Innate and Adaptive Defenses of the Immune System: Anatomy & Physiology Study Notes

Chapter 21: The Immune System – Innate and Adaptive Body Defenses

Overview of the Immune System

The immune system provides resistance to disease and is composed of two main defense systems: innate (nonspecific) and adaptive (specific) defenses. These systems work together to protect the body from pathogens and harmful substances.

• Innate defenses: Present at birth, provide immediate but nonspecific protection.

• Adaptive defenses: Develop after exposure, provide specific and long-lasting protection.

Key Point: Both systems release and recognize many of the same defensive molecules, and their responses are intertwined.

Classification of Immune Defenses

• Innate Defenses

  • Surface barriers: Skin, mucous membranes

  • Internal defenses: Phagocytes, natural killer cells, inflammation, antimicrobial proteins, fever

• Adaptive Defenses

  • Humoral immunity: B cells

  • Cellular immunity: T cells

Innate Defenses

First Line of Defense: Surface Membrane Barriers

The first line of defense consists of physical and chemical barriers that prevent pathogens from entering the body.

• Physical barriers: Skin and mucous membranes form mechanical barriers against pathogens.

• Chemical barriers: Secretions such as sweat, sebum, and mucus contain antimicrobial substances.

Structure/Associated Substance	Protective Mechanism
Intact epidermis	Forms mechanical barrier that prevents entry of pathogens and other harmful substances into body
Acid mantle of skin	Skin secretions (sweat and sebum) make epidermal surface acidic, which inhibits bacterial growth; also contain various bactericidal chemicals
Keratin	Provides resistance against acids, alkalis, and bacterial enzymes
Mucosae	Form mechanical barrier that prevents entry of pathogens
Mucus	Traps microorganisms in respiratory and digestive tracts
Nasal hairs	Filter and trap microorganisms in nasal passages
Cilia	Propel debris-laden mucus away from nasal cavity and lower respiratory passages
Gastric juice	Contains concentrated hydrochloric acid and protein-digesting enzymes that destroy pathogens in stomach
Vaginal secretions	Inhibit growth of most bacteria and fungi in female reproductive tract
Lacrimal secretion (tears) and saliva	Continuously lubricate and cleanse eyes (tears) and oral cavity (saliva); contain lysozyme, an enzyme that destroys microorganisms
Urine	Normally acidic pH inhibits bacterial growth; cleanses the lower urinary tract as it flushes from the body

Second Line of Defense: Internal Cellular and Chemical Defenses

If pathogens breach surface barriers, the second line of defense is activated, involving various cells and chemicals.

• Phagocytes: White blood cells that ingest and digest foreign invaders.

• Natural Killer (NK) cells: Non-phagocytic lymphocytes that target virus-infected and cancer cells.

• Inflammatory response: Involves macrophages, mast cells, white blood cells, and inflammatory chemicals.

• Antimicrobial proteins: Interferons and complement proteins.

• Fever: Elevated body temperature as a systemic response to infection.

Phagocytosis

Phagocytosis is the process by which phagocytes engulf and destroy pathogens. The main steps are:

1. Phagocyte adheres to pathogens or debris using receptors.

2. Phagocyte forms pseudopods that engulf the particles, forming a phagosome.

3. Phagosome fuses with a lysosome, forming a phagolysosome.

4. Acid hydrolase enzymes and toxic compounds destroy pathogens.

5. Indigestible and residual material is exocytosed from the phagocyte.

Opsonization enhances phagocytosis by coating pathogens with antibodies or complement proteins, making them easier to recognize and ingest.

Natural Killer (NK) Cells

• Large granular lymphocytes that patrol blood and lymph.

• Can kill cancer and virus-infected cells before the adaptive immune system is activated.

• Induce apoptosis (programmed cell death) in target cells.

• Release potent chemicals called chemokines to enhance the inflammatory response.

Inflammation: Tissue Response to Injury

Inflammation is triggered whenever body tissues are injured by trauma, heat, chemicals, or infection. Its main functions are:

• Prevents spread of damaging agents

• Disposes of cell debris and pathogens

• Alerts the adaptive immune system

• Sets the stage for tissue repair

Cardinal signs of acute inflammation:

• Redness (rubor)

• Heat (calor)

• Swelling (tumor)

• Pain (dolor)

• Impairment of function (sometimes)

Major steps of inflammation:

1. Inflammatory chemical release (e.g., histamine, kinins, prostaglandins, cytokines, complement)

2. Vasodilation and increased vascular permeability

3. Phagocyte mobilization

Phagocyte Mobilization

Phagocyte mobilization is a multi-step process that brings white blood cells to the site of injury:

1. Leukocytosis: Release of neutrophils from bone marrow.

2. Margination: Neutrophils cling to the walls of capillaries in the inflamed area.

3. Diapedesis: Neutrophils squeeze through capillary walls into tissues.

4. Chemotaxis: Inflammatory chemicals attract neutrophils to the injury site.

Monocytes follow neutrophils and become macrophages, replacing dying neutrophils and remaining to clean up debris.

Antimicrobial Proteins

Antimicrobial proteins enhance innate defenses by attacking microorganisms directly or hindering their ability to reproduce.

• Interferons (IFNs): Proteins released by virus-infected cells to warn neighboring cells and inhibit viral replication.

• Complement proteins: A group of about 20 blood proteins that enhance both innate and adaptive immunity.

Complement System

The complement system can be activated by three pathways:

1. Classical pathway: Triggered by antibodies bound to pathogens.

2. Lectin pathway: Triggered by lectins binding to specific sugars on pathogens.

3. Alternative pathway: Triggered by spontaneous activation on pathogen surfaces.

All pathways converge on the activation of protein C3, which splits into C3a and C3b, leading to:

• Enhancement of inflammation

• Promotion of phagocytosis (opsonization)

• Cell lysis via the membrane attack complex (MAC)

Fever

Fever is an abnormally high body temperature that is a systemic response to infection. Pyrogens released by leukocytes and macrophages act on the hypothalamus to raise body temperature.

• Causes liver and spleen to sequester iron and zinc, limiting microbial growth

• Increases metabolic rate, speeding up tissue repair

Adaptive Defenses

Humoral and Cellular Immunity

The adaptive immune system provides specific defense against particular pathogens and consists of:

• Humoral immunity: Mediated by B cells and antibodies

• Cellular immunity: Mediated by T cells

Adaptive responses take longer to activate but provide long-lasting and specific protection.

Summary Table: Innate vs. Adaptive Defenses

Defense Type	Main Components	Response Time	Specificity
Innate	Skin, mucous membranes, phagocytes, NK cells, inflammation, antimicrobial proteins, fever	Immediate	Nonspecific
Adaptive	B cells (antibodies), T cells	Delayed (days)	Specific to particular pathogens

Key Terms and Definitions

• Pathogen: Disease-causing microorganism

• Phagocyte: White blood cell that engulfs and digests foreign particles

• Opsonization: Process of coating pathogens to enhance phagocytosis

• Apoptosis: Programmed cell death

• Inflammation: Localized tissue response to injury or infection

• Interferon: Protein that interferes with viral replication

• Complement: Group of proteins that enhance immune responses

• Fever: Elevated body temperature as a defense mechanism

Relevant Equations

• Complement Activation Cascade:

$C3 \rightarrow C3a + C3b$

Example: C3b acts as an opsonin, enhancing phagocytosis, while C3a stimulates inflammation.

Additional info: The above notes expand on the brief points and diagrams in the slides, providing definitions, examples, and context for each major concept in the innate and adaptive immune systems.