Because other micro-organisms have different antigens, and the immune system must start again from scratch. Different strains have different antigens on the outside, so the body's immune system may not be prepared for that specific strain, so that it can get into cells and reproduce. The body's immune system alone was not enough to prevent epidemics of these diseases, and treatment with antibiotics tipped the balance in our favour. If strains of these pathogens are selected due to improper use of antibiotics, there will be fewer medical alternatives. Never say that a bacterium is immune to, or has immunity to, an antibiotic, for instance.
Resistance applies to the ability of a micro-organism to survive the effects of an antibiotic.
Since resistance to antibiotics is likely to be genetically based, there is fear that populations of resistant strains may become established. Passive immunity is based on ready-made antibodies. Passive immunity may also be obtained via serum - derived from blood of organisms exposed to a pathogen. The specificity of this antigen-antibody reaction can be of even more use to Man when it is further refined and developed. Since several different lymphocytes are involved in the production of a cocktail of antibodies, these are known as polyclonal antibodies .