Which antibody is the first to be formed after exposure to an antigen?

Immunoglobulin M (IgM) is the first antibody to be produced in response to an initial exposure to an antigen. This occurs because IgM is the primary antibody formed during the early stages of the immune response. When the immune system encounters a pathogen or antigen for the first time, B cells are activated and differentiate into plasma cells that secrete IgM.

IgM plays a crucial role in the body's defense against infections, as it can effectively neutralize pathogens and activate the complement system to enhance the immune response. Its structure as a pentamer allows for multiple binding sites, making it highly effective in clumping antigens together and facilitating their removal.

In contrast, while other antibody types like IgG, IgA, and IgE have important roles in the immune system, they appear later in the immune response or are involved in different functions, such as long-term immunity (IgG), mucosal immunity (IgA), and allergic responses (IgE). Therefore, the answer points to IgM as the initial responder to foreign antigens following the first exposure.