Immune System Study Guide: Structure and Function

The Immune System

Features, Lines, Goals, and Pathogens

The immune system is a complex network of cells, tissues, and organs that defends the body against pathogens such as bacteria, viruses, fungi, and parasites. Its primary goal is to recognize and eliminate foreign invaders while distinguishing them from the body's own cells.

• Lines of Defense: The immune system is organized into three main lines of defense: first, second, and third.

• Pathogens: Disease-causing agents include bacteria, viruses, fungi, and parasites.

• Goals: Prevent infection, neutralize threats, and maintain homeostasis.

Adaptive vs Innate Immunity

• Innate Immunity: Non-specific, immediate response; includes physical barriers and general cellular responses.

• Adaptive Immunity: Specific, slower response; involves recognition of specific antigens and memory formation.

• Example: Skin is an innate barrier; antibodies are adaptive responses.

First Line Defenses

Components, Characteristics, and Features

The first line of defense consists of physical and chemical barriers that prevent pathogens from entering the body.

• Physical Barriers: Skin, mucous membranes.

• Chemical Barriers: Lysozyme in saliva, acidic pH in stomach, antimicrobial peptides.

• Features: Non-specific, always present, rapid action.

• Example: Tears contain lysozyme, which destroys bacteria.

Second Line Defenses

Phagocytes: Types, Mechanisms, Opsonization

Phagocytes are cells that engulf and digest pathogens. Opsonization enhances phagocytosis by marking pathogens for destruction.

• Types: Neutrophils, macrophages, dendritic cells.

• Mechanisms: Recognition, engulfment, digestion.

• Opsonization: Antibodies or complement proteins coat pathogens, making them easier for phagocytes to recognize.

• Example: Macrophages ingest bacteria after opsonization by antibodies.

Cellular Defenses: Respiratory Burst, Actions of Leukocytes

• Respiratory Burst: Rapid release of reactive oxygen species by phagocytes to destroy pathogens.

• Leukocytes: Basophils release histamine; Natural Killer (NK) cells destroy infected or abnormal cells.

• Example: NK cells induce apoptosis in virus-infected cells.

Inflammation and Fever: Induction, Signs, Benefits

• Induction: Triggered by tissue injury or infection.

• Signs: Redness, heat, swelling, pain.

• Benefits: Increases blood flow, recruits immune cells, promotes healing.

• Fever: Elevated body temperature inhibits pathogen growth and enhances immune activity.

Antimicrobial Proteins: Interferons and Complement System

• Interferons: Proteins released by virus-infected cells to warn neighboring cells and inhibit viral replication.

• Complement System: A group of proteins that enhance immune responses via four main branches: classical, alternative, lectin, and terminal pathways.

• Process Names: Activation, opsonization, cell lysis, inflammation.

• Cells/Proteins Involved: C3, C5, MAC (membrane attack complex).

• Effects: Opsonization, chemotaxis, cell lysis.

• Basic Activation: Complement proteins are activated in a cascade, leading to pathogen destruction.

• Notation: C3a, C3b, C5a, etc.

Branch	Activation Trigger	Main Effect
Classical	Antibody binding	Opsonization, lysis
Alternative	Pathogen surface	Opsonization, lysis
Lectin	Mannose-binding lectin	Opsonization, lysis
Terminal	Final cascade	Membrane attack complex (MAC)

Third Line Defense

Humoral vs Cell-Mediated Immunity

The third line of defense is specific and adaptive, involving lymphocytes and antigen recognition.

• Humoral Immunity: Mediated by B cells and antibodies; targets extracellular pathogens.

• Cell-Mediated Immunity: Mediated by T cells; targets intracellular pathogens and abnormal cells.

• Example: Antibody neutralization (humoral); cytotoxic T cell killing (cell-mediated).

Antigens, Antibodies, and Epitopes

• Antigen: Any substance that triggers an immune response.

• Antibody: Protein produced by B cells that binds to specific antigens.

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

Characteristics of Key Players

• B Cells: Produce antibodies; mature in bone marrow.

• T Cells: Include helper (CD4+) and cytotoxic (CD8+) types; mature in thymus.

• APCs (Antigen Presenting Cells): Present antigens to T cells; include dendritic cells, macrophages, B cells.

• MHC Proteins: Major histocompatibility complex; present antigens on cell surfaces.

Lymphocytes: Characteristics, Types

• Lymphocytes: White blood cells central to adaptive immunity.

• Types: B cells, T cells, NK cells.

• Characteristics: Specificity, memory, diversity.

RAM: Generic vs Humoral vs Cell-Mediated

• RAM: Recognition, Activation, Memory.

• Generic: Applies to all adaptive responses.

• Humoral: B cell activation, antibody production.

• Cell-Mediated: T cell activation, cytotoxic response.

B Cell Activation - RAM

• Recognition: B cell binds antigen.

• Activation: Helper T cell stimulates B cell.

• Memory: Formation of memory B cells.

Classes of Antibodies

• IgG: Most abundant, crosses placenta.

• IgM: First produced, pentamer structure.

• IgA: Found in mucosal areas.

• IgE: Involved in allergies and parasitic infections.

• IgD: Functions mainly as a B cell receptor.

Class	Main Function	Location
IgG	Long-term immunity	Blood, tissues
IgM	Primary response	Blood
IgA	Mucosal protection	Secretions
IgE	Allergy, parasites	Blood
IgD	B cell receptor	Surface of B cells

T Cell Activation - RAM

• Recognition: T cell binds antigen presented by APC via MHC.

• Activation: Co-stimulation by APC.

• Memory: Formation of memory T cells.

Mechanisms of Antibody Attack

• Neutralization: Antibodies block pathogen activity.

• Agglutination: Antibodies clump pathogens together.

• Opsonization: Antibodies mark pathogens for phagocytosis.

• Complement Activation: Antibodies trigger complement cascade.

Primary vs Secondary Responses and Memory Cells

• Primary Response: First exposure; slower, lower antibody levels.

• Secondary Response: Subsequent exposure; faster, higher antibody levels due to memory cells.

• Memory Cells: Long-lived B and T cells that respond rapidly upon re-exposure.

Types of Immunity, Hypersensitivities, and Autoimmune Disease

Types of Immunity

• Active Immunity: Acquired through exposure to antigen or vaccination.

• Passive Immunity: Acquired through transfer of antibodies (e.g., maternal antibodies).

Hypersensitivities

• Type I: Immediate (allergy, IgE-mediated).

• Type II: Cytotoxic (antibody-mediated).

• Type III: Immune complex-mediated.

• Type IV: Delayed (cell-mediated).

Autoimmune Disease

• Definition: Immune system attacks self-tissues.

• Examples: Type 1 diabetes, rheumatoid arthritis, lupus.

Lymphatic System

Overview and Functions

The lymphatic system supports immune function by transporting lymph, housing immune cells, and filtering pathogens.

• Components: Lymph nodes, lymph vessels, spleen, thymus, tonsils.

• Functions: Fluid balance, immune surveillance, absorption of fats.

• Example: Lymph nodes filter pathogens and activate lymphocytes.

Additional info: The immune system and lymphatic system are closely integrated, with lymphatic organs serving as sites for immune cell maturation and activation.