免疫學(第2版．供醫學各專業本科生、留學生、長學制、研究生用)（簡體書）

Clearance of the initial infection provides the basis for protection againstfuture infection as well. In addition to the generation of increased numbersof virus-specific CD4~ and CD8~ T cells, generation of virus-specific an-tibodies also occurs, although usually too late to participate in clearanceof the original infection. However, the generation of neutralizing antibod-ies to inhibit viral infectivity is the primary means of limiting or preventingreinfection (see Chapter 11). During reexposure to a particular virus, thenumber of viral particles able to enter host cells is drastically reducedby neutralizing antibodies, and viruses that do succeed in entering hostcells are rapidly dealt with by heightened secondary cell-mediated re-sponses. Resistance to reinfection can be so effective that no noticeaLlesigns or symptoms develop.
2. Bacteria: Most bacteria spend their entire existence in a fluidextracellular environment. Others spend much of their time withinhost cells (intracellular), although they must spend some time inan extracellular environment as they establish an initial infection ormove on to infect additional cells.
a.Extracellular bacteria: Many extracellular pathogenic bacteria, including Staphylococcus, Streptococcus, Neisseria,Bordetella, and Yersinia, infect humans. These organisrns,once they are within the host, are constantly exposed to hu-moral host defenses (complement and antibodies) as well.