Mycobacterium tuberculosis is the most common cause of tuberculosis. Primary infection begins with inhalation of 1 to 10 aerosolized bacilli. The pathogenicity of the organism is determined by its ability to escape host immune responses as well as eliciting delayed hypersensitivity. Alveolar macrophages engulf the invading cells but are unable to mount an effective defense. Several virulence factors are responsible for this apparent failure; most notably in the mycobacterial cell wall are the cord factor, lipoarabinomannan, and the 65 kd heat shock protein or HSP65. The emergence of new strains of resistant Mycobacterium tuberculosis has created new interest in clinical diagnosis. Studies have shown immunohistochemical techniques to be superior to conventional special stains. Thus the demonstration of mycobacterial antigens are not only useful in establishing mycobacterial aetiology, but can also be used as an alternative method to the conventional Ziehl-Neelsen method.