Mycobacterium tuberculosis causes TB in the lungs or other tissues of human body. Robert Koch first discovered the organism in 1882. Organism is strict aerobe, so servive in tissues with high oxygen like apex of the lung.
Mode of transmission:- It can transmitted by-
- Inhalation of organism present in cough
- Inoculation into the skin
- Ingestion of bacilli from milk of infected cow.
- Transplacental via infected mother to foetus.
- Tubercle bacilli as enters into pulmonary capillary, neutrophils activated by initial response but rapidly destroy by organism.
- Infiltration by macrophages after 12hrs, because bacilli coated with serum complement factors C2a or C3b & attract the macrophages.
- Phagocytosis occurs , bacilli become die but further proliferate.
- Body’s immune response activate the T or B cells. Activated CD4+T cell develop cell mediated hypersensitivity reaction, B cell forms antibodies.
- Within 2-3 days macrophages changes structurally like a epithelial hence called epithelioid cell due to immune response –as cytoplasm become pale or eosinophillic & nuclei become elongated.
- Epithelioid form tight clusters (granuloma) or release cytokines IF,IL,TNF etc. In response to activated CD4+T cells.
- Some of macrophages form multinucleated giant cell, by fusion of cell, specially of langhan’s type in which nuclei arranged in peripherally like horseshoe or clustered at 2 opposite pole,or may be foreign body type- nuclei arranged centrally.
- Around the epithelioid cells or giant cell there is zone of lymphocyte, plasma cell or fibroblast . the leison called hard tubercle.
- Within 10-14 days, central mass begins to undergo caseation necrosis (cheesy appearance) & form soft tubercle.
Types:- Depending upon the types of tissues response or age it is of 2 main types:
- Primary TB:– The individual who has not been previously infected, also known as Ghon’s complex or childhood TB.
It is the leison produced in the tissues of portal of entry with foci in draining lymphatic vessel & lymph node. Most commonly involved are tissues of lungs or hilar lymph node.Incidence commonly seen in immune compromised persons.
Ghon’s complex in lungs consist of 3 component:-
- Pulmonary component:- Leison is 1-2 c.m. solitary area located peripherally in upper part of lower lobe.
- Lymphatic vessel component:– Draining lung lesion contain phagocyte, may develop beaded, military tubercles along the path of hilar lymph node.
- Lymph node component:– Affected lymph node shows caseation necrosis. It consist of enlarged hilar & tracheobronchial lymph node.
Fate of primary TB:-
- The lesion do not progress but heal by fibrosis & undergo calcification.
- In some primary focus continue to grow caseous material disseminated through bronchi to other part of lung is progressive primary TB.
- Bacilli enter the circulation by erosion in a blood vessel & spread to various tissues, called primary miliary TB. Lesion are seen in liver spleen, kidney, bone etc.
- In some lowered resistance host, healed lesion of primary TB get reactivated cause secondary progressive TB.
- Secondary TB:- The infection of an individual who has been previously infected is called post primary TB, reinfection or chronic TB.The lesion in secondary TB begins as 1-2 c.m. apical area of consolidation, which may occur by haematogenous spread of infection from primary complex to apex, where high O2 tension favoured the growth of bacilli.
Fate of secondary PTB:-
- The lesion may heal fibrous scaring or calcification.
- Leison may coalesce together & produce progressive secondary PTB with extra pulmonary involvement like-
- Fibrocaseous TB:- Area of tuberculous pneumonia undergo central caseation necrosis may braek into a bronchus from a cavity (cavitary open TB ) or remain soft caseous lesion with out drainage into bronchus produce non cavitary lesion.
- Tuberculous Caseous Pneumonia:- Caseous material may spread to rest of the lung produces caseous pneumonia.
- Miliary TB:- Due to spread of infection through lymphohaematogenous route to form multiple tiny leison called military TB. Expand to involve large area, lesion resemble millet seeds. It can confine to lungs or extrapulmonary spread.