Chapter 24 – Immune System

Overview of the Immune System

The immune system is a complex network of cells, tissues, and organs that protects the body from pathogens and maintains homeostasis. It is divided into innate and adaptive components, each with distinct roles and mechanisms.

• Major Functions:

  • Distinguishing self from non-self

  • Defending against pathogens (bacteria, viruses, fungi, parasites)

  • Removing damaged or abnormal cells

• Physical, Mechanical, and Chemical Barriers:

  • Physical barriers: Skin, mucous membranes

  • Mechanical barriers: Cilia, coughing, sneezing, flushing actions (tears, urine)

  • Chemical barriers: Enzymes in saliva, stomach acid, lysozyme in tears

Immune Response Steps

If pathogens bypass the initial barriers, the immune system responds in a series of steps:

1. Detection: Recognition of pathogen-associated molecular patterns (PAMPs)

2. Activation: Recruitment of immune cells and release of signaling molecules

3. Destruction: Elimination of pathogens via phagocytosis, cytotoxicity, or antibody-mediated mechanisms

4. Resolution: Removal of debris and restoration of tissue integrity

Innate vs. Adaptive Immunity

The immune system is classified into two main branches: innate and adaptive immunity.

• Innate Immunity:

  • Non-specific, immediate response

  • Includes barriers, phagocytic cells, natural killer cells, and inflammation

  • No memory of previous encounters

• Adaptive Immunity:

  • Specific, slower response

  • Involves lymphocytes (B cells and T cells)

  • Develops memory for faster response upon re-exposure

Mechanisms of Pathogen Destruction

Immune cells use several mechanisms to destroy microorganisms:

• Phagocytosis: Engulfment and digestion of pathogens by phagocytes (e.g., neutrophils, macrophages)

• Cytotoxicity: Killing of infected cells by cytotoxic T cells and natural killer cells

• Antibody-mediated: Neutralization and opsonization by antibodies produced by B cells

• Complement activation: Cascade of proteins leading to pathogen lysis

Monocytes and Macrophages

Monocytes are circulating white blood cells that migrate into tissues and differentiate into macrophages. These cells are essential for phagocytosis and antigen presentation.

• Locations: Monocytes are found in blood; macrophages reside in tissues (e.g., alveolar macrophages in lungs, Kupffer cells in liver).

• Role: Engulf pathogens, present antigens to T cells, and secrete cytokines to modulate immune responses.

Dendritic Cells and Antigen Presentation

Dendritic cells are professional antigen-presenting cells that bridge innate and adaptive immunity. They capture antigens and migrate to lymph nodes to activate T cells.

• Role: Initiate adaptive immune responses by presenting antigens via MHC molecules.

Recognition of Non-Self

Immune cells recognize non-self through pattern recognition receptors (PRRs) and antigen receptors. This ensures that pathogens are targeted while self-tissues are spared.

• PRRs: Detect conserved microbial patterns (e.g., Toll-like receptors)

• B and T cell receptors: Recognize specific antigens

Phagocytosis

Phagocytosis is the process by which cells ingest and destroy foreign particles.

• Phagocytic cells: Neutrophils, macrophages, dendritic cells

• Steps: Recognition, engulfment, digestion

Inflammation

Inflammation is a protective response to infection or injury, characterized by redness, heat, swelling, and pain.

• Functions:

  • Recruit immune cells to site of infection

  • Contain and eliminate pathogens

  • Initiate tissue repair

Cytokines

Cytokines are signaling proteins released by immune cells to coordinate responses.

• Types: Interleukins, interferons, tumor necrosis factors

• Functions: Promote cell activation, inflammation, and communication between immune cells

MHC Class I vs. MHC Class II

Major Histocompatibility Complex (MHC) molecules present antigens to T cells and are critical for immune recognition.

B and T Cell Binding vs. PRRs

B and T cells bind specific antigens via their unique receptors, enabling adaptive immunity. This differs from PRRs of innate cells, which recognize general patterns.

• B cells: Bind free antigens via B cell receptors

• T cells: Bind processed antigens presented by MHC molecules

• PRRs: Recognize broad pathogen-associated patterns

Self-Tolerance

Self-tolerance is the immune system's ability to avoid attacking the body's own tissues. It is enforced through central and peripheral mechanisms.

• Central tolerance: Elimination of self-reactive lymphocytes during development (in thymus and bone marrow)

• Peripheral tolerance: Suppression or inactivation of self-reactive cells in the periphery

Loss of self-tolerance can lead to autoimmune diseases.

Additional info: Academic context and definitions have been expanded for clarity and completeness.