All artificial immunizations lead to more or less specific reactions. These reactions may occur at the site of administration of immunising agent: regionally in the area around it: at sites far removed, in the viscera specifically concerned with immune response and those to which the dissemination of the immunising agent may occur.

Normal BCG Reaction: In the uninfected individuals the reaction starts about 2-4 weeks after vaccination and all the stages from erythema, nodule, pustule, ulcer, crust and scar formation are over in 4-6 weeks period in that sequence at the site of BCG vaccination. Regional lymph glands enlargement depends upon the exact location of injection. Entire sequence of occurrences are painless and uneventful. In previously infected persons or repeat BCG vaccination results in exaggerated and accelerated reaction of similar nature as described above. The reaction starts within hours of vaccination and healing of ulcer to scar formation is over within 2 weeks time without any lymph gland involvement. BCG organisms are also absorbed into the blood stream and produce multiple reactions at different sites.

Abnormal Reactions: Reactions other than those described above or those which do not conform to the stated time schedule are recognised as abnormal reactions or complications. The causes of these complications are attributed to the degree of attenuation, dose, concentration of vaccine, subcutaneous or muscular instead of intradermal, infection of ulcer, constitution of the subject and tuberculo hypersensitivity manifestiations anywhere in the body. The complications are of three types. i) local, ii) regional and iii) general. Local Complications: Undulating ulcer, abscess formation due to deep vaccination, Eczematous lesion, lupoid reaction and keloid formation. Regional Complications: enlargement of lymphnode, sinus formation. General Complications: Erythema nodosum, phlyctenular conjunctivitis, BCG Osteoitis, generalised tuberculosis disease among children with compromised or deficient immune system etc. The abnormal reactions are very uncommon and many of them rare.