<prompt> A 45-year old patient presented with fever, night sweats and weight loss. On X-ray, a mass was seen in apical lobe. On histopathology, caseous necrosis was present. What is the name of underlying process?Enzymatic degenerationAcute decrease in blood supplyDecreased supply of growth factorHypersensitivity reaction with modified macrophages, lymphocytes and giant cells  <response>: Ans. d. Hypersensitivity reaction with modified macrophages, lymphocytes and giant cells Ref: Robbins 9e p208. 375,376; Anananantyan 9e p162Clinical features like fever; night sweats and weight loss and X-ray finding of mass in apical lobe with histopathological findings of caseous necrosis are highly suggestive of tuberculosis. The underlying process is type IV hypersensitivity, which is characterized by presence of modified macrophages, lymphocytes giant cells.Schematic illustration of the events that give rise to the formation of granulomas in cell-mediated type IV hypersensitivity reactions.TuberculosisAntigenic property :Group specificity is due to polysaccharideQ while type specificity is due to protein antigenQ.Antibodies are not useful for diagnosis and immunity.Pathogenecity:Macrophages are the primary cells infectedQ by M. tuberculosis.M. tuberculosis replicated within the macrophages phagosome by blocking fusion of phagosome and IysosomeQ.It is due to escape of kiliing by macrophagesQ and induction of type IV hypersensitivity.Factors contributing in pathogenesis: Cord factor, Lipoarabinomannan, Complement system, M. tuberculosis heat shock proteins.Risk of acquiring infection is determined mainly by exogenous factorsQ while risk of developing disease depends largely on endogenous factorsQ.Most potent risk factor- HIV coinfectionQClinical Features:Pulmonary TBExtrapuimonary TBPrimary diseasePost primary disease MC site: Lymph nodeQ MC cervical supraclavicular Involvement of genitourinary tract sterile pyuria in acidic urine Skeletal TB MC site spine, hip, knee TB meningitis paresis of cranial nerves especially ocular, is frequent findingQ Gl TB MC site terminal ileum caecumQ Tuberculous pericarditis MC cause of chronic constrictive pericarditisQ Usually localized in middle lower zonesQ Primary focus is usually peripheral in subpleural region and is accompanied by draining lymphatics, inflamed regional LNs which are collectively called Primary complex Ghons focusQ. Occult hematogenous dissemination to apex of lung Simons FocusQ Also called adult type or reactivation or secondary tuberculosis or chronic pulmonary TB Usually localized to apical posterior segments of upper lobe due to high O2 concentration Puhls lesionQ MC hematologic finding: mild anemia leukocytosis Infraclavicular lesion is called Assmans FocusQ 