UNITED STATES—Mouse IgG antibodies are commonly used in immunology to detect a protein or study how a mouse fights off a virus. However, IgG is not a single type of molecule. It is a family divided into four subclasses, including IgG1, IgG2a/c, IgG2b, and IgG3.
Each subclass shares the same Y-shaped structure and can target a specific antigen. However, their effector functions are different. The reaction of the body after an antibody targets an antigen depends on the subclass.
Some subclasses simply attach to the target while others trigger an immune response to completely destroy the target. Choosing the wrong subclass can lead to misleading results in experiments. Therefore, it is important for researchers to understand the unique biological profile and functional properties of each subclass.
Structural Architecture and Genetics
The Hinge Region
The length and flexibility of the hinge depend on the subclass. For example, IgG3 has an exceptionally long and rigid hinge, while IgG1 and IgG2 subclasses are shorter. The length and flexibility determine how well the antibody can reach and bind to targets.
The IgG2a vs. IgG2c Debate
IgG2a and IgG2c are different alleles (variants) of the same subclass. BALB/c mice have the gene for IgG2a, and C57BL/6 mice have the gene for IgG2c. While both are functionally identical, the key difference is in choosing secondary antibodies. If you use an anti-IgG2a secondary on a sample from a C57BL/6 mouse, it may not bind at all.
Glycosylation
Glycosylation is a post-translational modification in which a sugar chain is attached to the heavy chain of the IgG antibody. The antibody becomes structurally unstable without this modification. If the glycosylation is wrong, the antibody might bind to its target but fail to trigger an immune response.
Individual Subclass Profiles
IgG1: The Th2 Mediator
IgG1 is the most common subclass in mice, and it is primarily involved in Th2-type immune responses used to fight off parasites or handle allergic reactions. While it binds to its target really well, it fails to trigger a chemical reaction that breaks the membrane of the invading cell. As a result, it cannot destroy the invading cell. It prefers to bind to a specific receptor called FcγRIII. Researchers use it to label a cell or block a protein without killing the cell itself.
IgG2a & IgG2c: The Th1 “Warriors”
IgG2a and IgG2c also trigger a Th1-type immune response, but unlike the IgG1 subclass, this subclass targets and destroys the invading cells. They induce Antibody-Dependent Cellular Cytotoxicity (recruiting natural killer cells) to execute a target cell that has been flagged. This subclass is useful in the development of drugs meant to kill cancer cells in a mouse model.
IgG2b: The Flexible Responder
While it also plays a role in a Th1-type immune response, IgG2b is less aggressive. It is produced in response to many different types of threats, including those that do not always involve T cells. It has a very flexible hinge region, allowing it to bind to antigens that are hard to reach on a cell. It binds effectively to FcγRIV (an activating receptor) but also to FcγRIIB (an inhibitory receptor) to help the immune system stay balanced and prevent it from overreacting.
IgG3: The Early Defense
This is the first subclass produced in the body once a bacterial infection is detected, especially bacteria with sugar-coated outer layers. It targets and stops the invader from entering a healthy cell.
It has a unique ability to clump together with other IgG3 molecules, which form a strong web over a bacterial surface. This makes it easy for the immune system to spot and clear away the invader. However, this clumping also makes it difficult to work with this subclass in the lab.
Applications in Research
Antibody Purification
Researchers use Protein A or Protein G to isolate antibodies from serum. Protein A binds strongly to IgG2a and IgG2b, but it has a very weak affinity for IgG1. When purifying a mouse polyclonal antibody, this difference is a major factor. When the buffer is not perfectly calibrated, researchers may wash away the IgG1 portion. The result is a purified mouse polyclonal antibody that does not represent the original immune profile of the mouse. By adjusting the pH of the wash buffers, researchers can choose to either isolate the entire mouse polyclonal antibody pool or specifically separate the subclasses from one another.
Subclass-Specific Multiplexing
Each subclass has a unique genetic sequence. Researchers can use secondary antibodies that target only one specific subclass. This helps detect multiple proteins in a single sample.
Functional Assays and Cell Depletion
The subclass plays an important role in the success of experiments in live-animal studies. Researchers use an IgG2a antibody if the study requires the depletion of a specific cell type.
