1. Because a specific Ab is required, immunity from an initial infection is delayed while the plasma cells mature and proliferate.
2.. Its specificity oc means that large numbers of identical plasma cells are produced that synthesize a myriad of monoclonal antibody, which can neutralize, agglutinate, or coagulate a lot of the insulting pathogen. HOWEVER, the specificity means that if the pathogen mutates esp if significantly [like Omicron in the current enigma faced by us humans] then its effectivity is reduced or even absent.
3. Some types of Ab, in particular IgM [immunoglobulin M - a large Ab with multiple binding sites for Ag] CANNOT cross the placental barrier, so that the neonate may not be afforded full immunity [babies can only synthesize their own Ab-s approx from the age of 6 months, hence the start of vaccination e.g. DTP around this stage ].