Immunity

• Disease & pathogens: infectious disease = caused by a transmissible pathogen (bacteria, some fungi, all viruses).
• Tuberculosis (TB): caused by Mycobacterium tuberculosis; spread by droplets; can lie dormant in tubercles then reactivate.
• HIV: a retrovirus that infects and destroys helper T cells, eventually causing AIDS.
• Routes of entry: vectors, inhalation, ingestion, indirect contact, direct contact, inoculation.
• Barriers to entry: skin, gut/skin flora, stomach acid, lysozyme in secretions.
• Non-specific response: inflammation, interferons, phagocytosis — same reaction to ANY pathogen.
• Specific response: antigens trigger lymphocytes (T cells & B cells) that target ONE particular pathogen.
• Antibodies: Y-shaped proteins with variable regions that bind specific antigens; they neutralise, agglutinate, and block pathogens.
• Primary vs secondary response: first exposure is slow (symptoms appear); memory cells make re-exposure fast (often symptom-free).
• Active vs passive immunity: active = your body makes antibodies (long-term); passive = antibodies given to you (short-term).
• Vaccines: deliberately introduce antigens to trigger active immunity without causing disease.
• Evolutionary arms race: pathogens evolve evasion tricks (e.g. antigenic variation, hiding antigens) faster than hosts can keep up.

Tuberculosis (TB)

TB is caused by the bacterium Mycobacterium tuberculosis. Think of it like a burglar who doesn't just break in — it breaks in, hides inside your own security guards (phagocytes), and can stay dormant for years before causing trouble.

The clever (and dangerous) part: TB bacteria don't just get destroyed by phagocytes like most bacteria do — they can survive and reproduce inside phagocytes. A healthy immune system usually walls off these infected phagocytes into structures called tubercles, where the bacteria sit dormant (the primary infection). But if the immune system later becomes compromised — for example by an HIV infection — the bacteria can reactivate and overpower the body. This is the active phase, and it's why TB is a classic "opportunistic infection" in AIDS patients.

HIV

retrovirus reverse transcriptase helper T cells

⚠️ Exam wording trap:

1. attaches

mast cells