strength and recovery. Good protein and calorie intake helps protect muscle, and it is worth asking about nutrition support if you are losing weight during treatment.
How the Response Changes From Babies to Older Adults
After surgery, infants older than about 4 months showed rising IL-6 within 2 to 6 hours and CRP at 24 hours, but newborns mostly showed no rise.41Pediatric Research. Different Acute-Phase Response in Newborns and Infants Undergoing Surgery A normal CRP in a very young baby therefore doesn’t rule out a serious problem. Babies’ immune cells lean toward calming signals and gradually shift to stronger inflammatory ones over the first years.42PubMed Central. Innate Immune Sensing by Toll-like Receptors in Newborns and the Elderly
A newborn’s immune system is still developing and relies partly on antibodies passed from the mother, one reason very young babies are more vulnerable to serious infection. Blood tests that work well in older children and adults are less reliable in the first weeks of life, so doctors rely more on a baby’s appearance, behaviour and examination, together with other tests. Any fever in a newborn is taken seriously regardless of results, and poor feeding or other symptoms matter more when markers are weak. Seek care promptly rather than waiting for a fever to build.
Frail older adults often have constantly raised IL-6, CRP and TNF.43PubMed Central. Innate Immune Sensing by Toll-like Receptors in Newborns and the Elderly Their baseline readings may be higher before any illness, so comparing CRP to a person’s own earlier results is more useful than judging it against a general range. Fever may be mild or absent despite infection, and confusion or weakness can be the first clue.