When Fat Cells Behave Like Immune Cells: From Local Stress to Bioenergetic Impedance

We usually think of adipocytes as storage cells. They take up excess energy, package it into triglycerides, and release fuel when the body needs it. In obesity, the conventional story then runs in the opposite direction: adipocytes become too large, inflammation develops, insulin resistance follows, and metabolic disease emerges.

A new Cell Metabolism Perspective by Julia Zinngrebe, Konstantin Sparrer, and Pamela Fischer-Posovszky invites us to look at adipocytes very differently. The authors argue that fat cells should also be considered active participants in host defense. They can sense pathogens, produce antimicrobial molecules, alter metabolism during infection, communicate with immune cells, and under some conditions even present antigens.

We think this emerging view has implications beyond adipose biology. It supports a broader idea developed in our recently accepted Perspective on bioenergetic impedance: immune signaling may be one component of a much larger cellular communication system that reports when local biological capacity is becoming constrained.

In other words, inflammation may sometimes begin not with a classical immune cell, but with an ordinary tissue cell saying, in molecular terms:

“Something here is exceeding my capacity to cope.”

Adipocytes are not passive storage containers

The Cell Metabolism Perspective brings together evidence showing that adipocytes possess many capabilities usually associated with immunity.

They express pattern-recognition machinery capable of detecting microbial signals. They can activate inflammatory and interferon pathways, produce nitric oxide and antimicrobial peptides, release cytokines and chemokines, use autophagy, and communicate extensively through their secretome. In certain contexts, adipocytes can also present lipid or peptide antigens to immune cells.  

This does not mean that an adipocyte is equivalent to a macrophage or dendritic cell. Professional immune cells remain highly specialized for functions such as phagocytosis, migration, antigen presentation, and systemic immune coordination.

But the distinction is important.

Innate immune competence is not exclusive to classical immune cells.

The authors note that almost all nucleated cells can mount some form of innate immune response, although the strength and repertoire of those responses differ between tissues.

That changes how we might think about immunity at the tissue level.

Instead of imagining the immune system as a separate force that arrives after tissue damage occurs, we can think of the tissue itself as part of the sensing network.

The adipocyte senses.The epithelial cell senses.The endothelial cell senses.The fibroblast senses.

Professional immune cells then enter a system in which local cells have already begun interpreting, signaling, and adapting to disturbance.

Fat storage itself may be a stress-buffering function

This becomes especially interesting when we consider what adipocytes actually do.

Their job is not simply to “hold fat.” They buffer fluctuations in nutrient availability.

After a meal, adipose tissue safely removes energy substrates from circulation and stores them. During fasting or increased demand, those substrates can be released again.

Healthy adipose tissue therefore helps prevent large excursions of energy-rich molecules into tissues that may be poorly equipped to store them.

But storage capacity is not unlimited.

As adipocytes enlarge, the surrounding tissue must expand with them. Blood flow has to increase. Oxygen and nutrient delivery must remain adequate. Extracellular matrix has to remodel. New adipocytes may need to differentiate. Waste products have to be removed.

Mitochondrial and cellular maintenance systems must continue functioning despite increasing load.

At first, the tissue responds adaptively.

Adipocytes communicate with endothelial cells to support vascular remodeling. They signal to stromal and immune cells. Adipogenesis may increase. Metabolic pathways change.

These are not necessarily signs of disease.

They are attempts to preserve function.

The Perspective provides striking examples of such adaptive remodeling. During skin infection, adipocyte precursors proliferate and differentiate in a process termed reactive adipogenesis, producing antimicrobial peptides that help contain Staphylococcus aureus.

Something similar can occur around the intestine. In Crohn's disease, mesenteric adipose tissue can expand around injured bowel segments as “creeping fat.” Rather than treating this tissue expansion as intrinsically pathological, the authors highlight evidence that it may initially serve a containment function, limiting microbial dissemination from a compromised intestinal barrier.

Expansion, in other words, can be part of defense.

The important question is whether the system can eventually resolve the demand that triggered the expansion.

From storage capacity to “storage impedance”

Our bioenergetic-impedance framework offers another way to interpret this transition.

We use the term bioenergetic impedance to describe a time-dependent constraint on biological energetic throughput and recovery relative to demand.

Adipose tissue provides an intuitive example.

As long as adipocytes can expand safely, recruit sufficient support, maintain perfusion, and store incoming substrate, the system can accommodate metabolic demand.

But eventually the rate of incoming substrate may begin to exceed the tissue's ability to safely process and store it.

We might call this storage impedance.

The problem is not simply that there is “too much fat.” The deeper problem is that storage demand is rising faster than storage capacity can adapt.

The tissue initially compensates.

More angiogenic signaling.More remodeling.More adipogenesis.Changes in endocrine signaling.Recruitment of neighboring cells.Changes in insulin sensitivity and substrate handling.

But if these adaptations cannot restore adequate capacity, compensation itself begins to change the behavior of the tissue.

When an adipocyte stops accepting the load

Insulin normally encourages adipocytes to accept and store substrate while suppressing lipolysis.

When adipose storage becomes chronically constrained, insulin signaling may become progressively less effective. Lipolysis is then less completely suppressed, allowing fatty acids to leave the adipocyte even while circulating energy availability remains high.

It is tempting to call this simply “metabolic dysfunction.”

But viewed dynamically, another interpretation becomes possible.

The overloaded adipocyte is effectively becoming less willing—or less able—to accept additional substrate.

The Perspective itself emphasizes that adipocyte metabolism and immunity are tightly intertwined. During infection and other stress states, metabolic substrates can be deliberately redistributed, and lipolysis can play active roles in host defense. Fatty acids can sometimes contribute to antimicrobial activity, whereas in other contexts microbes exploit host lipids for their own replication.

So the key question is not whether lipolysis or insulin resistance is always good or always bad.

The question is:

What problem was the system originally trying to solve, and did that response successfully resolve it?

Impedance does not stay local

This brings us to what may be one of the most important implications.

A constrained adipocyte does not remain an isolated problem.

If it can no longer safely absorb incoming substrate, that substrate has to go somewhere.

Fatty acids enter the circulation. The liver receives more substrate. Skeletal muscle receives more substrate. Lipid begins to accumulate in tissues that are less specialized for long-term storage.

At the same time, the adipocyte sends information outward.

Adipocytes release cytokines, chemokines, interferons, adipokines, extracellular vesicles, lipids, metabolites, and other signaling molecules capable of changing the behavior of neighboring and distant cells.

This suggests two parallel routes through which local impedance can spread.

One is material propagation: substrate, metabolites, lipids, damaged cellular products.

The other is informational propagation: inflammatory mediators, stress signals, endocrine signals, vascular signals, extracellular vesicles.

Together they allow a local constraint to become a network-level event.

Storage impedance in adipose tissue can therefore become substrate overload in the liver, metabolic stress in muscle, altered vascular signaling, pancreatic compensation, and systemic immune activation.

The original problem has propagated.

Immune signaling may be part of impedance signaling

This is where the new Perspective connects particularly well with our forthcoming work.

We usually divide biology into systems: metabolism, immunity, endocrine signaling, vascular regulation, stress responses.

Cells may not experience these divisions.

A cell experiencing excessive metabolic load can change mitochondrial function, produce reactive oxygen species, alter nutrient sensing, remodel its secretome, activate pattern-recognition pathways, induce inflammatory signaling, communicate with neighboring cells, and recruit immune cells.

From the cell's perspective, these may all be parts of the same response to a constraint.

We therefore propose thinking of some immune signaling as part of a broader category of impedance signaling.

An inflammatory cytokine is still an inflammatory cytokine. We are not proposing that every immune response is caused by bioenergetic impedance.

Rather, we are suggesting that when cellular throughput, repair capacity, substrate handling, oxygen delivery, or energetic recovery becomes constrained, immune pathways may be among the channels through which that constrained state is communicated to the surrounding network.

The adipocyte is a particularly useful example because the same cell combines nutrient storage, endocrine signaling, metabolic adaptation, pathogen recognition, and immune communication.

Inflammation may initially be information

This perspective also changes how we interpret inflammation.

Inflammation is often treated as though it were itself the disease.

But acute inflammatory signaling performs essential functions: warning neighboring cells, recruiting support, reallocating resources, controlling infection, removing damaged material, and coordinating repair.

In that setting, inflammation contains information.

It says:

There is an unresolved problem here. Resources are required. Behavior must change.

If the underlying disturbance is removed, the inflammatory response should eventually resolve.

The sequence is then:

challenge → constraint → alarm → compensation → repair → recovery → signal withdrawal.

But what happens if the initiating impedance persists?

The tissue continues signaling because the problem has not gone away.

Compensation continues.

Substrates continue to be rerouted.

Immune cells continue to be recruited.

Extracellular matrix continues to remodel.

Neighboring tissues take on additional burden.

And those tissues can eventually develop their own constraints.

Now the sequence becomes:

persistent impedance → persistent signaling → compensatory overload → propagation → secondary impedance → further signaling.

At that point, inflammation is no longer simply an acute defense program.

It has become embedded in a self-reinforcing network state.

Chronic inflammation as unresolved impedance

This leads to a different interpretation of chronic inflammation.

Chronic inflammation may sometimes represent more than excessive immune activation.

It may reflect failure to resolve the biological constraint that originally required the inflammatory response.

Trying to suppress the signal without understanding the unresolved constraint would then be somewhat analogous to silencing an alarm while leaving the underlying problem untouched.

This does not mean that chronic inflammatory signaling is harmless. Once persistent, inflammatory pathways can themselves damage tissue, disrupt mitochondrial function, alter vascular regulation, drive fibrosis, and create additional energetic demands.

The alarm can eventually become part of the problem.

That is precisely how impedance may become self-propagating.

A local capacity constraint generates compensatory signaling. Persistent signaling alters neighboring systems. Those systems become constrained in turn. What began as an adaptive response gradually becomes a stable maladaptive network.

The larger lesson from the adipocyte

The most important contribution of the Cell Metabolism Perspective may therefore extend beyond fat biology.

Adipocytes remind us that the boundary between metabolism and immunity is artificial.

A fat cell can store energy, sense pathogens, communicate danger, mobilize substrate, recruit immune responses, participate in tissue repair, and alter adaptive immunity.

The authors conclude that appropriate host defense depends less on the sheer quantity of adipose tissue than on having healthy, functional adipocytes. Both loss of adipose tissue and excessive dysfunctional adiposity can be associated with impaired host defense.

We would extend that idea one step further.

Perhaps many of the signals we call “metabolic,” “inflammatory,” or “stress” signals are different languages through which cells report the same fundamental problem:

the demand being placed on the system is exceeding its current capacity to respond, adapt, and recover.

When that mismatch is temporary, signaling organizes adaptation.

When capacity catches up, the system resolves.

But when the mismatch persists, signals propagate, compensations accumulate, and the network can become trapped in a new state.

That is the transition we describe as bioenergetic impedance moving from local adaptation toward propagation and lock-in.

And adipose tissue may offer one of the clearest biological examples of how that transition happens.

References:

Zinngrebe, J., Sparrer, K. M. J., & Fischer-Posovszky, P. (2026). Adipocytes: Hidden forces of the immune system? Cell Metabolism. Advance online publication. https://doi.org/10.1016/j.cmet.2026.08.020

Tippairote, T., & Hoonkaew, P. (2026). Bioenergetic impedance: A network physiology framework for stress adaptation, maladaptation, and recovery. Frontiers in Network Physiology, 6, 1967461. https://doi.org/10.3389/fnetp.2026.1967461