**5.4 Longevity of antibody responses to malaria antigens**

Among people living in endemic areas, levels of antibodies to many malaria antigens vary with the seasonality of malaria transmission, often being higher during periods of high malaria transmission than at the end of a low transmission season (Cavanagh *et al* 1998, Giha *et al* 1998, Nebie *et al* 2008). Second, levels of antibodies to malaria antigens often tend to be higher in individuals who also have malaria parasites at the time when their antibodies are measured than in those without parasites (al-Yaman *et al* 1995, Bull *et al* 2002, Kinyanjui *et al* 2004). These phenomena are typically seen in young children, probably because adults typically have much higher antibody levels that take longer to decay appreciably even in the absence of an infection [(Fruh *et al* 1991, Riley *et al* 1993, Taylor *et al* 1998). These observations and those from other longitudinal studies where malaria antibodies fell from relatively high levels to low levels within a few weeks of treatment of a clinical episode (Branch *et al* 1998, Fonjungo *et al* 1999, Fruh *et al* 1991) suggest that antibody responses to many malaria antigens are relatively short-lived. The preponderance of IgG3 subclass, which has a shorter half-life than the other IgG subclasses, might, in part, explain the brevity of antimalarial antibody responses. However, detailed kinetics studies on the decay of antimalarial antibodies suggest that even the other subclasses decline at a rate that is faster than can be explained by normal catabolic decay (Kinyanjui *et al* 2003). This brevity of circulating antibody responses might explain the rapid re-infection seen among individuals living in endemic areas after malaria treatment.
