Ahh the Granuloma - a great example of topic that overlaps multiple subjects and any topic that brushes borders with the likes of pathology, immunology and microbiology make for great questions. Here are 5 steps to the granuloma in perfect 3rd grader doodle form:
Step 1: Inhale the red, ACID FAST rod
M. tuberculosis is only acid fast why?
B/c of the MYCOLIC ACID in the cell wall resists decolorization with acid-alcohol and so it remains red which is the color of the initial stain, carbol fuchsin.
What else stains acid fast? Nocardia which is “partially acid fast”
Step 2: Phagocytosis by alveolar macrophages
Circulating monocytes roll on the vascular endothelium (d/t selectins) and adhere to it (d/t ICAMs) and then transmigrate into the affected area where they are called tissue macrophages.
Macrophages are the main players in the formation of a granuloma
Step 3: Antigen Presentation
Step 4: T Cell Activation
CD4 T Cells are involved
The TH1 subtype
No CD4 T Cells? No Granuloma. AIDS patients who loose their CD4 t-cells well before loosing their CD8 T Cells will make very weak if any granulomas.
Step 5: Macrophage Activation
The Caseous Granuloma
Note that the caseous necrosis is cottage cheese like and is due to the destruction of the M. TB organism's cell wall's mycolic acids (mycolic acids are lipids).