Immunological memory is our reservoir of ready-to-use antibodies and memory B cells. Because of immunological memory a secondary infection will be very light or not occur at all. Antibodies and cells, generated in the germinal center in response to the first encounter with antigen, are highly specific, remain in the organism virtually forever and are mostly of IgG isotype. Long lived plasma cells homing to the bone marrow ensure the constant production of protective antibodies, whereas switched memory B cells proliferate and differentiate in response to secondary challenge. IgM memory B cells represent our first-line defense against infections. They are generated by a T-cell independent mechanism probably triggered by Toll-like receptor-9. They produce natural antibodies with anti-bacterial specificity and the spleen is indispensable for their maintenance. We will review the characteristics and functions of IgM memory B cells that explain their importance in the immediate protection from pathogens. IgM memory B cells, similar to mouse B-1a B cells, may be a remnant of a primitive immune system that developed in the spleen of cartilaginous fish and persisted throughout evolution notwithstanding the sophisticated tools of the adaptive immune system.