Poor host immunity

Pathogenesis of TB is a product of interaction between bacterial activity and immune responses in the host.

Generally, in a person with good cellular immunity, the clearance of Mycobacterium tuberculosis (Mtb) is quite good and the chances of developing the active disease are less.

An activity against Mtb is conceivable considering that T lymphocytes CD8⁺ recognize Mtb antigens through class I molecules of the Major Histocompatibility Complex (MHC) and produce Interleukin-2 (IL-2), Interferon-gamma (IFN-γ) and Tumour Necrosis Factor-alpha (TNF-α), which have a well-known role in controlling Mtb.

However, upon airborne infection with Mtb, the acquired cellular immune response is generally slow to be induced. This slowing down establishes the infection and the organs suffer considerable damage by that time.

If there is superimposed immunosuppression, the bacterial response overcomes the weak cellular immune response. In these conditions of waning or poor immune response in the host, the risk of TB disease increases.

The common conditions where there is poor host immune response and hence, a higher risk of TB are:

• Human Immunodeficiency Virus (HIV) infection: Cellular immunity is weakened by the retroviral infection thereby increasing the risk of developing active TB by 18 times
• Undernutrition: 3 times more risk
• Alcohol use disorder: 3.3 times more risk
• Tobacco smoking: 1.6 times more risk
• Diabetes mellitus
• Occupational diseases, like silicosis
• Immunosuppressive therapy in autoimmune diseases, following organ transplant etc.
• Cancer chemotherapy
• Severe kidney disease

Resources

• TB Risk Factors, Centres for Disease Control & Prevention.
• Tuberculosis: Key Facts, World Health Organization.
• Cell-mediated Immune Responses in Tuberculosis, Annual Review of Immunology, 2009. 

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