Tuberculosis complex includes Mycobacterium Tuberculosis and  Bovis.Mycobacterium tuberculosis resembles likes  slender,straight and slightly curved rod with rounded end.The bacilli are non motile,non sporing and non capsulated.they are acid fast.they are seen in pairs.they have a gram positive cell wall,how ever,difficuilt to stain with gram staining as the wall is impermeable to stain because of the presence of mycolic acid in their cell wall.they can be stained with ziehl neelsen method of acid fast staining and they resist decolourization by 20%sulphuric acid and absolute alcohol for 10 min.

ANTIGENIC STRUCTURE:

Myco bacterium possess two types of antigen.They are cell wall antigen and cyto plasmic antigen.The cell wall consist of lipids,protein and poly saccharides.Cyto plasmic antigen are mostly protien antigen and include antigen 5,antigen 6 antigen 14,  antigen 19,antigen32,antigen 38 and antigen 60.

PATHOGENESIS:

The transmission is by direct inhalation of droplets nuclei containing tubercule bacilli.

the initial infection with m.tuberculosis is reffered to as a primary infection.subsequent disease is known as post primary,secondary,reinfection tuberculosis and both of the disease are difficuilt in pathological features.

PRIMARY TUBERCULOSIS:

primary tuberculosis is the initial infection by tubercle bacilli and usually occurs in young children.

the inhalation of bacilli,the infection is first established in the lungs.the bacilli is phagocytosed by alsveolar macrophages which multiply and give rise to a sub pleural lesion located either in the lower lobe or the lower part of the upper lobe leading to form initiallesion or ghon focus.some bacilli enter in to the hilar lymph nodes causing additional foci of infection.the ghon focus,together with the enlarged hilar lymph nodes,forms the primary complex or ghon complexex.this event occurs in 3-8 weeks from the time of infection.in additional,baccilli are disseminated in many organs and tissues.it is difficuilty for the immune system to eliminate the bacteria as they are intrecellular pathogen.a hard tubercle or granuloma may be formed,finally developing in to a chronic granuloma.a chronic granuloma contains a central area of large,multi nucleated gaint cells containing tubercle ,a mid zone of pale epitheloid cell,often arranged radially with a peripheral zone of fibroblast,lymphocytes and monocytes.then,peripheral fibrous tissues develop,and then the central area under goes caseation necrosis.such a  lesion is called a tubercle.