When an antigen is administered for the first time to an animal or a human being who has never been exposed to it, the first antibody to develop –
**Core Concept**
The primary immune response to a novel antigen involves the activation of antigen-presenting cells (APCs), such as dendritic cells, which process and present antigenic peptides to T-cells. This leads to the activation of naive T-cells, which then differentiate into effector T-cells and B-cells. The B-cells undergo clonal expansion and differentiate into plasma cells, which produce antibodies.
**Why the Correct Answer is Right**
The first antibody to develop after primary exposure to an antigen is IgM. IgM is a pentameric antibody with high avidity, which allows it to effectively neutralize and remove pathogens from the body. The production of IgM is initiated by the activation of the B-cell receptor complex, which triggers the activation of the B-cell receptor-associated signaling molecules. This leads to the activation of the transcription factor NF-κB, which drives the expression of IgM gene.
**Why Each Wrong Option is Incorrect**
**Option A:** IgG is a monomeric antibody that is produced later in the immune response, typically after the initial IgM response. It is responsible for long-term immunity and is the most abundant antibody isotype in the serum.
**Option B:** IgE is an antibody involved in allergic reactions and is not the first antibody to develop after primary exposure to an antigen.
**Option C:** IgA is an antibody that provides mucosal immunity and is not the first antibody to develop after primary exposure to an antigen.
**Clinical Pearl / High-Yield Fact**
The initial IgM response is often referred to as the "primary immune response." It is characterized by a rapid increase in antibody production, which peaks within 7-10 days after antigen exposure. This response is typically short-lived and is followed by a secondary immune response, which is mediated by IgG antibodies and provides long-term immunity.
**Correct Answer:** A. IgM