The Lymphatic System and Immunity: Innate and Adaptive Defenses

The Immune System: Overview

Introduction

The immune system protects the body from pathogens such as viruses, bacteria, protozoa, fungi, and multicellular parasites. It consists of innate (nonspecific) and adaptive (specific) defenses, each with distinct mechanisms and cellular components.

• Innate Immunity: Present at birth, provides immediate, nonspecific defense against pathogens.

• Adaptive Immunity: Develops after exposure to specific antigens, involving lymphocytes for targeted responses.

Defense Mechanisms

Innate (Nonspecific) Immunity

Innate immunity is the body's first line of defense, acting against any invading agent in a consistent manner.

• Physical Barriers: Skin, hair, mucosal linings, and secretions (sweat, mucus, urine) that flush away or destroy pathogens.

• Phagocytes: Cells that engulf and digest pathogens and debris.

• Immune Surveillance: Natural killer (NK) cells monitor and destroy abnormal cells.

• Interferons: Chemical messengers that trigger antiviral protein production.

• Complement System: Circulating proteins that enhance pathogen destruction.

• Inflammatory Response: Localized tissue response to injury or infection.

• Fever: Elevated body temperature that accelerates defenses and inhibits pathogens.

Adaptive (Specific) Immunity

Adaptive immunity targets specific pathogens and relies on lymphocytes. It develops after exposure to antigens and provides long-term protection.

• Cell-Mediated Immunity: T cells defend against abnormal cells and pathogens inside cells.

• Antibody-Mediated (Humoral) Immunity: B cells produce antibodies to neutralize antigens in body fluids.

Major Categories of Innate Immunity

1. Physical Barriers

   • Outer layer of skin

   • Hair

   • Epithelial layers of internal organs (mucosa)

   • Secretions (sweat, mucus, urine) flush away materials

   • Secretions (enzymes, antibodies, stomach acid) kill or inhibit microorganisms

2. Phagocytes

   • Microphages: Neutrophils and eosinophils, found in blood, enter tissues by chemotaxis

   • Macrophages: Derived from monocytes, distributed throughout body, part of the monocyte-macrophage system

   • Specialized macrophages: Microglia (CNS), Kupffer cells (liver), Alveolar macrophages (lungs)

3. Immune Surveillance

   • Carried out by NK cells

   • NK cells identify and attach to abnormal cells, release perforins to lyse target cells

   • Targets include cancer cells (with tumor-specific antigens), virus-infected cells, and transplanted cells (with MHC I)

4. Interferons

   • Chemical messengers that trigger antiviral protein production

   • Types: Alpha (produced by virus-infected cells), Beta (by fibroblasts, slows inflammation), Gamma (by T cells and NK cells, stimulates macrophages)

5. Complement System

   • 30 plasma proteins form the complement system

   • Activated via classical (antibody-dependent) or alternative (antibody-independent) pathways

   • Leads to formation of membrane attack complex (MAC) causing cell lysis

6. Inflammatory Response

   • Triggered by tissue injury

   • Cardinal signs: swelling, redness, heat, pain, loss of function

   • Effects: temporary repair, barrier formation, retardation of pathogen spread, mobilization of defenses, and tissue regeneration

   • Products: necrosis (tissue destruction), pus (debris/necrotic tissue), abscess (pus in enclosed space)

7. Fever

   • Body temperature rises above 37.2°C in response to pyrogens

   • Increases metabolism, accelerates defenses, inhibits some pathogens

   • Excessive fever can denature enzymes and cause harm

Forms of Immunity

Innate vs. Adaptive Immunity

• Innate Immunity: Present at birth, genetically determined, nonspecific

• Adaptive Immunity: Develops after birth, specific to antigens

Active and Passive Immunity

Type	How Acquired	Example
Active Immunity	Exposure to antigen	Infection (natural), vaccination (artificial)
Passive Immunity	Transfer of antibodies	Maternal antibodies (natural), injection of antibodies (artificial)

Specific Defenses: T Cells and B Cells

T Cells (Cell-Mediated Immunity)

• Defend against abnormal cells and pathogens inside cells

• Four major types:

  • Cytotoxic T cells (TC): Attack virus-infected cells

  • Memory T cells: Provide rapid response to previously encountered antigens

  • Helper T cells (TH): Stimulate T and B cell function

  • Suppressor T cells (TS): Inhibit T and B cell function

• T cells recognize antigens presented by MHC proteins on cell surfaces

  • MHC I: All nucleated cells, present endogenous antigens

  • MHC II: Antigen-presenting cells (APCs), present exogenous antigens

B Cells (Antibody-Mediated Immunity)

• Produce antibodies to neutralize antigens in body fluids

• Millions of B cell populations, each with unique antibody molecules

• B cell activation requires antigen binding and helper T cell stimulation

• Activated B cells differentiate into:

  • Plasma cells: Secrete antibodies

  • Memory B cells: Provide long-term immunity

Summary Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Nonspecific	Specific
Memory	None	Present
Cells Involved	Phagocytes, NK cells	T cells, B cells
Response Time	Immediate	Slower (after exposure)

Example:

• Innate Immunity: Skin prevents entry of bacteria; neutrophils engulf bacteria in a wound.

• Adaptive Immunity: After vaccination, B cells produce antibodies specific to the vaccine antigen.

Additional info: The immune system's ability to distinguish "self" from "nonself" is crucial for preventing autoimmune diseases. Toll-like receptors (TLRs) on immune cells recognize pathogen-associated molecular patterns (PAMPs), initiating immune responses.