B lymphocytes, or B cells, and antibodies are designed to recognize extracellular invaders and their byproducts, such as toxins. To accomplish this, they recognize epitopes on intact antigens, usually proteins, triggering the adaptive immune response. Isotypes are biochemically different classes of antibodies that perform unique functions in an immune response, allowing precise targeting of epitopes of microbes. Immunological memory involves memory B cells, long-lived plasma cells, and persistence of antigen, and supports the immune response for repeated infections or those invaders that persist after initial responses. This memory response depends on the replicative strategies of the invader or, in the case of vaccination, the method and composition of the vaccine.