Pathogen defence strategies against immunity
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agent can evade immune surveillance is by altering its antigens; this is particularly important for extracellular pathogens, against which the principal defense is the production of antibody against their surface structures. There are three ways in which antigenic variation can occur. First, many infectious agents exist in a wide variety of antigenic types. There are, for example, 84 known types of Streptococcus pneumoniae, an important cause of bacterial pneumonia. Each type differs from the others in the structure of its polysaccharide capsule. The different types are distinguished by serological tests and so are often known as serotypes. Infection with one serotype of such an organism can lead to type-specific immunity, which protects against reinfection with that type but not with a different serotype. Thus, from the point of view of the adaptive immune system, each serotype of S. pneumoniae represents a distinct organism. The result is that essentially the same pathogen can cause disease many times in the same individual (Fig. 11.1).