The article deals with primary complex and BCG vaccination. Lodgement or implantation of tubercle bacilli, at any site, in the body of an animal or human being is called primary infection. The tissue response by accumulation of polymorphonuclear leucocytes at the site of primary infection is termed as primary focus. The tubercle bacilli are transported from the primary focus to the lymph node through lymphatics. The primary focus, the lymphangitis and regional lymphadenitis together constitute primary complex. In 95% of cases it occurs in the lung: the initial polymorphic leucocytic reaction in the primary focus and the lymph nodes are soon augmented by large monocytes then epitheloid cells and the Langhans' giant cells. In about 2-4 weeks the reticuloendothehal system develops cell mediated immunity and tuberculo hypersensitivity. Most of the primary complexes (lesions) become innocuous after a short time harbouring the tubercle bacilli with arrested activity, but live and potentially virulent. There is always a lurking danger of these bacilli flaring up in the future to progressive tuberculous disease. BCG vaccination is aimed at establishing a controlled primary complex by intradermal injection of attenuated (harmless) live, bovine strain of tubercle bacilli in an attempt to forestall the infection with virulent tubercle bacilli among the uninfected persons. At the site of vaccination, the lower half of the left deltoid region, a primary focus is created from where some bacilli are transported to axillary lymph node through the lymphatics and complete the formation of primary complex. In 2-4 weeks time cell mediated immunity and delayed hypersensitivity are initiated and is completed in about 6-8 weeks time and the vaccinated persons show positive reaction to tuberculin test. The BCG lesion heals in 4-6 weeks time.

The advantages of primary complex established with BCG vaccination prior to a chance of natural infection are: i) primary tuberculosis disease caused by it can be ruled out; ii) there is no chance of spread of disease to adjoining parts i.e., haematogenous dissemination of disease leading to milliary, meningeal, bone tuberculosis etc., is prevented; iii) also the danger of future local flare up and thereby chances of disease after infection are reduced. To obtain maximum advantage from the BCG vaccination, it should be given at the earliest possible time in life of an individual.