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Infection key concepts

7 revision pages for Infection, each with the key facts, a memory aid where one helps, and the exam traps.

  1. Gastric acid as a chemical barrier

    Stomach acid, at a pH of around 2, is a chemical barrier: it makes the stomach hostile to most swallowed bacteria without recognising any particular one.

  2. MHC class I versus MHC class II: antigen source and T-cell partner

    MHC (major histocompatibility complex) class I and class II are the main groups of cell-surface molecules that show peptides to T lymphocytes, and each class has its own T-cell partner.

  3. Mucociliary clearance in the airways

    Mucociliary clearance is the innate (built-in) physical defence in which mucus traps inhaled microbes and cilia, tiny hair-like projections, sweep it up towards the throat.

  4. Opsonisation and phagocytosis of bacteria

    Opsonisation is the coating of a microbe with host molecules called opsonins, which helps phagocytes (cells that engulf and destroy microbes) to attach to it and take it up.

  5. SPUR: when to suspect an immunodeficiency

    SPUR is an initial guide to suspecting immunodeficiency (an immune system defect): severe, persistent (failing to respond to appropriate treatment), unusual or recurrent (more frequent than expected for age) infections.

  6. Staphylococcus versus Streptococcus on Gram stain: clusters versus chains

    On a Gram stain, which shows bacterial colour and shape, staphylococci are purple spherical bacteria usually arranged in grape-like clusters, whereas streptococci are purple spherical bacteria commonly arranged in chains.

  7. Why Gram-positive bacteria stay purple on a Gram stain

    Gram-positive bacteria look purple after a Gram stain because their thick peptidoglycan wall, a mesh of sugars and short protein chains, holds the crystal violet-iodine complex.

Practice Infection questions