Under a microscope, eczema’s most defining feature is spongiosis: fluid collecting between the skin cells of the outer layer (epidermis), pushing them apart and giving the tissue a sponge-like appearance. This is so characteristic that pathologists often call eczema “spongiotic dermatitis.” But the microscopic picture changes significantly depending on whether the eczema is fresh or long-standing, and several other features help distinguish it from conditions that look similar to the naked eye.
Spongiosis: The Hallmark Feature
The outer layer of your skin is made up of tightly packed cells called keratinocytes, held together by protein bridges. In eczema, immune cells migrate from the deeper skin layers into this outer layer and trigger the death of some keratinocytes. When these cells die, the protein bridges connecting neighboring cells stretch and weaken. Fluid rushes into the gaps, a process called intercellular edema. Under the microscope, the result looks like a sponge: widened spaces between cells, with the remaining bridges stretched thin like threads across the gaps.
In severe cases, so much fluid accumulates that tiny blisters (vesicles) form within the epidermis. These are visible microscopically even when the skin doesn’t show obvious blistering to the naked eye. Sitting on top of this disrupted epidermis, pathologists frequently see parakeratotic scales, meaning cells in the outermost layer that still contain their nuclei when they shouldn’t. In healthy skin, cells shed their nuclei as they reach the surface.
The Inflammatory Infiltrate
Below the epidermis, in the layer called the dermis, eczema produces a distinctive pattern of inflammation. Immune cells cluster around the small blood vessels near the skin’s surface, forming what pathologists call a perivascular infiltrate. The dominant cell type is the lymphocyte, a white blood cell central to the immune response. Mixed in with the lymphocytes, eosinophils (a type of white blood cell associated with allergic reactions) are commonly present. One study examining biopsies from people with atopic dermatitis found eosinophils in 30 out of 31 samples, making tissue eosinophilia a near-universal microscopic finding in eczema.
In the epidermis itself, specialized immune cells called dendritic cells aggregate alongside the T cells that have migrated upward from the dermis. These dendritic cells play a key role in presenting allergens to the immune system, and their clustering within the spongiotic areas is a commonly observed feature across different types of eczema. The surrounding tissue often shows mild swelling in the upper dermis, with dilated blood vessels contributing to the redness visible on the skin’s surface.
How Acute and Chronic Eczema Differ
The microscopic appearance shifts dramatically as eczema progresses from a new flare to a long-standing patch.
Acute eczema shows the most dramatic spongiosis. The outer layer of skin retains its normal “basket weave” pattern of surface cells, but the layers beneath are flooded with fluid and infiltrated by immune cells. This is the stage most likely to show microvesicles within the epidermis.
As eczema becomes chronic, the picture reverses. Spongiosis fades and may become only mild or patchy. Instead, the epidermis thickens substantially through two processes: acanthosis (the living cell layers multiply and expand downward in elongated ridges) and hyperkeratosis (the dead cell layer on the surface builds up and becomes abnormally thick). This thickening corresponds to what you can feel on the skin’s surface as the tough, leathery texture of chronic eczema, clinically called lichenification. Subacute eczema falls between these two extremes, showing moderate spongiosis with early signs of epidermal thickening.
How Eczema Looks Different From Psoriasis
Eczema and psoriasis can look strikingly similar on the hands and other areas, but under the microscope, several features help tell them apart. The most reliable distinction involves the granular layer, a thin zone just below the skin’s outermost surface where cells prepare to shed. In psoriasis, this layer is often lost entirely: one study found the granular layer absent in 62.5% of palmar psoriasis cases, compared to just 24.4% of hand eczema cases without atopic features and 0% of eczema cases associated with atopic or nummular dermatitis.
Psoriasis also produces its own signature findings that eczema lacks. Collections of neutrophils (a different type of white blood cell) form small abscesses within the upper epidermis, a feature essentially absent in eczema. The blood vessels in psoriasis tend to be more uniformly dilated and tortuous, and the epidermal ridges elongate in a very regular, rhythmic pattern. Eczema’s ridges are more irregular, and its inflammation is dominated by lymphocytes and eosinophils rather than neutrophils.
Skin Barrier Proteins Under the Microscope
Beyond standard staining, researchers use a technique called immunohistochemistry to visualize specific proteins in eczematous skin. One protein of particular interest is filaggrin, which acts as a kind of mortar between skin cells and is critical for maintaining the skin barrier. Using antibodies tagged with visible markers, scientists can see exactly where filaggrin is expressed in a tissue section.
In normal skin, filaggrin staining appears primarily in the granular layer, with some signal extending into the outermost surface layer. Eczematous skin tells a more complex story. Research in atopic skin models has shown that after allergen exposure, the skin actually ramps up filaggrin production, but it also degrades filaggrin more extensively than normal skin does. The net result is a compromised barrier despite the skin’s attempt to compensate. This type of molecular staining adds a layer of information beyond what standard microscopy reveals, helping explain why eczematous skin loses water and lets irritants penetrate so easily.
Why Biopsies Are Taken
Most eczema is diagnosed clinically, based on its appearance and distribution on the skin. Microscopic examination through a skin biopsy becomes useful when the diagnosis is uncertain, when a rash isn’t responding to treatment as expected, or when a pathologist needs to rule out other conditions like psoriasis, fungal infections, or cutaneous lymphoma. The biopsy itself is small, typically a few millimeters of tissue taken under local anesthesia, and it gives the pathologist a cross-sectional view of all the layers of skin at once. The combination of spongiosis pattern, inflammatory cell types, and epidermal changes usually provides enough information to confirm or exclude eczema with confidence.