Primary response: when the body first encounters an antigen and the immune response is triggered, the first class of antibody to be produced is IgM. After a few days, the B cells switch into production of IgG antibodies which circulate through the body, helping to clear the pathogen. During primary response, it can take more than a week before there is strong production of IgG antibody. The secondary response is quite different. When the body encounters an antigen the second time, memory B cells rapidly divide and differentiate into plasma cells and begin production of IgG and IgM antibodies. IgG production is particulary strong and far exceeds the primary response.