The immune system protects the body from various threats, including disease-causing microorganisms and abnormal cells. A significant part of this defense is Cell-Mediated Immunity (CMI), which plays a role in the body’s protective responses.
What is Cell-Mediated Immunity?
Cell-Mediated Immunity (CMI) is a type of immune response that relies on specialized cells rather than antibodies to defend the body. This arm of the adaptive immune system is primarily driven by T-cells, macrophages, and natural killer (NK) cells. T-cells, which mature in the thymus, are central to CMI, recognizing and targeting infected or abnormal cells.
Macrophages are large white blood cells that engulf and digest cellular debris, foreign substances, microbes, and cancer cells, also presenting antigens to T-cells to initiate an immune response. Natural killer cells identify and destroy infected cells and cancer cells directly. Together, these components allow CMI to target intracellular pathogens like viruses, certain bacteria, fungi, protozoans, and cancerous cells.
CMI contrasts with humoral immunity, which primarily involves B-cells producing antibodies that circulate in body fluids to neutralize extracellular threats. While distinct, these two branches of the adaptive immune system often cooperate to provide comprehensive protection. CMI is particularly effective against threats that reside within host cells, where antibodies cannot reach.
How Cell-Mediated Immunity is Measured
Assessing Cell-Mediated Immunity involves specialized laboratory tests that evaluate the activity and presence of specific immune cells. These tests, categorized as in vivo (within the living body) or in vitro (in a test tube or culture dish), aim to detect how effectively immune cells respond to specific antigens.
One common in vivo method is delayed-type hypersensitivity (DTH) skin testing. This involves injecting a small amount of antigen, such as purified protein derivative (PPD) for tuberculosis, under the skin. A positive reaction, characterized by localized inflammation and induration after 48 to 72 hours, indicates the presence of antigen-specific memory T-cells.
In vitro assays offer more detailed insights by analyzing immune cell behavior in a controlled laboratory setting. Techniques like lymphocyte proliferation assays measure the ability of T-cells to multiply in response to an antigen, often by tracking DNA synthesis. Another widely used method is flow cytometry, which quantifies different immune cell subsets, such as specific T-cell types (CD4+ and CD8+), and assesses their activation markers. Flow cytometry can also detect intracellular cytokine production, which indicates T-cell activation.
Enzyme-linked immunospot (ELISPOT) assays are highly sensitive for detecting cells that secrete specific cytokines, such as interferon-gamma (IFN-γ), in response to antigen stimulation. Measuring cytokine levels in cell culture supernatants using techniques like ELISA or multiplex assays also provides information on the immune response.
Interpreting CMI Test Results
Interpreting Cell-Mediated Immunity test results requires healthcare professional expertise, considering a patient’s medical history, symptoms, and other findings. CMI results are not standalone diagnostic values; a “normal” range varies by laboratory and assay.
Higher-than-normal CMI activity might indicate a robust immune response to an ongoing infection or a strong reaction to a vaccine. It could also suggest immune activation in autoimmune conditions, where the immune system mistakenly attacks the body’s own tissues. Conversely, lower-than-normal CMI could point to immunodeficiency, meaning the immune system is not responding effectively to threats. This might be seen in individuals with certain genetic conditions, those undergoing immunosuppressive therapies, or in the context of chronic infections that weaken the immune system.
The interpretation also considers whether the CMI response is specific to a particular pathogen or antigen, providing insight into the immune system’s memory and ability to target specific threats. For instance, a strong CMI response to a particular virus suggests prior exposure or successful vaccination. These results help guide further investigation and personalized treatment strategies.
When CMI Assessment Matters
Assessing Cell-Mediated Immunity is relevant in various medical scenarios, offering insights into immune function and disease progression. One significant application is in monitoring immune function in transplant recipients, where CMI tests can help manage immunosuppressive medications to prevent organ rejection while minimizing infection risk. These tests can also evaluate the immune response to certain infectious diseases, such as tuberculosis or cytomegalovirus, helping diagnose active infections or determine past exposure.
CMI assessment also plays a role in evaluating the effectiveness of vaccines, especially those designed to induce cellular rather than antibody-based immunity. In cancer treatment, CMI tests can gauge a patient’s immune response to tumors and monitor the efficacy of immunotherapies. CMI profiling can contribute to understanding and managing autoimmune conditions, where immune cells mistakenly attack healthy tissues.