What is the first antibody made in response to an antigen?

The first antibody produced in response to an antigen is IgM. When the immune system detects an invader, such as a virus or bacterium, it initiates an immune response. IgM is typically the first antibody type generated during the primary immune response.

IgM is a pentameric antibody, meaning it consists of five subunits that are linked together. This structure allows it to effectively bind to multiple antigens at once, providing an early and robust response to new pathogens. It is produced by B cells shortly after they encounter an antigen and is important for activating complement and marking pathogens for destruction.

In contrast, other antibody classes like IgA, IgG, and IgE are produced later in the immune response or in specific contexts, such as IgG for long-term immunity and IgA for mucosal immunity. IgE is associated primarily with allergic reactions and responses to parasitic infections. Thus, while all these antibodies play important roles in the immune system, IgM is specifically recognized as the first antibody produced upon initial exposure to an antigen.