In conclusion, despite significant progress in the field of nucleic acid delivery in vitro as well as in vivo, there still is a long way to go before this will become a standard procedure in the clinic. Certain problems like endosomal escape are known for more than twenty years and still far from being resolved. In order to develop new strategies, more information about intracellular processes involved in nucleic acid trafficking is needed. Moreover, it would be desirable if the field would move from a qualitative description towards a quantitative evaluation preferentially using standardized model systems. This would allow for comparison of different approaches with one another. While animal studies are inevitable in the long run, there still is a lot of room for improvement on the cellular level. So it might be worthwhile to fathom how far we can push the different systems on this level.  Human pregnancy-associated malaria-specific B cells target polymorphic, conformational epitopes in VAR2CSA Pregnancy-associated malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes (IEs) that bind to chondroitin sulphate A (CSA) in the placenta by PAM-associated clonally variant surface antigens (VSA). Pregnancy-specific VSA (VSA(PAM)), which include the PfEMP1 variant VAR2CSA, are targets of IgG-mediated protective immunity to PAM. Here, we report an investigation of the specificity of naturally acquired immunity to PAM, using eight human monoclonal IgG1 antibodies that react exclusively with intact CSA-adhering IEs expressing VSA(PAM). Four reacted in Western blotting with high-molecular-weight (> 200 kDa) proteins, while seven reacted with either the DBL3-X or the DBL5-ε domains of VAR2CSA expressed either as Baculovirus constructs or on the surface of transfected Jurkat cells. We used a panel of recombinant antigens representing DBL3-X domains from P. falciparum field isolates to evaluate B-cell epitope diversity among parasite isolates, and identified the binding site of one monoclonal antibody using a chimeric DBL3-X construct. Our findings show that there is a high-frequency memory response to VSA(PAM), indicating that VAR2CSA is a primary target of naturally acquired PAM-specific protective immunity, and demonstrate the value of human monoclonal antibodies and conformationally intact recombinant antigens in VSA characterization. People living in areas of intense transmission of Plasmodium falciparum parasites acquire protective immunity to malaria during childhood, and the bulk of mortality and severe morbidity from P. falciparum malaria is therefore concentrated among young children. Protective immunity acquired in response to P. falciparum exposure appears to be mediated mainly by IgG antibodies specific for variant surface antigens (VSA) that mediate sequestration of infected erythrocytes (IEs) in various tissues (reviewed by Hviid, 2005) . Despite pre-existing protective immunity, women become highly susceptible to P. falciparum infection when they become pregnant, and pregnancyassociated malaria (PAM) is a major cause of mother/ offspring morbidity (Guyatt and Snow, 2001; 2004) . However, in areas of stable P. falciparum transmission, susceptibility to PAM rapidly declines with increasing parity, consistent with acquisition of PAM-specific protective immunity (reviewed by Hviid, 2004) . PAM is caused by P. falciparum-IEs selectively accumulating in the placental intervillous space through VSA PAM-mediated adhesion to chondroitin sulphate A (CSA). VSAPAM differ in several ways from VSA expressed on IEs obtained from males and non-pregnant females. Thus, only VSAPAM mediate binding to CSA in vitro (Fried and Duffy, 1996) and only VSAPAM-expressing IEs are consistently not recognized by IgG in the plasma of P. falciparum-exposed women who have never been pregnant or by IgG in plasma from similarly exposed men (Beeson et al., 1999; Ricke et al., 2000) . These observations, and the fact that plasma levels of VSAPAM-specific IgG increase with increasing parity (Fried et al., 1998; Ricke et al., 2000) , are consistent with evidence that these antibodies are the mediators of protective immunity to PAM (Duffy and Fried, 2003; Staalsoe et al., 2004) .