Organization of the Immune System: Overview and Cellular Components

Organization of the Immune System

Introduction

The immune system is a complex network of cells, tissues, and molecules that protects the body from pathogenic agents such as bacteria, viruses, fungi, and parasites. It is organized into innate and adaptive branches, each with distinct mechanisms and roles in host defense.

Overview of the Body's Defenses

Species Resistance

• Definition: Species resistance refers to the natural ability of a species to resist infection by certain pathogens due to physiological and biochemical incompatibilities.

• Mechanisms:

  • Absence of specific chemical receptors required by pathogens to infect human cells.

  • Human body conditions (e.g., temperature, pH) may not support the survival of certain pathogens.

• Example: Many plant and animal pathogens cannot infect humans due to these incompatibilities.

Immune System Functions

• Protects against a wide variety of human pathogenic agents, including bacteria, viruses, toxins, and parasites.

Host Response: Innate and Adaptive Immunity

Innate Immunity

Innate immunity provides the first line of defense against pathogens. It is non-specific and acts rapidly upon infection.

• Generalized response: Targets a broad range of pathogens.

• Immediate action: Responds within minutes to hours.

• Does not usually clear pathogens completely.

• Links to adaptive immunity: Initiates and shapes the adaptive response.

Adaptive Immunity

Adaptive immunity is highly specific and develops more slowly. It is responsible for long-term protection and immunological memory.

• Highly specific: Targets specific antigens.

• Delayed response: Takes days to become fully effective.

• Clears pathogens: Capable of eliminating infections.

• Immunological memory: Provides faster and stronger responses upon re-exposure to the same pathogen.

An Overview of the Body's Defenses

Innate Immunity Components

• First line of defense: Physical and chemical barriers (e.g., skin, mucous membranes).

• Phagocytosis: Engulfment and destruction of pathogens by phagocytic cells.

• Inflammation: Localized response to infection or injury, recruiting immune cells to the site.

• Complement system: A group of proteins that enhance phagocytosis and directly kill pathogens.

Adaptive Immunity Components

• Humoral immunity: Mediated by antibodies produced by B cells.

• Cell-mediated immunity: Involves T cells that destroy infected cells or help other immune cells.

• Antigen processing and presentation: Display of pathogen-derived peptides on cell surfaces for recognition by T cells.

Physical Barriers

Types of Physical Barriers

• Skin: Acts as a tough, impermeable barrier to most pathogens.

• Mucosa: Lines the gastrointestinal (GI), respiratory, and urogenital tracts, as well as the eyes.

• Other barriers: Include secretions (e.g., tears, saliva, mucus) that trap and eliminate microbes.

Hematopoiesis

Definition and Process

Hematopoiesis is the process by which all blood cells, including immune cells, are formed from hematopoietic stem cells in the bone marrow.

• Common lymphoid progenitor: Gives rise to B cells, T cells, and NK cells.

• Common myeloid progenitor: Gives rise to granulocytes (neutrophils, eosinophils, basophils), monocytes, macrophages, dendritic cells, mast cells, erythrocytes, and platelets.

Cytokines

Role and Function

Cytokines are small, soluble proteins released by immune cells that regulate immunity, inflammation, and hematopoiesis.

• Producer cells: Cells that secrete cytokines in response to stimuli.

• Effector cells: Cells that respond to cytokines by altering gene expression and function.

• The network of cytokine signals is called the cytokine network.

Main Types of Immune System Cytokines

• Interleukins (ILs): Mediate communication among leukocytes and other tissues.

• Interferons (IFNs): Antiviral proteins that may also act as cytokines.

• Growth factors: Stimulate stem cell division and differentiation.

• Tumor necrosis factor (TNF): Secreted by macrophages and T cells to kill tumor cells and regulate immune responses and inflammation.

• Chemokines: Direct the movement of leukocytes to sites of infection or injury (chemotaxis).

Mechanism of Cytokine Action

• Cells have specific receptors for cytokines.

• Binding of cytokines to their receptors triggers intracellular signaling, leading to changes in gene expression and cell behavior.

• Cells may be stimulated to produce additional cytokines, amplifying the immune response.

Summary Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Non-specific	Highly specific
Response Time	Immediate (minutes to hours)	Delayed (days)
Memory	None	Immunological memory present
Main Components	Physical barriers, phagocytes, complement, NK cells	B cells, T cells, antibodies
Major Functions	Initial defense, inflammation, pathogen containment	Pathogen clearance, long-term protection