Eosinophils stain red because their cytoplasmic granules are packed with strongly basic (positively charged) proteins that attract eosin, an acidic dye that is itself bright red. The name “eosinophil” literally means “eosin-loving,” coined by Paul Ehrlich in 1879 when he first noticed certain white blood cells greedily soaking up the dye while other cell types ignored it. That vivid reddish-orange color under the microscope is not just pretty; it is the starting point for identifying eosinophils in blood smears and tissue biopsies, which in turn helps clinicians spot allergic reactions, parasitic infections, autoimmune conditions, and rarer disorders where eosinophils cause organ damage.
How Eosin Finds Its Target
Eosin is a synthetic acidic dye, meaning it carries a negative charge in solution. It was first produced in 1874 by a German chemist working at a chemical company, and Ehrlich adopted it a few years later as part of a new technique for staining blood films with coal tar dyes.1PubMed. The early history of the eosinophil The principle is straightforward: negatively charged dye molecules are drawn to positively charged targets. Acid dyes like eosin bind to basic (cationic) proteins, while basic dyes like azure B bind to acidic structures such as DNA. The pH of the staining solution matters because it affects how strongly proteins carry their charge, which in turn controls how much dye they pick up.2International Review of Cytology. Factors Which Control the Staining of Tissue Sections with Acid and Basic Dyes
In the standard Romanowsky-type stains used in most clinical labs, eosin and azure B are applied together. Eosin colors red blood cells pink and gives eosinophil granules their signature red-orange. Azure B, meanwhile, stains chromatin blue, turns platelet granules violet, and colors the cytoplasm of cells rich in ribosomes a bluish hue. Some structures end up purple because azure B and eosin form a combined complex inside them. The selectivity of this process depends on how quickly eosin can penetrate into structures already occupied by azure B.3PubMed. Understanding Romanowsky staining. I: The Romanowsky-Giemsa effect in blood smears Eosinophil granules, however, are so rich in basic proteins that they grab eosin immediately and hold it tightly, producing that unmistakable red-orange that sets them apart from the pale lilac granules of neutrophils or the deep blue-violet of basophils.
Why Eosinophil Granules Are So Basic
The reason eosinophil granules attract eosin so intensely comes down to what they contain. The dominant protein inside each granule is major basic protein-1, or MBP-1, a small molecule of about 117 amino acids that carries a powerful positive charge. Its calculated isoelectric point is around 11.4, with a net charge of roughly +15 at physiological pH.4bioRxiv. In situ crystalline structure of the human eosinophil major basic protein-1 That is exceptionally basic for a biological protein. Most cellular proteins hover near neutral, so when eosin washes over a blood smear, the eosinophil’s granules stand out like magnets in a tray of plastic chips.
MBP-1 is stored inside the granule as a nanocrystal, tightly packed in an ordered lattice. When the cell is activated by an immune signal, those crystals expand and can be extruded from the granule entirely.5PubMed Central. In situ crystalline structure of the human eosinophil major basic protein-1. But MBP-1 is not alone. Each granule also houses eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), and eosinophil-derived neurotoxin (EDN). All four are cationic, which reinforces that affinity for eosin and explains why the staining is so consistently intense across different eosinophils and different patients.
What Those Granule Proteins Actually Do
The proteins that give eosinophils their color are also their primary weapons. MBP-1 is directly toxic to the lining of airways and to parasitic worm larvae. In lab experiments on guinea pig tracheal tissue, MBP caused dose-dependent damage to the epithelium at concentrations as low as 10 micrograms per milliliter. ECP was also damaging, though it required higher concentrations, while EDN did not damage the tissue at all despite being biochemically similar to ECP.6American Review of Respiratory Disease. Toxicity of Eosinophil Cationic Proteins for Guinea Pig Tracheal Epithelium In Vitro
ECP has a broader resume: it functions as an enzyme that chews up RNA and has been linked to tissue-scarring activity, nerve toxicity, and immune regulation. It also appears to have direct antimicrobial effects against bacteria and viruses.7PubMed Central. Analysing the eosinophil cationic protein–a clue to the function of the eosinophil granulocyte EPO generates reactive oxidants that can kill pathogens but also injure surrounding tissue when the reaction runs unchecked. Taken together, these granule proteins make the eosinophil a potent but somewhat indiscriminate fighter: effective against parasites, but also capable of collateral damage to the body’s own tissues.
Eosinophils and Parasitic Defense
Historically, eosinophils have been understood primarily as anti-parasite cells. Parasitic worms are too large for a single immune cell to engulf, so eosinophils use a different strategy: they coat the worm larva and release their toxic granule contents onto its surface. In the presence of specific antibodies or complement proteins, eosinophils can kill larval stages of parasitic worms in laboratory conditions.8PubMed Central. Eosinophils in Helminth Infection: Defenders and Dupes This host-protective role has been confirmed across a range of helminth infections.9PubMed Central. Eosinophil-mediated tissue inflammatory responses in helminth infection
Eosinophils also deploy a more recently discovered tactic called extracellular traps, or EETs. These are meshworks of DNA fibers studded with granule proteins like MBP and ECP, flung outward from the cell to ensnare pathogens. The DNA in these traps often comes from the eosinophil’s mitochondria rather than its nucleus, which means the cell can remain alive after releasing them.10PubMed. Eosinophil extracellular DNA traps: molecular mechanisms and potential roles in disease EET formation can also happen through a more dramatic process in which the cell dies while expelling its trap material.11PubMed Central. Eosinophil extracellular traps: heterogeneity of their stimuli, components, and functions
When Eosinophil Counts Climb
Because eosinophils are so recognizable under the microscope, an elevated count on a standard blood smear immediately narrows the diagnostic possibilities. The most common triggers for high eosinophils are allergic diseases (asthma, hay fever, eczema) and parasitic infections. In allergic inflammation, a signaling molecule called interleukin-5 (IL-5) is the most powerful driver of eosinophil production, activation, and survival. IL-5 is released by certain immune cells during an allergic response and essentially tells the bone marrow to make more eosinophils and tells existing eosinophils to stay alive longer and become more aggressive.12PubMed. The roles of IL-5 and anti-IL-5 treatment in eosinophilic diseases: Asthma, eosinophilic granulomatosis with polyangiitis, and eosinophilic chronic rhinosinusitis
When eosinophil counts stay persistently above about 1,500 cells per cubic millimeter of blood without an obvious allergic or parasitic cause, the condition is classified as hypereosinophilic syndrome (HES). HES is rare but serious because eosinophils infiltrate organs and cause direct damage. The heart is a frequent target: the hallmark cardiac complication is scarring of the inner lining of the heart muscle, called endomyocardial fibrosis, and patients can also develop blood clots inside the heart chambers.13PubMed Central. Cardiovascular manifestations of hypereosinophilic syndromes Mouse models of hypereosinophilia show that the damage worsens with age as the heart struggles to repair continuous eosinophil-driven injury, eventually progressing to heart failure.14PubMed Central. Characterization of a mouse model of hypereosinophilia-associated heart disease
When Eosinophil Counts Drop
Low eosinophil counts get less attention in popular health discussions, but they carry their own clinical significance. In hospitalized patients, a drop in circulating eosinophils, sometimes called eosinopenia, can signal bacterial infection or sepsis. A scoping review found that persistent eosinopenia beyond 48 hours in intensive care predicted higher mortality and readmission among patients with sepsis.15PubMed Central. The role of eosinophils in sepsis and acute respiratory distress syndrome: a scoping review A study in older hospitalized adults found that very low eosinophil counts were independently associated with acute bacterial infection, with odds roughly three to six times higher compared to patients whose eosinophils remained in the normal range.16PubMed Central. Association between low eosinophil count and acute bacterial infection, a prospective study in hospitalized older adults
The mechanism behind this drop is not entirely settled, but the prevailing idea is that stress hormones and inflammatory signals during acute infection shunt eosinophils out of the bloodstream or suppress their production. In practical terms, when a clinician sees a patient with signs of infection and a near-absent eosinophil count, it adds weight to the suspicion of a serious bacterial cause rather than a viral one.
Eosinophilic Diseases You Might Not Expect
Beyond classic allergies and parasites, eosinophils drive inflammation in some surprisingly specific conditions. Eosinophilic esophagitis (EoE) is an increasingly recognized disease in which eosinophils pile up in the lining of the esophagus, causing difficulty swallowing and food impaction. Diagnosis requires finding at least 15 eosinophils per high-power microscopy field on an esophageal biopsy, alongside symptoms of esophageal dysfunction and the exclusion of other causes.17PubMed Central. Eosinophilic esophagitis: diagnostic tests and criteria The staining characteristics described above are exactly how pathologists spot and count those eosinophils in the tissue sample. Without eosin, identifying them reliably would be far harder.
A rarer condition, eosinophilic granulomatosis with polyangiitis (formerly called Churg-Strauss syndrome), combines blood vessel inflammation with asthma and high eosinophil counts. It typically develops in middle-aged adults who already have severe asthma. The disease most frequently damages peripheral nerves and skin, and about a third of patients test positive for a specific autoantibody against an enzyme called myeloperoxidase.18PubMed. Diagnosis and classification of eosinophilic granulomatosis with polyangiitis (formerly named Churg-Strauss syndrome)
Eosinophils in Gut Health and Beyond the Immune Battlefield
The image of eosinophils as purely destructive cells has shifted over the past decade. A large population of eosinophils lives in the lining of the small intestine even in perfectly healthy people, arriving before the gut is colonized by bacteria after birth. Research in mice showed that without these resident eosinophils, the intestinal villi were blunted, fat absorption dropped, gut transit sped up, and the epithelial barrier was weaker. Intestinal cells released a signaling molecule (IL-33) in response to incoming microbes, and this activated the local eosinophils, which in turn helped maintain gut architecture, support macrophage maturation, and preserve the barrier between the body and its microbial tenants.19PubMed. Small intestinal resident eosinophils maintain gut homeostasis following microbial colonization So the same granule-laden cells that can shred airway tissue in an asthma attack seem to play a constructive housekeeping role in the gut.
Eosinophils and Cancer
Eosinophils have turned up inside tumors in ways researchers are still sorting out. Depending on the conditions, activated eosinophils can either promote or suppress tumor growth. Their granule proteins are directly toxic to some cancer cells, and eosinophils can also recruit and activate other immune cells within the tumor microenvironment. But under different signaling conditions, they can dampen anti-tumor immunity or even support the blood vessel growth that tumors need.20PubMed Central. Eosinophils in the tumor microenvironment: implications for cancer immunotherapy Figuring out which signals tip the balance one way or the other is an active area of research, with the hope that eosinophils could eventually be steered toward anti-tumor activity in clinical settings.21PubMed Central. Eosinophils in solid cancers: sentinels, predictors, and therapeutic allies
Drugs That Target Eosinophils Through IL-5
Because IL-5 is the central driver of eosinophil production and survival, it has become the main therapeutic target for diseases involving too many eosinophils. Three monoclonal antibodies are now in clinical use. Mepolizumab and reslizumab bind to IL-5 itself, preventing it from reaching eosinophils. Benralizumab takes a different approach: it binds to the IL-5 receptor on the eosinophil’s surface, which not only blocks the signal but also flags the cell for destruction by other immune cells, leading to a more dramatic reduction in eosinophil numbers.22PubMed. Targeting the IL-5 pathway in eosinophilic asthma: A comparison of anti-IL-5 versus anti-IL-5 receptor agents
These drugs have transformed the management of severe eosinophilic asthma, cutting exacerbation rates and allowing patients to reduce or stop oral steroid use. They are also being used or studied in hypereosinophilic syndrome, eosinophilic esophagitis, and eosinophilic granulomatosis with polyangiitis. The therapeutic logic is clean: if IL-5 is what keeps eosinophils alive and angry, blocking IL-5 calms them down or removes them altogether.23PubMed Central. Interleukin-5 in the Pathophysiology of Severe Asthma A lingering question, though, is whether aggressively depleting eosinophils carries a cost for their quieter housekeeping roles, such as the gut maintenance described above. So far, long-term safety data have been reassuring, but the research community has not stopped watching.
The Eosinophil’s Internal Clock
One of the more unexpected findings in eosinophil biology is that these cells have a functioning molecular clock. Like many cell types, eosinophils express proteins that oscillate over a roughly 24-hour cycle, governing when the cell is more or less active. In eosinophils from people with mild asthma, these clock-protein rhythms were disrupted: the oscillation periods shifted and the overall amplitude of several key clock proteins was lower compared to healthy donors.24PubMed Central. The molecular circadian clock of eosinophils: A potential therapeutic target for asthma This could help explain why asthma symptoms tend to peak at certain times of day and might eventually open a door to timing eosinophil-targeting treatments for maximum effect.
An Ancient Lineage
Eosinophil-like cells are not unique to mammals. Granulocytes with eosin-staining granules have been observed in various invertebrates, and these may represent evolutionary precursors of modern vertebrate eosinophils. Molecular evidence suggests that eosinophil peroxidase and the broader family of myeloperoxidases diverged somewhere around 60 to 70 million years ago, though the full evolutionary path from invertebrate granulocyte to human eosinophil remains poorly mapped.25Oxford Academic. Considerations on the evolutionary biology and functions of eosinophils: what the “haeckel”? The fact that eosin stains these cells across such distant branches of the animal kingdom underscores how conserved the underlying chemistry is: highly basic granule proteins paired with peroxidases seem to be a formula evolution has kept running for a very long time.