The term “Ig percent” in a blood test typically refers to the measurement of Immunoglobulins (Ig), which are specialized proteins also known as antibodies. These proteins are a central part of the body’s immune system, circulating in the blood and tissue fluids to defend against foreign invaders. The test, formally called a quantitative immunoglobulin assay, measures the total amount of these proteins and quantifies the specific amounts of the major classes. While “IG%” can also refer to the percentage of Immature Granulocytes, the clinical context of diagnosing immune disorders almost always relates to the immunoglobulin protein profile.
The Role of Immunoglobulins
Immunoglobulins are the core agents of the adaptive immune system, providing a highly specific defense mechanism against a vast array of threats. They are produced by specialized white blood cells called plasma cells, which are derived from B lymphocytes. The fundamental purpose of these Y-shaped proteins is to identify and neutralize foreign invaders, known as antigens.
Each antibody is designed to bind precisely to a unique target, much like a lock fitting a specific key. This binding action can directly neutralize a pathogen, such as by blocking a virus from entering a cell. Antibodies also serve as molecular flags, coating invaders to signal other immune cells to destroy the tagged threat. This response allows the body to effectively clear infections and establish long-term protection, often called immune memory.
Understanding the Five Classes of Antibodies
The body produces five major classes of immunoglobulins, each with a distinct structure and primary location. Immunoglobulin G (IgG) is the most abundant class, making up approximately 70 to 80 percent of all immunoglobulins in the blood. IgG provides long-term immunity, is active in tissue fluids, and is the only class capable of crossing the placenta to protect a developing fetus.
Immunoglobulin A (IgA) accounts for about 10 to 15 percent of the total and is primarily found in mucosal secretions like saliva, tears, and the lining of the respiratory and digestive tracts. Its main function is to guard body surfaces, acting as a first line of defense against inhaled or ingested pathogens.
Immunoglobulin M (IgM) is a large, pentameric structure that makes up around 5 to 10 percent and is the first antibody produced during a new infection. Its structure allows it to quickly clump together pathogens in the bloodstream while the body mounts a more targeted response.
The remaining classes are present in much smaller percentages. Immunoglobulin E (IgE) is found in trace amounts and is responsible for triggering allergic reactions by binding to mast cells and basophils, and is also involved in defending against parasitic infections. Immunoglobulin D (IgD) is found mainly on the surface of B cells, where it functions primarily as a receptor to help initiate B cell activation and maturation.
Why Doctors Order This Specific Test
A quantitative immunoglobulin test is ordered to assess the functional health of a patient’s humoral immune system. Doctors use the test when a patient presents with symptoms suggesting an underlying immune problem. These symptoms often include recurrent, severe, or unusual infections, particularly in the respiratory tract, such as chronic sinusitis or pneumonia.
Testing is necessary to investigate suspected primary immunodeficiency disorders, where the body fails to produce adequate levels of one or more antibody classes. Abnormal immunoglobulin levels can also be caused by acquired conditions or chronic infections. Additionally, the test is a standard tool for evaluating and monitoring plasma cell disorders, such as multiple myeloma.
Interpreting Abnormal Levels and Ratios
The levels of individual immunoglobulin classes, and their ratios to one another, provide specific clues about a patient’s health status. An elevated level of a single class, known as a monoclonal increase, may suggest a plasma cell malignancy like multiple myeloma. Conversely, an increase in all classes, called a polyclonal increase, is commonly seen in chronic infections or autoimmune diseases, reflecting a broad immune system activation.
A significantly high IgM level often indicates a recent or acute infection, since it is the first antibody produced in response to a new antigen. Low levels of IgG can be a sign of an immunodeficiency disorder or may indicate a loss of protein from the body due to kidney or gut disease. An elevated IgE level is a strong indicator of an active allergic process or a parasitic infection. Interpreting these results requires looking at the overall pattern and correlating it with the patient’s clinical history.