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What is Host Response to Infection?
In the realm of biology and medicine, the term "host" refers to an organism that harbors another organism. The presence of a “guest,” or symbiont, can affect a human host in several ways, but pathogenic relationships are of greatest concern in medicine. Pathogens are organisms that damage the host and ultimately cause disease. When a pathogen invades a host, it may cause an infection—the invasion and multiplication of the pathogen within the host's body—disrupting normal bodily functions, causing harm, and leading to disease.
A Very Brief Overview of the Immune Response—How the Host Responds to Infection
The immune system is comprised of various cells and tissues working together to recognize and neutralize harmful pathogens. It can broadly be divided into two coordinating mechanisms—innate immunity, which provides an immediate barrier at the initial site of invasion, and acquired immunity, a slower but more specific process. When a pathogen invades the body, the immune system responds through a complex series of steps:
- Recognition: Macrophages and other phagocytes—part of innate immunity—detect the presence of foreign invaders and engulf them.
- Innate Response: The engulfed pathogen is destroyed, and cytokines are released, causing inflammation. Monocytes and neutrophils are activated during the inflammation process, stimulating the activated monocytes and neutrophils to release even more cytokines, most of which are interleukins (IL) that act as chemical signals between white blood cells. For example, IL-1 is released into the blood, goes to the brain, and activates the fever response, while IL-8 is released to recruit and activate more neutrophils. IL-6, which under normal conditions is a key indicator for B-cell maturation, T-cell development, and antibody production, acts as a key pro-inflammatory cytokine, triggering inflammation in response to infection. At the same time, the complement system is activated directly by the pathogen.
- Acquired Response: Antigens from the pathogen are presented to T and B cells by dendritic cells to identify and activate the correct cells. Some activated T cells (cytotoxic T cells) identify virally infected cells and kill them, other T cells (T helper cells) stimulate the correct B cells to produce antibodies. The specific antibodies produced do things like neutralizing toxins and causing agglutination.
- Resolution: The immune system eliminates the pathogen, and the body repairs damage.
- Memory: Where producing antibodies after the first exposure to a pathogen can take several days to weeks, T and B cells retain a memory of the pathogen for quicker response in future encounters.
What to Look for in Host Response to Infection?
The host’s response to infection is a critical factor in determining the outcome and severity of disease. Host response to infection diagnostics focus on understanding and measuring the host's biological response to an infection rather than solely detecting the pathogen itself. By analyzing biomarkers, gene expression, and immune response profiles, host response to infection diagnostics provide a comprehensive view of how the body is reacting to the infection. This approach can offer insights into infection type and severity, disease progression, and immune response effectiveness.
Difference Between Host Response to Infection and Pathogen Identification
Sometimes, to effectively treat an infection, it is necessary to identify the specific pathogen and its antimicrobial susceptibility through methods such as cultures, molecular tests, mass spectrometry, and antigen detection. However, these methods only confirm that the pathogen is present but do not provide insight into the immune response of the patient. Obtaining results can also be time consuming, leaving clinicians with diagnostic uncertainty in time-critical situations.
Host response to infection diagnostics, on the other hand, do what the name suggests—they focus on the host’s response. These methods target changes in the host’s immune system such as increases in inflammatory markers and fluctuations in other physiological parameters that occur due to infection.
Are There Differences in Interpreting Host Response to Infection?
Host response diagnostics can be single biomarkers or panels. Single biomarkers are typically reported as a discrete measured value with a regulatory cleared cutoff and reference ranges, so interpretation is relatively straightforward. Biomarker panels aggregate multiple measurements into a single reported value, such as a risk score, so interpretation is more nuanced.
Emerging Host Response to Infection Diagnostic Tools
In addition to established biomarkers such as WBC, PCT, CRP and novel biomarkers such as Monocyte Distribution Width (MDW), technological innovations are paving the way for new tools and methods.
- Complex biomarker assays: For example, MeMed BV® computationally integrates the levels of three host immune proteins—TRAIL, IP-10 and CRP—into a single score indicating the likelihood of a bacterial immune response or co-infection as compared to a viral immune response.
- Next-Generation Sequencing (NGS): This technology allows for comprehensive analysis of the host’s genomic response to infections
- Artificial Intelligence (AI): AI and machine learning algorithms can analyze vast amounts of data from host response to infection diagnostics and other health information to predict disease outcomes and guide treatment decisions
The future of host response to infection diagnostics is promising. By focusing on the host’s response, these diagnostics offer a more holistic approach to treatment and promise to provide advancements in understanding and managing infectious diseases, paving the way for better patient outcomes.
