Part 2: Immunity – Ability to Resist Infection and Disease

Overview of Immunity

Immunity refers to the body's ability to resist infection and disease through various defense mechanisms. These mechanisms are classified into two main systems: the nonspecific (innate) defense system and the specific (adaptive) defense system.

• Innate Immunity: Provides immediate, nonspecific defense against pathogens.

• Adaptive Immunity: Provides specific, targeted responses to particular pathogens, involving memory and specificity.

Nonspecific Defense System: Innate Immunity

Physical Barriers

The first line of defense against pathogens consists of physical barriers that prevent entry of infectious agents.

• Skin: The epidermis acts as a tough, protective layer that blocks pathogen entry.

• Hair: Traps microbes and particulates, especially in the nose and ears.

• Mucous Membranes: Line the respiratory, digestive, urinary, and reproductive tracts, trapping microbes and particulates in mucus.

• Normal Flora: Beneficial bacteria that inhabit the body and compete with pathogens for nutrients and space.

Chemical Barriers

Chemical barriers enhance physical defenses by creating hostile environments for pathogens.

• Sebum & Sweat: Sebum creates an acidic mantle on the skin, inhibiting microbial growth.

• Lysozyme: An enzyme found in saliva, nasal secretions, tears, sweat, and other fluids; breaks down bacterial cell walls.

• Tears: Lubricate eyes and contain lysozyme and IgA antibodies.

• Acidity: Stomach acid (HCl) and vaginal acidity destroy ingested pathogens.

• Urine: Flushes microbes from the urinary tract.

• Mucus: Sticky substance that traps microorganisms.

• Saliva: Contains lysozyme and IgA, lubricates mouth, and helps eliminate microbes.

Cellular Defense

Cells of the innate immune system act to identify and eliminate pathogens through various mechanisms.

• Phagocytes: White blood cells such as monocytes, macrophages, neutrophils, and eosinophils engulf and digest pathogens.

• Natural Killer (NK) Cells: Lymphocytes that provide immune surveillance by destroying infected or abnormal cells.

Chemical & Protein Defense

Certain chemicals and proteins in the body provide additional layers of defense against infection.

• Basophils & Mast Cells: Release histamine and other chemicals that promote inflammation.

• Antimicrobial Proteins:

  • Interferons (IFNs): Proteins released by virus-infected cells that inhibit viral replication and activate immune cells.

  • Complement System: A group of plasma proteins that enhance phagocytosis, promote inflammation, and directly destroy pathogens.

  • Defensins: Small proteins that provide a chemical barrier, have direct antimicrobial activity, and support immune functions.

• Fever: An increase in body temperature that accelerates metabolic activity and enhances body defenses.

First Line of Defense: Preventing Entry of Infectious Agents

Skin and Mucosal Membranes

These structures provide physical, chemical, and biological barriers to infection.

• Skin:

  • Prevents growth of pathogens due to its dry, acidic environment.

  • Exfoliation removes attached microbes.

  • Sebaceous glands secrete sebum, creating an acidic mantle.

  • Normal flora compete with pathogens for resources.

• Mucosal Membranes:

  • Trap microbes in mucus and move them out via cilia or peristalsis.

  • Contain antimicrobial substances such as lysozyme and IgA.

  • Gastric glands secrete HCl, destroying ingested microbes.

  • Defecation and vomiting eliminate microbes from the body.

Second Line of Defense: Internal Nonspecific Responses

Phagocytosis

Phagocytes recognize, engulf, and destroy pathogens through a process called phagocytosis.

• Phagocytes are attracted to sites of infection by chemical signals (chemotaxis).

• Pathogens are engulfed into a phagosome, which fuses with a lysosome to form a phagolysosome, where the pathogen is destroyed.

Natural Killer (NK) Cells

NK cells patrol the body and destroy infected or abnormal cells by releasing cytotoxic substances.

• Provide rapid responses to virus-infected cells and tumor formation.

• Do not require prior activation by antigens.

Inflammation

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

• Triggered by chemicals such as histamine, kinins, prostaglandins, and leukotrienes.

• Increases blood flow and vascular permeability, allowing immune cells and proteins to access the affected tissue.

• Promotes healing and removal of pathogens.

Steps of Inflammation

1. Release of inflammatory chemicals from damaged tissues.

2. Vasodilation and increased permeability of blood vessels.

3. Migration of phagocytes to the site of injury (chemotaxis).

4. Phagocytosis and removal of pathogens and debris.

5. Tissue repair and resolution of inflammation.

Antimicrobial Proteins

These proteins enhance the innate immune response by targeting pathogens directly or supporting other immune mechanisms.

• Interferons (IFNs): Inhibit viral replication and activate immune cells.

• Complement System: Consists of plasma proteins that form a cascade to destroy pathogens, enhance phagocytosis (opsonization), and promote inflammation.

• Defensins: Directly attack microbes and support immune cell function.

Complement Pathways

Fever

Fever is a systemic response to infection, characterized by an elevation in body temperature above 37.2°C (99°F).

• Triggered by pyrogens released from leukocytes and pathogens.

• Increases metabolic rate, accelerates immune reactions, and inhibits growth of some pathogens.

• Regulated by the hypothalamus.

Summary Table: Innate Immunity Components

Additional info: Adaptive immunity (specific defense system) involves B lymphocytes (humoral immunity) and T lymphocytes (cellular immunity), which provide targeted responses and immunological memory. These are covered in detail in other sections of the course.