Neutralizing antibody titer and viral load impact the ability to prevent infection of susceptible cells. (a) With a high antibody titer and a low viral load, neutralizing antibody may be able to block all of the binding sites on the virus and prevent viral entry and HIV infection. (b) With a low antibody titer and a low viral load, neutralizing antibody may not be able to prevent infection. (c) Although the antibody titer is high, if the viral load is high, the neutralizing antibody may not be able to prevent infection. (d) A low antibody titer and a high viral load is the worst possible situation, in which the virus is most likely to infect even in the presence of neutralizing antibody