Antibodies bind to and block, or neutralize, the infectivity of microbes and the interactions of microbial toxins with host cells (Fig. 1). Antibodies in mucosal secretions in the gut and airways block the entry of ingested and inhaled microbes (discussed later in the chapter). After microbes enter the host, they use molecules in their envelopes or cell walls to bind to and gain entry into host cells. Antibodies may attach to these microbial surface molecules, thereby preventing the microbes from infecting host cells. The most effective vaccines available today work by stimulating the production of neutralizing antibodies that block initial infection. Microbes that are able to enter host cells may replicate inside the cells and then be released and go on to infect other neighboring cells. Antibodies can neutralize the microbes during their transit from cell to cell and thus also limit the spread of infection. If an infectious microbe does colonize the host, its harmful effects may be caused by endotoxins or exotoxins, which often bind to specific receptors on host cells in order to mediate their effects. Antibodies prevent binding of the toxins to host cells and thus block their harmful effects. Emil von Behring and Shibasaburo Kitasato’s demonstration of this type of protection mediated by the administration of antibodies against diphtheria toxin was the first formal demonstration of therapeutic immunity against a microbe or its toxin, then called serum therapy, and the basis for awarding Behring the first Nobel Prize in Physiology or Medicine in 1901. 

Fig1. Neutralization of microbes and toxins by antibodies. A, Antibodies at epithelial surfaces, such as in the gastrointestinal and respiratory tracts, block the entry of ingested and inhaled microbes, respectively. B, Antibodies prevent the binding of microbes to cells, thereby blocking the ability of the microbes to infect host cells. C, Antibodies block the binding of toxins to cells, thereby inhibiting the pathologic effects of the toxins.