IL-4

IL-4 helps coordinate immune responses involved in allergies, asthma, and eczema. Its role in these diseases is well established. The usefulness of measuring it in your blood is much less certain, because activity in your skin or airways may not be reflected in the amount circulating.

Blood IL-4 is an exploratory biomarker. Laboratories offer clinical measurements, but there are no widely accepted disease-specific cutoffs or proven targets for preventive health. A result cannot reliably identify your immune system's balance, explain symptoms, or show whether you need treatment.

What This Messenger Does

Cytokines are proteins that cells use to communicate. IL-4 is produced by several immune cells, including a group of helper T cells involved in allergic responses. Mast cells, basophils, and eosinophils can also produce it. Together with other signals, IL-4 helps antibody-making B cells switch to producing IgE. IgE is the antibody involved in many immediate allergic reactions.

IL-4 can also suppress some inflammatory responses and change how macrophages behave. Macrophages are immune cells that remove debris and help coordinate tissue responses. Laboratory and animal studies show that IL-4 can promote repair-related activity in these cells, although prolonged signaling can also contribute to tissue scarring. Calling it either an inflammatory or an anti-inflammatory signal leaves out part of its biology.

What the Blood Measurement Captures

This test reports the concentration of IL-4 in a blood sample in pg/mL. The catalog does not specify whether the laboratory analyzes serum or plasma, or which measurement method it uses. Serum is the liquid remaining after blood clots. Plasma is separated from blood that has been kept from clotting. Results from these preparations and from different laboratory methods should not be assumed to be interchangeable.

Studies also measure IL-4 released by cells after laboratory stimulation, IL-4 gene activity, and the receptors that respond to IL-4. Those measurements answer different questions from the concentration already circulating in your blood.

Circulating IL-4 is often present in very small amounts. A study of 728 healthy adults in eastern China found no statistically significant differences in plasma IL-4 by sex or age within that study population. Its laboratory-specific findings do not establish a universal healthy level or a target linked to better health.

Allergies, Asthma, and Eczema

Several studies have found higher circulating IL-4 in people with allergic disease than in comparison groups. These include serum measurements in allergic rhinitis and childhood asthma, and plasma measurements in a small study that included people with rhinitis, eczema, or asthma. These are group-level associations. They do not establish how accurately an individual result identifies allergic disease or measures its severity.

In a study that followed 64 babies, serum IL-4 rose after birth and peaked around 6 to 9 months. Levels were higher among infants who developed allergic disease during the first 18 months, sometimes before symptoms appeared. That small study did not establish an infant screening test.

Other findings show why the blood measurement needs caution. In the small study of adults with allergic disease, higher plasma IL-4 was not accompanied by increased activity of selected IL-4-responsive genes in circulating white blood cells. In another study of adults with breathing symptoms and normal spirometry, blood IL-4 was frequently below the assay's detection limit and was excluded from further biomarker analyses. The study therefore could not adequately assess its ability to distinguish early airway inflammation.

An elevated result can occur with allergic disease, but it cannot establish that allergic inflammation is active throughout your body. A low or undetectable result also cannot rule out asthma, eczema, or an allergy.

Weight, Fat Tissue, and Insulin Resistance

A small human study found significantly lower plasma IL-4 in adults with obesity than in lean adults. IL-4 gene activity in fat tissue was also lower in the obesity group. The study also found fewer eosinophils in fat tissue among participants with obesity, and those tissue-cell measurements were associated with insulin resistance. Insulin resistance develops when cells respond less effectively to insulin.

Experiments using human fat tissue and cultured fat cells found that IL-4 reduced leptin production. Leptin is a hormone produced by fat cells that helps regulate appetite. These experiments suggest a possible mechanism, but they do not show that a low blood IL-4 result diagnoses insulin resistance or that raising circulating IL-4 improves metabolic health.

Why There Is No Single Good Direction

IL-4 has different effects in different tissues and diseases. Its concentration in blood is also an incomplete measure of its activity. The available studies do not establish that a higher or lower result is generally better, or that changing the number improves health.

Advanced Dry Macular Degeneration

Geographic atrophy is an advanced form of dry age-related macular degeneration in which areas of retinal tissue are lost. In a study of 435 people, the median plasma IL-4 level was roughly three times higher in those with geographic atrophy than in controls. The pattern was not a steady rise with disease stage: people with intermediate disease had a lower median than controls. Because the groups were compared at one point in time, the study does not show whether IL-4 predicts future disease or progression.

A separate study measured serum IL-4 in people with intermediate or wet macular degeneration and people with certain bone marrow disorders. It found no significant IL-4 difference among several of those groups. One subgroup stood out: people with bone marrow disorders who also had drusen had higher IL-4 than those without drusen and than people with wet macular degeneration. Drusen are small deposits that build up under the retina. The different blood preparation and comparison groups prevent a direct comparison with the geographic atrophy study. Together, these findings do not establish a stage-specific screening test.

Depression

In a study of 110 adults with major depression and 107 healthy comparison participants in Bangladesh, average serum IL-4 was roughly three times higher in the depression group and was associated with symptom severity. The researchers also explored whether it could distinguish the groups, but assessed that performance in the same sample used to develop the classification. This does not establish accuracy in people seeking an evaluation for depression.

Earlier studies, cited by the same researchers, found lower serum IL-4 in major depression. The direction of the association is inconsistent across studies. Blood IL-4 has no established role in diagnosing depression or selecting treatment, and the association does not show that IL-4 causes depression.

Findings in Other Conditions

Researchers have reported differences in circulating IL-4 in several other conditions. These findings come from different populations, blood preparations, and measurement methods, so they cannot be combined into a diagnostic guide for your result.

• Higher serum levels in some studies: lupus, systemic sclerosis, brucellosis, and chronic hepatitis C. Systemic sclerosis can cause thickening and scarring of skin and internal organs. Brucellosis is an infection transmitted from animals.
• Conflicting findings in endometrial cancer: one study found higher serum IL-4 in endometrial cancer, while a larger multicenter study of women with abnormal uterine bleeding found that low IL-4 was linked to a higher likelihood of endometrial cancer.
• Mixed findings after brain bleeding: one study linked higher serum IL-4 after a ruptured brain aneurysm with poorer outcomes. Other research on IL-4 in spinal fluid has reported different associations, including with favorable outcomes, so the direction is not settled. Spinal fluid measurements are a separate test.
• Lower levels in severe malaria, with low confidence: a meta-analysis found lower circulating IL-4 in severe than in uncomplicated malaria, but results varied enormously between studies, and the authors concluded IL-4 should not be used to predict malaria severity. A later umbrella review also raised concern that published IL-4 results may overstate the association.
• No consistent difference in several other settings: a systematic review found no consistent circulating IL-4 difference in autism, and a meta-analysis found no significant difference in sarcopenia. Sarcopenia involves loss of muscle strength and muscle mass. A separate pediatric study found no significant serum IL-4 difference between complicated and uncomplicated appendicitis.

The first endometrial cancer study illustrates the limitations of classifying disease from this number. Its proposed IL-4 classification incorrectly flagged half of the comparison group, who had benign endometrial conditions. It has not been validated as a screening test.

When Results Can Be Misleading

• Laboratory method and blood preparation: results can depend on the assay and whether serum or plasma was analyzed. Published values span an enormous range. A flow cytometry study of healthy adults reported plasma values in the single digits of pg/mL, a depression study reported group averages in the hundreds of pg/mL, and a macular degeneration study using serum reported medians below 1 pg/mL. Gaps this large mean your number cannot be compared with figures from published studies, even when both use pg/mL.
• Sample handling: separating the liquid portion from blood cells, storage, and transport must follow the performing laboratory's instructions. Problems with collection or handling can compromise interpretation.
• Values near the detection limit: an undetectable result means the assay could not reliably detect IL-4 at that concentration. It does not prove that your body produces none. Small changes near this limit can produce misleading percentage changes.
• Local immune activity: IL-4 can act within tissues without producing a measurable rise in circulating blood. A blood result cannot reliably describe activity in your skin, airways, or fat tissue.
• Dupilumab treatment: this drug blocks signaling through a receptor component shared by IL-4 and IL-13. In a small study of people with eczema, serum IL-4 rose during treatment even as symptoms improved. A separate study found higher IL-4 released by stimulated blood cells in treated patients, a different measurement that points the same way. A rising concentration during treatment does not by itself indicate treatment failure.
• Illness and injury: infections and major injury have been associated with changes in circulating IL-4. A result obtained during these events may not represent your usual condition.