high levels of antibodies, with the same range of antigen specificities as their mother. Breast milk contains antibodies (mainly IgA) that are transferred to the gut of the infant, protecting against bacterial infections, until the newborn can synthesize its own antibodies. This is passive immunity because the fetus does not actually make any memory cells or antibodies: It only borrows them. Short-term passive immunity can also be transferred artificially from one individual to another via antibody-rich serum. Active memory In general, active immunity is long-term and can be acquired by infection followed by B cell and T cell activation, or artificially acquired by vaccines, in a process called immunization. Immunization Historically, infectious disease has been the leading cause of death in the human population. Over the last century, two important factors have been developed to combat their spread: sanitation and immunization. Immunization (commonly referred to as vaccination) is the deliberate induction of an immune response, and represents the single most effective manipulation of the immune system that scientists have developed. Immunizations are successful because they utilize the immune system's natural specificity as well as its inducibility. The principle behind immunization is to introduce an antigen, derived from a disease-causing organism, that stimulates the immune system to develop protective immunity against that organism, but that does not itself cause the pathogenic effects of that organism. An antigen (short for antibody generator), is defined as any substance that binds to a specific antibody and elicits an adaptive immune response. Most viral vaccines are based on live attenuated viruses, whereas many bacterial vaccines are based on acellular components of microorganisms, including harmless toxin components. Many antigens derived from acellular vaccines do not strongly induce an adaptive response, and most bacterial vaccines require the addition of adjuvants that activate the antigen-presenting cells of the innate immune system to enhance immunogenicity. Immunological diversity Most large molecules, including virtually all proteins and many polysaccharides, can serve as antigens. The parts of an antigen that interact with an antibody molecule or a lymphocyte receptor, are called epitopes, or antigenic determinants. Most antigens contain a variety of epitopes and can stimulate the production of antibodies, specific T cell responses, or both. A very small proportion (less than 0.01%) of the total lymphocytes are able to bind to a particular antigen, which suggests that only a few cells respond to each antigen. For the acquired response to "remember" and