Chapter 27: The Immune System

Learning Objectives

• LO1: Identify the organs of the immune system and describe two main types of invertebrate innate immunity.

• LO2: Describe the structure and function of the vertebrate immune system.

• LO3: Explain the steps of the inflammatory response.

• LO4: Describe the organs, structure, and function of adaptive immunity.

• LO5: Discuss adaptive immunity, including B cells, T cells, development, maturation, clonal selection, and naïve vs. activated cells.

• LO6: Compare primary and secondary immune responses in terms of strength and duration.

Types of Pathogens

Overview of Pathogens

The immune system protects the body from a wide variety of foreign, "non-self" pathogens that can cause disease. Pathogens include bacteria, fungi, protists, and viruses.

• Bacteria: Single-celled prokaryotic organisms that can cause infections such as strep throat and tuberculosis.

• Fungi: Eukaryotic organisms, including molds and yeasts, responsible for diseases like athlete's foot and ringworm.

• Parasitic Protists: Single-celled eukaryotes such as Plasmodium (malaria) and Trypanosoma (sleeping sickness).

• Parasitic Animals: Multicellular organisms such as worms (helminths) that can infect the body.

• Viruses: Non-cellular infectious agents that require host cells to replicate, causing diseases like influenza and HIV/AIDS.

Divisions of the Immune System

Layered Defenses

The immune system is organized into multiple layers of defense to block entry and eliminate invaders.

• Innate Immunity: Non-specific, immediate defense mechanisms present from birth. Includes physical barriers and cellular responses.

• Adaptive Immunity: Specific, acquired defense mechanisms that develop after exposure to pathogens. Provides long-term protection and immunological memory.

Innate Immunity

Physical and Chemical Barriers

Physical and chemical barriers are the first line of defense against pathogens.

• Skin (Integumentary System): Nearly impenetrable barrier preventing pathogen entry.

• Mucous Membranes: Line respiratory, digestive, and urogenital tracts; secrete mucus to trap pathogens.

• Cilia: Hair-like extensions in the respiratory tract that move trapped particles out of the body.

• Secretions: Sweat, tears, and saliva contain enzymes (e.g., lysozyme) that destroy pathogens.

Cellular Defenses

• Phagocytic Cells: White blood cells (e.g., macrophages, neutrophils) that engulf and digest pathogens.

• Natural Killer (NK) Cells: Destroy infected or cancerous cells by inducing apoptosis.

• Complement Proteins: A group of proteins that enhance phagocytosis, lyse pathogens, and recruit immune cells.

Inflammatory Response

Steps of the Inflammatory Response

The inflammatory response is a non-specific reaction to tissue damage or infection, characterized by redness, heat, swelling, and pain.

1. Recognition: Damaged cells release chemical signals (e.g., histamine).

2. Vasodilation: Blood vessels widen, increasing blood flow to the area.

3. Recruitment: Phagocytes and other immune cells migrate to the site of infection.

4. Phagocytosis: Immune cells engulf and destroy pathogens.

5. Resolution: Tissue repair and return to homeostasis.

Fever: Cytokines released during inflammation can induce fever, which helps inhibit pathogen growth.

Adaptive Immunity

Organs and Cells of Adaptive Immunity

Adaptive immunity involves specialized organs and cells that recognize and remember specific pathogens.

• Lymphoid Organs: Bone marrow (site of B cell development), thymus (site of T cell maturation), spleen, and lymph nodes.

• Lymphocytes: White blood cells responsible for adaptive responses. Two main types: B cells and T cells.

B Cells and T Cells

• B Cells: Responsible for humoral immunity; produce antibodies that neutralize pathogens in body fluids.

• T Cells: Responsible for cell-mediated immunity; include helper T cells (activate other immune cells) and cytotoxic T cells (destroy infected cells).

Development and Maturation

• Clonal Selection: Upon encountering an antigen, specific lymphocytes proliferate and differentiate into effector and memory cells.

• Naïve Cells: Lymphocytes that have not yet encountered their specific antigen.

• Activated Cells: Lymphocytes that have recognized an antigen and initiated an immune response.

Antibodies

Structure and Function

Antibodies are Y-shaped proteins produced by B cells that specifically bind to antigens.

• Variable Region: Binds to specific antigens.

• Constant Region: Determines the class of antibody and mediates effector functions.

Functions of Antibodies:

• Neutralize pathogens by binding to them.

• Opsonize pathogens, enhancing phagocytosis.

• Activate complement proteins, leading to pathogen lysis.

Primary and Secondary Immune Responses

Comparison of Responses

Immune System Disorders

Autoimmune Diseases

Occur when the immune system attacks the body's own tissues. Example: Type 1 diabetes (destruction of insulin-producing cells).

Allergies

An inappropriate immune response to harmless substances (allergens), resulting in symptoms such as inflammation and swelling.

Key Terms and Concepts

• Pathogen: Disease-causing organism or agent.

• Antigen: Molecule recognized as foreign by the immune system.

• Phagocytosis: Process by which cells engulf and digest particles.

• Apoptosis: Programmed cell death.

• Immunological Memory: Ability of the immune system to respond more rapidly and effectively to pathogens it has encountered before.

Important Equations and Processes

• Clonal Selection Equation:

• Complement Activation:

Additional info: The notes have been expanded to provide context and definitions for key terms, as well as to clarify the steps and mechanisms of immune responses. The table comparing primary and secondary immune responses is inferred from standard immunology knowledge.