Chapter 21 - The Immune System

What are the two main types of immunity?

1. Innate (non-specific): Immediate defense against all pathogens.
2. Adaptive (specific): Slower, targeted response against specific pathogens with memory.

List the components of the First Line of Defense (Innate Immunity).

Skin: Physical barrier.

• Mucous Membranes: Trap and remove microbes.

• Other Barriers: Tears, saliva, sebum, acidity, commensal bacteria.

List the components of the Second Line of Defense (Innate Immunity).

Phagocytes (macrophages, neutrophils) & Non-phagocytes (NK cells).

• Inflammation

• Antimicrobial Proteins (Complement, Interferon)

• Fever

What is the process of Phagocytosis?

Chemotaxis
2. Adherence
3. Ingestion (forms phagosome)
4. Fusion with lysosome (forms phagolysosome)
5. Digestion
6. Exocytosis

What are the four cardinal signs of inflammation, and what causes them?

Redness & Heat: Caused by vasodilation.

• Swelling: Caused by increased permeability.

• Pain: Caused by swelling and chemicals.

What are the two main branches of Adaptive Immunity?

Humoral Immunity: Mediated by B cells and antibodies. Fights extracellular pathogens.
2. Cellular (Cell-Mediated) Immunity: Mediated by T cells. Fights intracellular pathogens and cancer.

Define Immunocompetence and Self-Tolerance.

Immunocompetence: The ability of lymphocytes to recognize and respond to antigens.

• Self-Tolerance: The ability to recognize "self" and not attack the body's own cells.

Where do B cells and T cells originate and mature?

Origin: Both originate in the red bone marrow.

• Maturation:

  • B cells: Mature in bone marrow.

  • T cells: Mature in the thymus.

Describe the steps of the Humoral Immune Response.

Antigen binds to B cell receptor (BCR).
2. B cell internalizes, processes, and presents antigen on MHC II.
3. Helper T cell activates B cell.
4. B cell clones into Plasma Cells (make antibodies) and Memory B Cells.

Draw and label the basic structure of an antibody.

(Imagine a Y-shaped molecule)

• Two Heavy Chains

• Two Light Chains

• Variable Regions at tips of Y (antigen-binding site)

• Constant Region at stem (determines class/function)

What are the five classes of antibodies and their key functions?

• IgG: Most common; crosses placenta; enhances phagocytosis.

• IgM: First responder; pentamer; agglutination.

• IgA: In secretions (tears, saliva, mucus); mucosal protection.

• IgD: B cell receptor; role in activation.

• IgE: Allergies; anti-parasitic.

What are the effects of antibody binding to an antigen?

• Neutralization

• Agglutination

• Opsonization

• Activation of Complement

• Antibody-dependent Cell-mediated Cytotoxicity (ADCC)

Compare the roles of CD4+ (Helper T) cells and CD8+ (Cytotoxic T) cells.

• CD4+ T cells: Recognize antigen on MHC II. "Orchestrate" the immune response by activating B cells, T cells, and macrophages.

• CD8+ T cells: Recognize antigen on MHC I. Directly kill infected or cancerous cells using perforins and granzymes.

How are Natural Killer (NK) Cells and Cytotoxic T Cells alike and different?

• Alike: Both can kill target cells (virus-infected, cancer).

• Different:

  • NK Cells: Innate, non-specific, no memory.

  • Cytotoxic T Cells: Adaptive, highly specific, have memory.

Compare Active vs. Passive Immunity and give examples.

• Active: Body produces its own antibodies. Long-lasting.

  • Natural: Infection (e.g., chickenpox).

  • Artificial: Vaccine (e.g., MMR).

• Passive: Receive pre-made antibodies. Short-term protection.

  • Natural: Maternal antibodies (placenta/milk).

  • Artificial: Immune serum (e.g., antivenom).

Define and give examples of Immunodeficiency and Autoimmune Diseases.

• Immunodeficiency: Weakened immune system.

  • Example: AIDS (from HIV), SCID.

• Autoimmune Disease: Immune system attacks self.

  • Example: Rheumatoid arthritis, Type 1 Diabetes, Multiple Sclerosis.

Compare the Primary and Secondary Immune Response.

• Primary Response:

  • Slow (days to weeks)

  • Peak level of antibodies is lower

  • Mainly IgM, then IgG

• Secondary Response:

  • Rapid (hours to days)

  • Peak level of antibodies is much higher

  • Mainly IgG

What is Immunological Memory?

The ability of the adaptive immune system to mount a faster and stronger response upon a second encounter with a pathogen, due to the presence of Memory B and T cells.

Give three proven scientific reasons why vaccines are effective.

1. Disease Eradication/Control: Vaccines have eradicated smallpox and nearly eradicated polio.

2. Herd Immunity: Protects vulnerable individuals who cannot be vaccinated.

3. Cost-Effectiveness: Prevent millions of deaths and save billions in healthcare costs (e.g., 154 million deaths averted since 1974).

What is the evidence that the MMR vaccine does not cause autism?

A large-scale 2015 study of over 95,000 children found no link between the MMR vaccine and autism, even in high-risk children. The original 1998 study proposing a link was retracted due to fraud and scientific misconduct.

What are MHC Proteins and what are their two main classes?

Major Histocompatibility Complex (MHC) = "Self-identification" proteins.

• MHC I: Found on all nucleated cells. Presents endogenous antigens to CD8+ T cells.

• MHC II: Found on antigen-presenting cells (macrophages, B cells, dendritic cells). Presents exogenous antigens to CD4+ T cells.

What is the function of Interferon?

A chemical secreted by virus-infected cells. It signals nearby cells to produce antiviral proteins, helping to limit the spread of the infection. It is part of the innate immune system.

What is the function of the Complement System?

A group of >20 plasma proteins that, when activated, can:

1. Form a Membrane Attack Complex (MAC) to lyse microbes.

2. Enhance inflammation.

3. Opsonize pathogens for phagocytosis.