Immune System

Components of the Immune System

The immune system is a complex network of organs, cells, and tissues that work together to defend the body against harmful pathogens and maintain homeostasis.

• Organs: Includes lymph nodes, spleen, thymus gland, and mucosa-associated lymphoid tissue (MALT).

• Cells: Includes various types of white blood cells (leukocytes) such as granulocytes, phagocytes, and lymphocytes.

Functions of the Immune System

• Defense: Protects the body from external and internal threats, such as bacteria, viruses, and abnormal cells.

• Absorption of Nutrients: Assists in the absorption of dietary fats and fat-soluble vitamins via the lymphatic system.

• Fluid Balance: Maintains fluid balance by returning excess interstitial fluid to the bloodstream.

Lymphatic System (Network of Organs in the Immune System)

Components of the Lymphatic System

• Vessels: Lymphatic vessels transport lymph fluid throughout the body.

• Nodes: Lymph nodes filter lymph and house immune cells that detect and respond to pathogens.

• Non-encapsulated Lymph Tissue (MALT): Mucosa-associated lymphoid tissue found in mucosal layers of many body areas (e.g., GI tract, vaginal tract).

• Types of MALT: GALT (gut-associated), NALT (nasal-associated), BALT (bronchus-associated), depending on location.

• Leukocytes: White blood cells that monitor for foreign substances.

• Spleen: Filters blood, removes old red blood cells, and supports immune responses.

• Thymus Gland: Site of T-lymphocyte maturation.

• Microbes in the Intestine: Interact with immune cells in the gut-associated lymphoid tissue.

Functions of Lymphatic Vessels and Nodes

• Pick up excess filtered fluid at the capillary bed and return it to the bloodstream.

• Absorb fat in the intestines and move it to the bloodstream.

• Expose contents of fluid to white blood cells for immune surveillance.

• Lymph nodes: Filter lymph, remove waste, bacteria, and viruses; contain reticular fibers that trap particles.

• Number of nodes: Approximately 450 in the human body.

White Blood Cells (Leukocytes)

Types of Leukocytes

• Granulocytes: Contain granules in their cytoplasm.

  • Neutrophils: Most abundant; phagocytic; first responders to infection.

  • Eosinophils: Red granules; involved in allergic reactions and parasitic infections.

  • Basophils: Least abundant; involved in allergic reactions.

• Agranulocytes: Lack visible granules.

  • Phagocytes: Engulf and digest pathogens.

  • Lymphocytes: Carry out many immune responses (e.g., B cells, T cells, NK cells).

Spleen

Functions and Characteristics

• Destroys/removes old defective red blood cells and acts as a blood reservoir.

• Red pulp: Contains reticular fibers that squeeze old RBCs, causing them to break and be phagocytized.

• White pulp: Site where WBCs reside and monitor for foreign particles.

Thymus Gland

Functions

• Secretes thymosin, a hormone important for T-lymphocyte maturation.

• T-lymphocytes mature in the thymus.

• Shrinks with age (involution).

Main Arms of Immunity

Innate (Non-specific) Immunity

• Effective at birth and does not require prior exposure to a pathogen.

• Response is the same regardless of the pathogen.

• Does not improve with repeated exposure.

Adaptive (Acquired & Specific) Immunity

• Requires first exposure to a pathogen to be effective.

• Response is specific to the pathogen encountered.

• Improves with repeated exposure (immunological memory).

First Line of Defense (Innate Immunity)

• Physical Barriers:

  • Intact skin (cannot be broken easily)

  • Ciliary escalator in the trachea

  • Lacrimal gland (washes the eye when you blink)

  • Nose hairs

• Chemical Barriers:

  • Lysozyme (in tears and saliva) breaks bacterial cell walls

  • Urine flow flushes out microbes

  • HCl in the stomach destroys pathogens

  • Saliva contains antimicrobial enzymes

• Reflexes: Coughing and sneezing expel pathogens from the respiratory tract.

Second Line of Defense (Innate Immunity)

Pattern Recognition Receptors (PRRs)

• Located on white blood cells and recognize PAMPs (pathogen-associated molecular patterns).

• PRRs bind to PAMPs and induce lysis of the cell with the PAMP.

• Example: Toll-like receptors (TLRs).

Inflammation

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

• Symptoms:

  • Redness (vasodilation increases blood flow)

  • Swelling (increased hydrostatic pressure and filtration in capillaries)

  • Warmth

  • Pain

• Chemical Mediators:

  • Prostaglandins

  • Kinins

  • Leukotrienes

• Leukocyte Recruitment:

  • WBCs have CAMs (cell adhesion molecules) that help them adhere to capillary walls at injury sites (margination).

  • After margination, WBCs exit the capillary via diapedesis and carry out phagocytosis.

Phagocytosis

Phagocytosis is the process by which phagocytes engulf and digest pathogens.

1. Chemotaxis and adherence of microbe to phagocyte

2. Ingestion of microbe by phagocyte

3. Formation of phagosome

4. Fusion of phagosome with lysosome to form a phagolysosome

5. Digestion of ingested microbe by enzymes

6. Formation of residual body containing indigestible material

7. Discharge of waste materials

• Opsonin: A substance that binds to pathogens and increases the likelihood of phagocytosis.

Complement System

The complement system is a group of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens.

• Activation Pathways:

  • Classical pathway: Triggered by the presence of antibodies.

  • Alternate pathway: Triggered by surface molecules on pathogens.

• Membrane Attack Complex (MAC): Forms holes in the pathogen membrane, leading to lysis and death of the pathogen.

Interferons

• Produced by virus-infected cells to interfere with viral replication in other cells.

• Induce production of antiviral proteins in uninfected cells.

Natural Killer (NK) Cells

• Type of lymphocyte that acts in immune surveillance.

• Recognize and kill tumor cells and virus-infected cells by releasing cytotoxic agents.

Fever

• Thermostat is located in the hypothalamus.

• Pyrogens: Substances that induce fever.

• Effects of fever:

  • Increase in body temperature

  • Increase in phagocytosis

  • Can inhibit bacterial growth

Summary Table: Innate vs. Adaptive Immunity