Chapter 43: Immunology – Innate and Adaptive Immune Responses

Immunology: Overview

Immunology is the study of the immune system, which protects organisms from infection by recognizing and responding to pathogens. The immune system is divided into two main branches: innate immunity and adaptive immunity.

Innate and Adaptive Immunity

Comparison of Innate and Adaptive Immunity

• Innate Immunity:

  • Present from birth; acts immediately upon infection.

  • Non-specific: recognizes traits shared by broad ranges of pathogens using a small set of receptors.

  • Includes physical barriers (skin, mucous membranes), chemical defenses, and blood cells.

  • Rapid response.

• Adaptive (Acquired) Immunity:

  • Develops after exposure to pathogens.

  • Highly specific: recognizes specific features of particular pathogens using a vast array of receptors.

  • Involves lymphocytes (B cells and T cells).

  • Slower initial response, but generates memory cells for rapid future responses.

Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Non-specific	Highly specific
Response Time	Immediate	Slower (days to weeks)
Memory	None	Immunological memory
Main Cells	Phagocytes, NK cells	B cells, T cells

Concept 1: Innate Immunity

Recognition and Response in Innate Immunity

• Relies on traits common to groups of pathogens.

• Types of phagocytic cells:

  • Neutrophils: Engulf and destroy pathogens; attracted to infection sites.

  • Macrophages: Large phagocytes found throughout the body; also activate adaptive immunity.

  • Dendritic cells: Activate adaptive immunity by presenting antigens.

  • Eosinophils: Produce destructive enzymes, especially against parasites.

• Lymphatic system: Network that distributes lymph (fluid) and houses immune cells.

Inflammatory Response

• Localized response to injury or infection, characterized by redness, swelling, heat, and pain.

• Mast cells release histamine, increasing blood vessel permeability.

• Macrophages and neutrophils release cytokines to enhance the immune response.

• Pus: Accumulation of white blood cells, dead pathogens, and cell debris.

The Complement System

• Composed of about 30 proteins in blood, usually inactive.

• Functions:

  • Opsonization: Enhances phagocytosis.

  • Activates mast cells: Promotes inflammation.

  • Cytolysis: Forms pores in pathogen membranes, causing lysis.

  • Neutralizes antigens.

Concept 2: Adaptive Immunity

Pathogen-Specific Recognition

• Relies on lymphocytes:

  • B cells: Mature in bone marrow.

  • T cells: Mature in thymus.

• Key characteristics:

  • Diversity: Billions of antigen receptors.

  • Self-tolerance: Avoids attacking self.

  • Proliferation: Clonal selection after activation.

  • Immunologic memory: Enhanced response to repeated exposure.

Antigen Recognition

• Antigen: Substance that elicits a lymphocyte response.

• Epitope: Specific part of antigen recognized by receptors.

• B and T cell receptors have similar components but recognize antigens differently.

B Cell Receptors and Antibodies

• B cell receptor: Y-shaped, with two heavy and two light chains.

• Constant (C) region: Little variation among B cells.

• Variable (V) region: High diversity, provides antigen specificity.

• Activated B cells secrete antibodies (immunoglobulins, Ig).

T Cell Receptors and MHC

• T cell receptor: Two polypeptide chains (α and β).

• Recognizes antigen fragments presented by MHC (major histocompatibility complex) on host cells.

• Antigen presentation: Process of displaying antigen fragments on cell surface via MHC.

Concept 3: Adaptive Immunity Defends Against Infection

Branches of Acquired Immunity

• Humoral immune response:

  • Antibodies neutralize or eliminate toxins and pathogens in blood and lymph.

• Cell-mediated immune response:

  • Specialized T cells destroy infected host cells.

  • Helper T cells activate both humoral and cell-mediated responses.

Antibody Functions

• Opsonization: Enhances phagocytosis.

• Complement activation: Triggers protein cascade, leading to cell lysis.

• Attack complex: Forms pores in foreign cell membranes.

• Neutralization: Blocks pathogen/toxin from binding to host cells.

Immunity Types

• Active immunity:

  • Develops after exposure to antigen (natural infection or vaccination).

  • Involves memory cell formation.

• Passive immunity:

  • Immediate, temporary protection (e.g., maternal antibodies via placenta or breast milk).

Classes of Immunoglobulins (Antibodies)

• IgD: Membrane-bound form.

• IgM: First soluble class produced.

• IgG: Most abundant, second soluble class.

• IgA and IgE: Other soluble classes with specialized functions.

Concept 4: Disruptions in Immune System Function

Allergies

• Exaggerated immune responses to harmless antigens (allergens).

• IgE antibodies bind to mast cells, triggering histamine release.

• Severe reactions can cause anaphylactic shock.

Autoimmune Diseases

• Immune system attacks body's own cells.

• Examples: lupus, rheumatoid arthritis, type 1 diabetes, multiple sclerosis.

Other Factors Affecting Immunity

• Moderate exercise enhances immune function.

• Psychological stress and lack of rest can impair immunity.

HIV and Immune System Suppression

• HIV infects helper T cells, impairing both humoral and cell-mediated responses.

• Leads to AIDS due to loss of immune function.

• HIV evades immunity via antigenic variation and latency.

Cancer and Immunity

• Impaired adaptive immunity increases cancer risk.

• Some cancers are virus-associated (e.g., HPV and cervical cancer).

• Vaccines can prevent certain virus-induced cancers.

Key Terms and Concept Map Suggestions

• Lymphocyte, antigen, receptor, helper T cell, B cell, cytotoxic T cell, memory cell, antigen-presenting cell (APC), plasma cell, MHC class I/II, heavy chain, light chain, variable region, constant region, CD4, macrophage, phagocytosis, humoral response, cell-mediated response.