What is the first and largest antibody produced in response to an infection?

The first and largest antibody produced in response to an infection is IgM. This class of immunoglobulin is typically the first to be secreted by B cells when the body encounters a pathogen. Upon initial exposure to an antigen, B cells undergo a process called class switching, which allows them to produce different types of antibodies; however, IgM is the first to be generated during the immune response.

The structure of IgM is pentameric, meaning that it consists of five monomeric units connected together. This unique structure allows for a high degree of binding and makes IgM particularly effective at agglutinating pathogens and activating the complement system. Because of its early production and effectiveness, IgM plays a crucial role in the body's initial defense against infections.

In contrast, IgG is more prevalent in later stages of immune response, primarily providing long-term immunity and memory. IgA is mainly found in mucosal areas and bodily secretions, including saliva and breast milk, providing localized protection. IgE is associated primarily with allergic reactions and responses to parasitic infections. Each type of antibody plays a specific role in the immune system, but in terms of initial response and size, IgM holds a unique position.