1 Host Defense and Immune Cells

Host Defense and Immune Cells

12.1 Lines of Defense

• Natural Barriers, Innate Immunity, Adaptive Immunity

  • Host defense is subject to multiple layers of strategies against pathogens, categorized into three main lines of defense:

  1. Natural barriers

  2. Innate immunity

  3. Adaptive immunity

Pathogen Needs for Successful Infection

• A pathogen must accomplish the following for infection to be successful:

  • Portal of entry

  • Colonization

  • Evade defenses

  • Reproduction

  • Portal of exit

Three Lines of Defense Against Foreign Invaders

• 1st Line: Natural Barriers

  • Immediate Response

  • Function: Limits entry, colonization, and growth of pathogens

  • Characteristics: Nonspecific (no recognition)

    • Examples: Skin, mucus, acidic environments, beneficial microflora

• 2nd Line: Innate Immunity

  • Quick Response Systemic Circulation

  • Function: Eliminates microbes and limits spread

  • Characteristics: Nonspecific recognition of Pathogen-Associated Molecular Patterns (PAMPs)

    • Examples: Inflammation, fever, phagocytosis

• 3rd Line: Adaptive Immunity

  • Slow Response Systemic Circulation

  • Function: Eliminates microbes and resolves infections

  • Characteristics: Specific recognition of antigens; also enhances barriers and innate response

    • Components: Antibodies, T cells

    • Memory Response: Faster response upon subsequent exposures

Immune Cells Overview

12.2 Immune Cells

• Circulation and Hematopoiesis

  • Circulating Immunity: The movement of leukocytes in search of threats and pathogens.

  • Hematopoiesis: The process of blood cell production from stem cells in red bone marrow, including:

    • Erythrocytes (RBCs): Transport blood gases.

    • Thrombocytes (Platelets): Limit blood loss during injury.

    • Leukocytes (WBCs): Fight infection.

Types of Leukocytes

1. Granulocytes:

   • Types:

     • Neutrophils

       • Characteristics: Lobed nucleus, large cytoplasmic granules

       • Role: Most abundant WBC; early responders to inflammation; engage in phagocytosis and contain digestive enzymes and toxic substances.

     • Eosinophils

       • Role: Attach to large eukaryotic parasites and produce toxic chemicals for destruction.

     • Basophils

       • Role: Few in circulation; release chemical mediators of inflammation, such as histamine.

   • Mast Cells: Similar to basophils but more plentiful and localized in tissues; play a major role in inflammation and allergic responses.

2. Agranulocytes:

   • Types:

     • Monocytes

       • Role: Secondary responders to inflammation; differentiate into macrophages in tissue and participate in phagocytosis; present antigens to activate T-cell responses.

     • Lymphocytes

       • Role: Major component of adaptive immunity (further discussed in Chapter 13).

       • Composition:

         • B cells and Plasma Cells

         • Helper T Cells

         • Cytotoxic T Cells

         • Natural Killer (NK) Cells: Part of innate immunity.

       • Representation: 20-35% of circulating leukocytes and 10% of all body cells in lymphoid and connective tissues.

Summary of Leukocyte Functions:

• Granulocytes (Innate): Neutrophils, Basophils, Eosinophils

• Agranulocytes:

  • Monocytes (become macrophages and dendritic cells for antigen presentation)

  • Lymphocytes (adaptive immune response, includes various cell types)

Immune Response Mechanisms

Surveillance and Pathogen Recognition

• Leukocyte Activity:

  • Circulate and search for pathogens/infection.

  • Have specific receptors to recognize harmful molecules (PAMPs) and nonself antigens.

Elimination Process

• Effector Leukocytes: Attack and destroy harmful, nonself microbes.

• Mechanism:

  1. Detection and Recognition: Foreign cell or virus detected through Pathogen Recognition Receptors (PRR) binding to PAMPs on the microbe.

  2. Destruction: Activated leukocytes eliminate the identified threats effectively using diverse approaches, including phagocytosis and chemical mediation.