Heterophils are a type of immune cell found in birds, reptiles, and some mammals. They are a component of the innate immune system, which provides immediate defense against infections. Functionally, they are the equivalent of neutrophils in humans and other mammals, playing a similar role in identifying and neutralizing foreign invaders. These specialized white blood cells protect an animal from a wide range of pathogens.
The Role of Heterophils in the Immune System
As first responders, heterophils are among the initial cells to arrive at a site of infection or tissue injury. They are drawn to these areas by chemical signals released by other cells and pathogens. This rapid migration allows them to quickly engage with invading bacteria, fungi, and other microorganisms to contain and eliminate these threats.
Upon reaching their target, heterophils use a process called phagocytosis to engulf and internalize pathogens. Once inside the heterophil, the microbe is trapped within a compartment called a phagosome. The heterophil then releases antimicrobial substances from its granules into the phagosome to destroy the invader. These granules contain enzymes and proteins, such as β-defensins, effective at breaking down pathogens.
Beyond directly killing pathogens, heterophils also help coordinate the broader immune response by releasing signaling molecules known as cytokines and chemokines. These molecules recruit other immune cells to the site of infection. In some cases, heterophils form extracellular traps, which are web-like structures of DNA and proteins. These traps can ensnare and kill pathogens outside the cell, preventing their spread.
Heterophils as Indicators of Animal Health
The number of heterophils in an animal’s bloodstream provides insight into its health. An elevated count, known as heterophilia, points to an active bacterial infection or significant inflammation. When the body detects an invader, the bone marrow produces and releases more heterophils to fight it. A blood test showing high heterophil levels alerts veterinarians to a potential underlying disease.
Conversely, a lower-than-normal count, or heteropenia, can indicate that the body is overwhelmed by a severe infection, using heterophils faster than they are produced. It might also suggest a problem with the bone marrow’s ability to generate new cells. Both scenarios signal a compromised immune state.
The Heterophil-to-Lymphocyte (H/L) ratio is a useful diagnostic tool and an indicator of chronic stress in animals. When an animal experiences prolonged stress, its body releases corticosteroids, which cause heterophil numbers to rise and lymphocyte (another type of white blood cell) numbers to fall. By calculating the H/L ratio from a blood sample, experts can assess the physiological impact of stressors like habitat loss, poor nutrition, or social pressure.
How Heterophils Differ From Human Neutrophils
While heterophils and human neutrophils perform the same job, they differ in appearance under a microscope after staining. Heterophil granules—the small packets within the cell containing antimicrobial substances—are larger, more distinct, and often rod-shaped or spindle-shaped. In contrast, neutrophil granules are smaller and less prominent, giving the cell’s cytoplasm a more uniform look.
These structural variations are linked to chemical differences in their granules. A distinction is that heterophils lack the enzyme myeloperoxidase, which is abundant in mammalian neutrophils. This enzyme produces bleach-like substances that help neutrophils kill pathogens. Heterophils compensate by relying on other non-oxidative antimicrobial proteins, like lysozyme and defensins, to destroy invaders.
It is also useful to distinguish the heterophil cell from a separate term used in human medicine: “heterophile antibody.” These antibodies are found in the blood of people with certain conditions, most notably infectious mononucleosis. They are detected through a “heterophile antibody test,” but they are an entirely different biological entity with no direct connection to heterophil immune cells.