The “CD” in CD64 stands for Cluster of Differentiation, a naming system for proteins on the surface of cells. These proteins act as cellular fingerprints to identify different cell types, particularly within the immune system. CD64 is a protein on specific immune cells that responds to threats. Its concentration provides information about the body’s state of alert.
CD64’s Function and Location in the Immune System
CD64 is scientifically known as Fc-gamma Receptor I (FcγRI). This protein is found on immune cells like monocytes, macrophages, and, under certain conditions, neutrophils. Monocytes and macrophages are the immune system’s cleanup crew. CD64 is not present on resting neutrophils, but its expression increases when these cells are activated by inflammatory signals, turning them into first responders.
The protein’s function is linked to antibodies, which are Y-shaped proteins that tag pathogens for destruction. The stem of this “Y” is the Fc region, and CD64 is a high-affinity receptor for it, meaning it binds with exceptional strength and specificity. This strong bond makes CD64 a trigger for immune activity.
When CD64 on an immune cell latches onto the Fc region of an antibody attached to a pathogen, it initiates a defensive response. This binding triggers two main actions. The first is phagocytosis, a process where the cell engulfs and digests the antibody-coated invader. The second is the release of cytokines, chemical messengers that recruit more immune cells and intensify the inflammatory response.
Using CD64 as a Diagnostic Marker for Sepsis
During a severe bacterial infection, the body’s immune system launches an inflammatory response, and CD64 expression on neutrophils increases dramatically. In a healthy state, neutrophils have very low levels of this receptor. In response to bacterial components, CD64 levels can increase more than tenfold within four to six hours, making it an early indicator of a developing systemic infection.
This characteristic makes CD64 a diagnostic marker for sepsis, a life-threatening condition caused by the body’s response to an infection. Its high sensitivity and specificity for bacterial infections are notable. Measuring neutrophil CD64 can help distinguish between sepsis and non-infectious inflammatory conditions, sometimes with greater accuracy than biomarkers like C-reactive protein (CRP) or procalcitonin (PCT). Combining CD64 with other markers can enhance diagnostic precision.
The level of CD64 on neutrophils is measured using flow cytometry. This technique uses a specialized instrument to analyze individual cells from a blood sample as they pass through a laser beam. The instrument counts the amount of CD64 protein on thousands of neutrophils, providing a quantitative result called a CD64 index. This rapid test aids in the early diagnosis of sepsis and helps guide antibiotic therapy.
CD64’s Role in Other Disease States
CD64’s relevance extends beyond acute infections to chronic autoimmune disorders. In conditions like systemic lupus erythematosus (SLE) and rheumatoid arthritis, the immune system mistakenly targets the body’s own tissues, causing persistent inflammation. During active phases, or “flares,” CD64 expression is often elevated on immune cells, contributing to the inflammation that causes tissue damage. Measuring CD64 levels can also help differentiate between a disease flare and a new infection in these patients.
CD64 is also a subject of investigation in oncology, particularly for blood cancers like acute myeloid leukemia (AML). In AML, CD64 is explored as a marker on leukemia cells and as a potential therapeutic target. The goal is to develop treatments that can specifically target cells expressing high levels of CD64 to attack and destroy cancer cells.