General Biology: The Immune System (Chapter 24) Study Notes

Chapter 24: The Immune System

Introduction to the Immune System

The immune system protects animals against disease-causing agents, including pathogens, abnormal body cells, and cancerous cells. It is essential for maintaining health and defending against infections.

• Pathogens: Microorganisms such as bacteria, viruses, and fungi that cause disease.

• Abnormal body cells: Cells that may become cancerous and threaten the organism.

Lines of Defense

The immune system has three cooperative lines of defense to counter threats:

• External barriers: Skin, mucous membranes, and secretions(分泌物) that prevent pathogen entry.

• Innate immunity(先天免疫): Non-specific responses including phagocytic cells, inflammation, and antimicrobial proteins.(抗菌蛋白)

• Adaptive immunity(後天免疫): Specific responses involving lymphocytes and antibodies.

Innate Immunity(先天免疫)

Innate immunity provides immediate, non-specific defense against pathogens.

• Phagocytic cells: Neutrophils(嗜中性白血球) and macrophages(巨噬細胞) engulf and destroy pathogens.

• Inflammatory response: Triggered by tissue damage; involves histamine release(組織胺釋放), increased blood flow(血流量增加), and recruitment of immune cells.(免疫細胞募集)

• Antimicrobial proteins（抗菌蛋白）: Interferons(干擾素) and complement proteins(補體蛋白) help fight infections.

The Lymphatic System

The lymphatic system is crucial for immune function and fluid balance.

• Lymphatic vessels: Transport lymph, a fluid containing immune cells and nutrients.

• Lymph nodes: Sites for fighting infection, packed with lymphocytes and macrophages.

• Other organs: Spleen, thymus, and tonsils contribute to immune defense.

Adaptive (Acquired) Immunity （後天免疫）

Adaptive immunity is a specific response to foreign molecules called antigens. It involves lymphocytes and produces immunological memory(產生免疫記憶).

• Antigens（抗原）: Molecules that elicit （引發） an immune response.

• Active （主動） immunity: Developed after exposure to antigens (infection or vaccination).

• Passive immunity: Temporary immunity acquired by receiving antibodies (e.g., from mother to child).

Lymphocytes and Immune Response

Lymphocytes are white blood cells central to adaptive immunity. Two main types are B cells and T cells.

• B cells: Produce antibodies; responsible for humoral immunity.

• T cells: Mediate cell-mediated immunity; attack infected or abnormal cells.

Antibodies and Antigen Recognition

Antibodies are proteins that bind specifically to antigens, marking them for elimination.

• Antigen-binding sites: Regions on antibodies that recognize specific antigen determinants.

• Antigen determinants (epitopes): Specific regions of antigens recognized by antibodies.

Clonal Selection and Immunological Memory

Clonal selection is the process by which lymphocytes that recognize an antigen proliferate and differentiate.

• Activated lymphocytes form effector cells (fight infection) and memory cells (provide long-term immunity).

• Upon re-exposure, memory cells mount a rapid and robust secondary immune response.

B Cells and Humoral Immunity

B cells differentiate into plasma cells that secrete antibodies. The lifespan of plasma cells is short, but memory B cells persist.

• Antibodies neutralize toxins, agglutinate pathogens, and activate complement proteins.

Monoclonal Antibodies

Monoclonal antibodies are laboratory-produced molecules that bind to specific antigens. They are used in diagnostics and cancer therapy.

• Produced by fusing B cells with tumor cells to create hybridomas.

• Used to target specific cells, such as cancer cells.

T Cells and Cell-Mediated Immunity

T cells recognize antigens presented by antigen-presenting cells (APCs) via major histocompatibility complex (MHC) molecules.

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

• Cytotoxic T cells: Destroy infected or cancerous cells.

Major Histocompatibility Complex (MHC)

• Class I MHC: Found on almost all nucleated cells; present antigens to cytotoxic T cells.

• Class II MHC: Found on specialized immune cells; present antigens to helper T cells.

Immune System and Cancer

Cytotoxic T cells may help prevent cancer by attacking cells with abnormal surface molecules.

• Immunotherapy uses immune cells or antibodies to target cancer cells.

Immune System Disorders

Malfunctions of the immune system can lead to disease.

• Autoimmune diseases: The immune system attacks the body's own cells (e.g., multiple sclerosis, rheumatoid arthritis).

• Immunodeficiency diseases: The immune system is weakened (e.g., Severe Combined Immunodeficiency, AIDS).

• Allergies: Overreaction to harmless substances (allergens).

• Physical and emotional stress can weaken immune responses.

AIDS and Immune System Failure

AIDS is caused by the HIV virus, which attacks helper T cells, crippling both cell-mediated and humoral immunity.

• No cure yet, but drugs and vaccines offer hope.

• Prevention includes practicing safer sex.

Herd Immunity

Herd immunity prevents the outbreak of infectious diseases by reducing the number of susceptible individuals in a population.

• Achieved through widespread vaccination.

Summary Table: Types of Immunity

Type	Mechanism	Key Cells	Example
Innate Immunity	Non-specific, immediate response	Phagocytes, NK cells	Inflammation, skin barrier
Adaptive Immunity	Specific, slower response, memory	B cells, T cells	Antibody production, cytotoxic T cell response
Passive Immunity	Transfer of antibodies	Antibodies	Maternal antibodies to infant

Key Equations and Concepts

• Antibody-Antigen Binding:

$\text{Antibody} + \text{Antigen} \rightarrow \text{Immune Complex}$

• Clonal Selection:

$\text{Activated Lymphocyte} \xrightarrow{\text{Proliferation}} \text{Effector Cells} + \text{Memory Cells}$

• Herd Immunity Threshold:

$\text{Herd Immunity Threshold} = 1 - \frac{1}{R_0}$

where $R_0$ is the basic reproduction number of the pathogen.

Additional info:

• Immunotherapy and monoclonal antibodies are rapidly advancing fields in medicine, offering new treatments for cancer and autoimmune diseases.

• Understanding the molecular basis of immunity is essential for vaccine development and disease prevention.