Only one of these is expressed at any one time by being However, contains about 1000 VSG genes, each encoding a protein with distinctĪntigenic properties. Response that rapidly clears most of the parasites. Variant-specific glycoprotein (VSG), which elicits a potent protective antibody The trypanosome is coated with a single type of glycoprotein, the The extracellular tissue spaces of the body and cause sleeping sickness in Trypanosomes are insect-borne protozoa that replicate in Trypanosomes, where changes in the major surface antigen occur repeatedly withinĪ single infected host.

The most striking example occurs in African The third mechanism of antigenic variation involves programmed rearrangements in Susceptible to the new virus, and severe infection results. The resulting virus is recognized poorly, if at all, by antibodies andīy T cells directed against the previous variant, so that most people are highly Host, leading to major changes in the hemagglutinin protein on the viral Genome of the influenza virus and related animal influenza viruses in an animal This happens when there is reassortment of the segmented RNA Major influenza pandemics resulting in widespread and often fatal disease occurĪs the result of the second process, which is termed antigenic With antibodies and T cells produced against the previous variant of the virus,Īnd therefore most of the population have some level of immunity (see Section 10-25). This causesĪn epidemic that is relatively mild because there is still some cross-reaction Immune to, the old variant are thus susceptible to the new variant. Individuals who were previously infected with, and hence are Particular, CD8 T cells, so that cells infected with the mutant virus alsoĮscape destruction. Other mutations affect epitopes that are recognized by T cells and, in That allow the virus to evade neutralization by antibodies in the population Every 2–3 years, a variant arises with mutations Is caused by point mutations in the genes encoding hemagglutinin and a second Neutralizing antibody that mediates protective immunity is directedĪt the viral surface protein hemagglutinin (H), which is responsibleįor viral binding to and entry into cells. Two types of variation allow repeated infection with type A The result is that essentially the same pathogen can causeĭisease many times in the same individual ( Fig. pneumoniae represents aĭistinct organism. Thus, from the point of view of the adaptive To type-specific immunity, which protects against reinfection with that type but Infection with one serotype of such an organism can lead Tests and so are often known as serotypes.

The different types are distinguished by serological Each type differs from the others in the structure of its Known types of Streptococcus pneumoniae, an important cause ofīacterial pneumonia. First, many infectiousĪgents exist in a wide variety of antigenic types.

There are three ways in which antigenic variation can occur. Its antigens this is particularly important for extracellular pathogens,Īgainst which the principal defense is the production of antibody against their One way in which an infectious agent can evade immune surveillance is by altering Antigenic variation allows pathogens to escape from immunity

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