Who puts the tubercle in tuberculosis?

Tuberculosis (TB), an illness that mainly affects the respiratory system, is one of the world's most pernicious diseases. TB currently infects one-third of the world's population and kills approximately 1.7 million people each year. Most infected individuals fail to progress to full-blown disease because the TB bacilli are 'walled off' by the immune system inside a tissue nodule known as a granuloma. The granuloma's primary function is one of containment and it prevents the dissemination of the mycobacteria. But what is the role of the TB bacillus in the progression of the granuloma? This Review explores how Mycobacterium tuberculosis influences granuloma formation and maintenance, and ensures the spread of the disease.