Chapter 43: The Immune System – Innate and Adaptive Immunity

The Immune System

Introduction

The immune system is a complex network of cells, tissues, and molecules that protects organisms from pathogens such as bacteria, viruses, and fungi. It consists of two main branches: innate immunity and adaptive immunity, each with distinct mechanisms and roles in defense.

Innate Immunity

Overview of Innate Immunity

• Innate immunity is the first line of defense, providing immediate, non-specific protection against pathogens.

• All animals possess innate immunity, which includes barrier defenses, phagocytosis, and antimicrobial peptides.

• Vertebrates have additional innate defenses such as natural killer cells, interferons, and the inflammatory response.

Barrier Defenses

• Physical and chemical barriers prevent pathogen entry.

• Skin and mucous membranes (respiratory, urinary, reproductive tracts) act as physical barriers.

• Mucus traps microbes for removal.

• Body fluids (saliva, mucus, tears) contain antimicrobial substances.

• Low pH of skin and digestive system inhibits bacterial growth.

Cellular Innate Defenses

• Innate immune cells detect, ingest, and destroy pathogens.

• Recognition is mediated by Toll-like receptors (TLRs), which bind to conserved molecular patterns on pathogens (e.g., dsRNA in viruses, flagellin in bacteria, mannans in fungi).

Types of Phagocytic Cells:

• Neutrophils: Circulate in blood; engulf and destroy pathogens.

• Macrophages: Migrate through tissues or reside in organs; perform phagocytosis and activate other immune cells.

• Dendritic cells: Stimulate development of adaptive immunity.

• Eosinophils: Discharge destructive enzymes against parasites.

Natural Killer (NK) Cells:

• Detect abnormal (infected or cancerous) cells and induce apoptosis (cell death).

• Release chemicals to inhibit spread of infection.

Lymphatic System: Many innate immune responses involve the lymphatic system, which transports lymph and houses immune cells.

Local Inflammatory Response

• Triggered by injury or infection; characterized by heat, swelling, redness, and pain.

• Mast cells release histamine, causing blood vessels to dilate and become more permeable.

• Cytokines recruit neutrophils and other immune cells to the site.

• Enhanced blood flow delivers antimicrobial peptides; accumulation of pus (dead cells, pathogens, debris) occurs.

• Excess fluid and pus are taken up as lymph and transported to lymph nodes, where pathogens are further processed.

Systemic and Chronic Inflammation

• Severe infection or tissue damage can cause a systemic inflammatory response (e.g., fever).

• Fever is induced by cytokines that reset the body's thermostat, potentially enhancing immune function.

• Overwhelming inflammation can lead to septic shock, a life-threatening condition.

• Chronic inflammation (e.g., Crohn's disease, ulcerative colitis) disrupts normal tissue function.

Antimicrobial Peptides and Proteins

• Pathogen recognition triggers production of peptides that attack or inhibit pathogens.

• Interferons: Proteins that inhibit viral replication and activate macrophages.

• Complement system: A group of ~30 plasma proteins that, when activated, cause lysis of pathogens and enhance inflammation.

Adaptive Immunity

Overview of Adaptive Immunity

• Adaptive immunity is specific and develops after exposure to pathogens.

• Relies on lymphocytes (white blood cells): B cells (mature in bone marrow) and T cells (mature in thymus).

• Features immunological memory and self-tolerance.

Antigens and Antigen Receptors

• Antigens: Foreign molecules (usually proteins or polysaccharides) that elicit an immune response.

• Antigen receptors: Proteins on B and T cells that specifically bind to antigens.

• Epitope: The specific part of an antigen recognized by a receptor.

Antigen Recognition by B Cells and Antibodies

• B cell receptors are Y-shaped molecules with two identical heavy chains and two identical light chains.

• Variable (V) regions confer antigen specificity; constant (C) regions are less variable.

• Binding of antigen activates B cells, leading to secretion of antibodies (immunoglobulins, Ig) with the same specificity.

• Antibodies are secreted, not membrane-bound.

Antigen Recognition by T Cells

• T cell receptors consist of two polypeptide chains (α and β), each with variable and constant regions.

• T cells recognize antigen fragments presented on host cells by major histocompatibility complex (MHC) molecules.

• Antigen presentation: MHC molecules bind and display antigen fragments on the cell surface for T cell recognition.

B Cell and T Cell Development

• Four major characteristics of adaptive immunity:

  • Immense diversity of lymphocytes and receptors

  • Self-tolerance (no reactivity to self molecules)

  • Proliferation after activation (clonal selection)

  • Immunological memory

The Basis of B Cell and T Cell Diversity

• Diversity arises from recombination of gene segments encoding antigen receptors.

• Immunoglobulin (Ig) genes have multiple V (variable), J (joining), and C (constant) segments.

• For the light chain: 40 V segments × 5 J segments = 200 combinations (not including heavy chain diversity).

• Recombination is mediated by the enzyme recombinase and is permanent for each lymphocyte.

Self-Tolerance

• Lymphocytes with receptors for self molecules are eliminated or inactivated during development (apoptosis or anergy).

Clonal Selection and Immunological Memory

• Upon antigen binding, lymphocytes proliferate to form clones of effector and memory cells.

• Primary immune response: First exposure to antigen; slower and less robust.

• Secondary immune response: Subsequent exposure; faster and stronger due to memory cells.

Summary Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Non-specific (recognizes broad classes of pathogens)	Highly specific (recognizes unique antigens)
Response Time	Immediate (minutes to hours)	Slower (days to weeks for first exposure)
Memory	None	Immunological memory (faster upon re-exposure)
Main Components	Barriers, phagocytes, NK cells, complement, interferons	B cells, T cells, antibodies

Example

When a virus enters the body, innate immunity responds first by blocking entry (skin, mucous membranes), then by phagocytosis and inflammation. If the virus persists, adaptive immunity is activated, producing specific antibodies and memory cells for long-term protection.

Additional info: This summary is based on Campbell Biology, Chapter 43, and is suitable for college-level General Biology students preparing for exams on the immune system.