Host Defenses Overview 13B

Host Defenses: Overview

General Approach to Host Defenses

  • Host defenses are categorized into two primary groups:
    • Innate (Nonspecific) Defenses: Immediate defense mechanisms.
    • Acquired (Specific) Defenses: Adaptive immunity developed over time.

Three Lines of Defense

First Line of Defense
  • Surface Protection: Composed of anatomical and physiological barriers.
    • Prevents microbial penetration into sterile body compartments.
Second Line of Defense
  • Cellular and Chemical Responses: Activated if infectious agents bypass the first line.
    • Involves various immune cells (e.g., phagocytes) and responses such as inflammation and fever.
Third Line of Defense
  • Specific Host Defenses: Developed uniquely for each microbe.
    • Utilizes specialized white blood cells (B cells and T cells) to establish immunity.

Adaptive Immunity: Key Characteristics

  • Acquired Specific Immunity: Generated after an immunizing event (e.g., infection).
  • Immunocompetence: Ability of the body to respond to countless foreign substances through lymphocyte specificity.

Antigens and Immunogens

  • Antigens: Molecules identified by the immune system, potentially leading to a response.
  • Immunogens: Specific antigens that trigger an immune response, generally proteins or polysaccharides from pathogens or environmental sources.

PAMPs vs. Immunogens

  • PAMPs (Pathogen-associated Molecular Patterns): Shared by many microbes; trigger innate responses.
  • Immunogens: Unique structure; stimulate adaptive immunity.

Commonality:

  • Both provoke defensive reactions from the host.

Major Functions of Immune System Markers

  • Recognizing self versus non-self antigens.
  • Facilitating communication via chemical signals.
  • Coordinating immune responses through Major Histocompatibility Complex (MHC).

Major Histocompatibility Complex (MHC)

  • MHC: Genes coding for markers on human cells, crucial for immune recognition.
    • Class I MHC Genes: Present on nucleated cells for self-recognition.
    • Class II MHC Genes: Found on immune regulatory cells (e.g., macrophages), presenting antigens to T cells.
    • Class III MHC Genes: Encode complement system proteins.

Stages of Lymphocyte Development

B-Lymphocyte Development
  • Maturation occurs in the bone marrow; includes clonal deletion.
  • Preparation to respond to a specific antigen.
T-Lymphocyte Development
  • Maturation in the thymus, includes markers like CD3, CD4, and CD8.
  • Differentiation into various T cell types upon activation.

Stages of Immune Response

  1. Lymphocyte Development and Differentiation: Initial programming of lymphocytes to recognize unique antigens.
  2. Antigen Presentation: Antigen-presenting cells (APCs) like dendritic cells and macrophages present antigens to T cells.
  3. Activation and Proliferation: B cells differentiate into plasma cells to secrete antibodies; T cells become cytotoxic or helper cells.

Response Mechanisms

T Cells
  • Helper T Cells: Activate other immune cells.
  • Cytotoxic T Cells: Destroy infected host cells and foreign cells.
  • Regulatory T Cells: Regulate immune responses and prevent overactivity.
B Cells
  • Upon activation, B cells proliferate and produce antibodies, tagging antigens for destruction.

Immune Tolerance

  • Mechanisms to prevent immune responses against self-antigens.
  • Clonal deletion eliminates potentially harmful T and B cells.

Key Takeaways

  • The immune response has multiple stages ensuring specificity and adaptability.
  • Understanding the roles of different cells, markers, and mechanisms is vital for comprehending how immunity functions.