IgM is a particular isotype of Antibody. When IgM is expressed as a membrane protein it is termed the "B-cell Receptor". In the membrane-bound form, binding of antigen to IgM can induces a variety of signal transduction cascades in the B-cell which indicate that the B-cell Receptor has encountered antigen. Consequently, expression of membrane IgM as B-cell receptor is the primary format in which B-cells express antibody during B-cell Development and the initial stages of the Humoral Immune Response.