Immune System Notes

Immune System Overview

• Definition: The immune system is the body's defense against pathogens (microorganisms and viruses).

Types of Immunity

Innate Immunity

• Characteristics:

  • Recognizes traits shared by broad categories of pathogens using a limited set of receptors.
  • Provides a rapid response to infections.

• Components:

  • Barrier Defenses:
  • Skin
  • Mucous membranes
  • Secretions (like mucus, saliva, tears)
  • Internal Defenses:
  • Phagocytic cells (engulf pathogens)
  • Antimicrobial proteins
  • Inflammatory response
  • Natural killer cells (destroy infected or cancerous cells)

Acquired Immunity

• Characteristics:

  • Recognizes specific traits of particular pathogens using a vast array of receptors.
  • Provides a slower response compared to innate immunity.

• Components:

  • Humoral Response: Antibodies defend against infections in body fluids.
  • Cell-mediated Response: Cytotoxic lymphocytes defend against infections in body cells.

Phagocytosis

• Process:
  • Phagocytic cells engulf pathogens through a process called phagocytosis.
  • Key Cells: Macrophages and other immune cells that ingest foreign substances.

Antibodies

• Structure:

  • Composed of antigen-binding sites that attach to specific antigens (foreign molecules).

• Types of Responses:

  • Viral Neutralization: Antibodies block virus entry into cells.
  • Opsonization: Antibodies mark pathogens for destruction by phagocytes.
  • Activates the complement system to enhance pathogen destruction.

Immune Responses

Primary Immune Response

• Initial Exposure:
  • After the first exposure to an antigen, naive B cells encounter the antigen, leading to the production of plasma cells and memory B cells.
  • Timeframe: Initially, antibodies begin to appear around day 7 to 14.

Secondary Immune Response

• Subsequent Exposure:
  • Memory B cells rapidly differentiate into plasma cells, producing antibodies more quickly and in larger quantities than during the primary response.
  • This leads to heightened immunity against previously encountered pathogens.

B Cell Activation

• Process:

  • B cells are activated when they encounter their specific antigen presented by an antigen-presenting cell (APC) with a Class II MHC protein.
  • Helper T cells support B cell activation and proliferation.

• Outcomes:

  • Formation of plasma cells (which produce antibodies).
  • Formation of memory B cells (which provide long-term immunity).

T Cell Activation and Immune Function

Helper T Cells

• Role: Activate B cells and cytotoxic T cells by recognizing antigens presented with Class II MHC molecules.

Cytotoxic T Cells

• Function: Destroy infected cells and cancer cells. They recognize antigens presented with Class I MHC molecules on target cells and release cytotoxic factors (perforins and granzymes) to induce cell death.

Summary of Immune Mechanisms

• Humoral Immunity: Defends against extracellular pathogens using antibodies to neutralize and mark them for destruction.
• Cell-mediated Immunity: Defends against intracellular pathogens and cancer by directly attacking the infected or malignant cells.