Specific antibody is highly effective, both by directing complement lysis and inducing opsonisation and phagocytosis. Some bacteria have evolved capsules which prevent recognition by innate mechanisms and require both antibody and complement opsonisation to promote efficient clearance by phagocytes. Where toxins are produced antibody is of course vital. Parasites Large, multicellular parasites present a special problem to the immune system and indeed are rather poorly eliminated. The mechanisms deployed include antibody directed complement attack and ADCC, in particular by eosinophils.
Innate immunity is generally ineffective.
Intracellular organisms Bacteria and Protozoa Many bacteria have evolved resistance to the constitutive killing mechanisms used by phagocytes. These pathogens actively replicate inside bacteria, either in the phagosome or, in some cases where a specific adaptive mechanism has been acquired, in the cytoplasm. This type of bacteria cannot be eliminated by immediate immune mechanisms. Antibody is generally ineffective in eliminating a first infection. Cytotoxic T cells also play a role in clearing many intracellular organisms.