Body Defense Mechanisms Quiz

Body Defense Mechanisms

• The immune system is the body's network of cells, tissues, and organs protecting against infection and disease. It defends against bacteria, viruses, fungi, and parasites.

• The immune system has two components:

  • Innate immunity: a rapid, non-specific first response.
  • Adaptive immunity: a slower, more specific response with memory for future encounters with pathogens.

First Line of Defense

• Physical barriers:

  • Skin: The largest organ, composed of tightly packed epithelial cells. Sebum and sweat contain antimicrobial substances. Dead skin cell shedding removes surface microbes.
  • Mucous Membranes: Line respiratory, gastrointestinal, and urogenital tracts. Mucus traps pathogens, and cilia moves trapped particles out of the body.

• Chemical Defenses:

  • Stomach Acid: Kills ingested pathogens.
  • Lysozyme: An enzyme, found in tears, saliva, sweat that breaks down bacterial cell walls.
  • Antimicrobial Peptides: Short proteins (like defensins) that disrupt microbial membranes.
  • Flushing mechanisms: Tears, urine, and mucus wash away microbes. Example: blinking.

Second Line of Defense: Innate Immunity

• Phagocytes:

  • Macrophages: Reside in tissues; engulf pathogens by phagocytosis, and release cytokines to recruit more immune cells.
  • Neutrophils: Most abundant white blood cells, rapidly recruited to infection sites. They engulf pathogens, release enzymes, and toxic substances.
  • Mechanism of Phagocytosis: Pathogens are engulfed into a phagosome, fuses with a lysosome, and enzymes degrade the pathogen.

• Natural Killer (NK) Cells: Recognize and destroy cells lacking self-markers (MHC class I molecules). Typically virus-infected or cancerous cells. Release perforins and granzymes that induce apoptosis.

• Inflammatory Response:

  • Tissues injured release histamine.
    • Blood vessels dilate causing redness and heat, allowing more immune cells to the area.
    • Blood vessels become more permeable, causing swelling.
  • Purpose: speeds up healing process and recruits immune cells to fight infection.
  • Clinical Example: Swelling and redness around a cut is inflammation.

• Fever: Pyrogens (substances) reset the body's thermostat in the hypothalamus to inhibit microbial growth and enhance immune response.

• Complement system: A set of about 30 proteins that work together to destroy microbes by punching holes in microbial membranes (Membrane Attack Complex - MAC). Enhances phagocytosis through opsonization, making pathogens easier for phagocytes to identify.

• Interferons: Released by virus-infected cells to warn neighboring cells, stimulating them to produce antiviral proteins.

Third Line of Defense: Adaptive Immunity

• Antigen Recognition: Foreign molecules (proteins, polysaccharides) recognized by the adaptive immune system.
• B Cells and Antibodies: B cells mature in the bone marrow and, upon antigen activation, differentiate into plasma cells that secrete antibodies, which provide long-term immunity.
• Types of immunoglobulins include IgG (most common), IgA (found in mucous secretions), and IgM (first antibody made).
• T Cells: Mature in the thymus, crucial for cell-mediated immunity; including helper T cells (CD4+) and cytotoxic T cells (CD8+).
• Helper T cells: Activate B cells and other immune cells.
• Cytotoxic T cells: Kill infected or abnormal cells.
• Memory cells: Responsible for faster response upon re-exposure to the same pathogens.

Role of the Lymphatic System

• Lymph nodes: Filter lymph fluid and trap pathogens.
• Spleen: Filters blood, removing old red blood cells and pathogens.
• Lymphatic vessels: Transport lymph fluid, carrying immune cells and waste products away from tissues.

Immune System Disorders

• Autoimmune diseases: The immune system mistakenly attacks the body's own cells (Examples: Multiple sclerosis, type 1 diabetes).
• Allergies: Hypersensitivity to non-harmful antigens, triggering an exaggerated immune response.
• Immunodeficiency: Primary (genetic) or secondary (acquired) conditions where the immune system is weakened, e.g., HIV/AIDS.

Vaccination and Immune Memory

• Vaccines: Introduce a harmless form of an antigen to the body, allowing it to produce memory cells without causing disease; useful against certain diseases.

• Herd immunity: A significant portion of a population that's immune to a disease prevents its spread to unvaccinated individuals.

Pathogen Evasion Mechanisms

• Antigenic variation: Pathogens change their surface proteins to escape immune cell recognition. Examples: Influenza virus, Trypanosoma brucei (sleeping sickness).
• Hiding in host cells: Pathogens hide within host cells to avoid immune detection. Examples: Mycobacterium tuberculosis, Herpes simplex virus.
• Inhibiting phagocytosis: Pathogens produce factors to prevent phagocytes from engulfing and destroying them. Examples: Streptococcus pneumoniae.
• Inhibiting complement system: The pathogen produces factors to inhibit the complement system.
• Disrupting cytokine signaling: Some pathogens interfere with the host's immune communication system.
• Molecular mimicry: Some pathogens produce molecules mimicking host proteins, blending in to avoid detection.
• Biofilm formation: Microorganisms form communities of cells shielded by a protective extracellular matrix, making them resistant to immune responses.

Clinical Examples

• Swollen lymph nodes during an infection: Indicates active immune response.