Chapter 9: Immunity

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Chapter Overview & SLOs

What is the primary role of the immune system? The immune system acts as a defense mechanism to protect the body against pathogens like bacteria, viruses, and fungi, as well as abnormal cells such as cancer cells.

What are innate and adaptive immunity?

• Innate Immunity (Non-specific): Immediate response, present from birth. Includes physical barriers (skin, mucus), chemical barriers (stomach acid, enzymes), and cellular responses (phagocytes). Does not improve with repeated exposure.
• Adaptive Immunity (Specific): Delayed but highly specific response. Develops after exposure to pathogens. Involves lymphocytes (B cells and T cells) and has memory (improves with repeated exposure).

What are the three lines of defense?

| Line | Type | Components | Specificity | |------|------|------------|-------------| | First line | Innate | Skin, mucous membranes, secretions (tears, saliva, stomach acid) | Non-specific | | Second line | Innate | Phagocytes (macrophages, neutrophils), inflammation, fever, antimicrobial proteins | Non-specific | | Third line | Adaptive | Lymphocytes (B cells, T cells), antibodies | Highly specific |

What are the main organs of the immune system?

• Bone marrow: Produces all blood cells including B lymphocytes (B cells mature here).
• Thymus: Gland where T lymphocytes (T cells) mature.
• Lymph nodes: Small organs that filter lymph, trap pathogens, and house lymphocytes.
• Spleen: Filters blood, removes old red blood cells, and contains lymphocytes.
• Tonsils and adenoids: Trap pathogens entering through mouth and nose.

How does a coordinated immune response work?

1. Antigen presentation: A macrophage (or dendritic cell) engulfs a pathogen and presents its antigen on MHC molecules.
2. T-helper cell activation: T-helper cells (CD4+) recognize the antigen and become activated.
3. B cell activation: Activated T-helper cells stimulate B cells to produce antibodies.
4. Cytotoxic T cell activation: T-helper cells activate cytotoxic T cells (CD8+) to kill infected cells.
5. Memory cell formation: Some B and T cells become memory cells for long-term protection.

How do memory cells ensure long-term protection? Memory B and T cells are created after an initial infection or vaccination. They "remember" the specific pathogen and initiate a faster and stronger response upon re-exposure (secondary immune response).

What are cytotoxic T cells? Cytotoxic T cells (CD8+ T cells) are specialized to recognize and kill virus-infected cells or tumor cells directly.

How do vaccines and booster doses work?

• Vaccines: Train the immune system to produce antibodies without causing illness. They contain weakened, inactivated, or parts of a pathogen.
• Booster doses: Strengthen immunity by increasing the population of memory cells and targeting new virus variants. Needed for some vaccines (e.g., COVID-19, tetanus).

What is the difference between primary and secondary immune responses?

| Feature | Primary Response | Secondary Response | |---------|-----------------|-------------------| | Timing | Slow (days to weeks) | Fast (hours to days) | | Magnitude | Lower antibody levels | Higher antibody levels | | Memory | None initially | Strong (memory cells) | | Example | First exposure to pathogen | Re-exposure to same pathogen |

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we describe the steps of a coordinated immune response? It begins when a macrophage engulfs a pathogen and presents its antigen via MHC, which then activates T-helper cells to stimulate B cells and cytotoxic T cells for a targeted attack.
• How do we differentiate between the three lines of defense? The first line includes physical barriers like skin and mucus, the second line involves internal non-specific responses like phagocytosis and fever, and the third line is the specific response involving lymphocytes (B cells and T cells).
• How do we identify the main organs of the immune system? Key organs include the bone marrow (where blood cells are produced), the thymus (where T cells mature), and lymph nodes (which filter lymph and trap pathogens).
• How do vaccines and booster doses protect against diseases? Vaccines train the immune system to produce antibodies without causing illness, while booster doses strengthen immunity by increasing the population of memory cells and targeting new virus variants.

Frequently Asked Questions (FAQ)