Chapter 22: The Immune System – Structure, Function, and Defense Mechanisms

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Immune System Overview

Introduction to the Immune System

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against infectious agents and disease. It is divided into two main branches: the innate immune system and the adaptive immune system. These systems collaborate to identify, target, and eliminate pathogens while maintaining tolerance to self-antigens.

Host defense system: Protects against infectious agents (pathogens) and disease.
Pathogens: Organisms that cause damage or death to the host. Major types include bacteria, viruses, fungi, protozoans, and multicellular parasites.

Types of Infectious Agents

Bacteria, Viruses, Fungi, Protozoans, and Multicellular Parasites

Pathogens are classified based on their structure and life cycle. Understanding their differences is crucial for targeted immune responses and treatments.

Bacteria: Single-celled prokaryotes with cell walls. Easily targeted by antibiotics due to differences from human cells.
Viruses: Acellular entities that require host cells to replicate. They hijack cellular machinery, often killing the host cell upon exit.
Fungi: Eukaryotic organisms that can cause superficial or systemic infections.
Protozoans: Single-celled eukaryotes, often causing diseases like malaria.
Multicellular parasites: Complex organisms such as worms that live and feed on hosts.

Comparison of prokaryotic and eukaryotic cells Viral replication cycle in a host cell Microscopic images of bacteria, viruses, fungi, protozoans, and worms

Examples of Diseases Caused by Infectious Agents

Bacterial: Strep throat, tuberculosis, tetanus, salmonella
Viral: Influenza, common cold, HIV/AIDS, coronavirus, mononucleosis
Fungal: Ringworm, athlete’s foot, yeast infections
Protozoan: Malaria, amoebic dysentery
Parasitic: Hookworms, tapeworms, pinworms

Immune System Organization

Distribution of Immune Cells

Immune cells are distributed throughout the body, residing in blood, secondary lymphatic structures, connective tissues, and organs exposed to the environment (e.g., lungs, skin, mucosal membranes).

Distribution of immune cells in the human body

Innate Immune System

First Line of Defense: Physical and Chemical Barriers

The first line of defense prevents entry of pathogens through physical and chemical barriers.

Skin: Acts as a physical barrier; secretes antimicrobial substances (proteins, enzymes, sebum).
Mucous membranes: Line body openings, produce mucus, and secrete antimicrobial enzymes and IgA.
Normal flora: Nonpathogenic microorganisms that outcompete pathogens for resources.

Second Line of Defense: Cellular and Chemical Responses

If pathogens bypass the first line, the second line of defense involves various immune cells and chemicals.

Phagocytic cells: Neutrophils, macrophages, and dendritic cells engulf and destroy pathogens.
Proinflammatory cells: Basophils and mast cells release chemicals (histamine, heparin, eicosanoids) to promote inflammation.
Apoptosis-inducing cells: Natural killer (NK) cells induce apoptosis in infected or abnormal cells using perforin and granzymes.
Parasite-destroying cells: Eosinophils release cytotoxic chemicals to kill parasites.
Complement system: Plasma proteins that opsonize pathogens, recruit immune cells, induce cytolysis, and link antibodies to red blood cells for clearance.
Interferons: Proteins that inhibit viral replication and recruit NK cells.
Fever: Increases metabolic rate and tissue repair, but high fevers can be dangerous.
Inflammation: Localized response characterized by redness, heat, swelling, and pain. Involves four steps: release of factors, vascular changes, recruitment of immune cells, and delivery of plasma proteins.

Phagocytic cells: neutrophil, macrophage, dendritic cell Dendritic cell presenting antigen to T cell Proinflammatory chemical-secreting cells: basophil and mast cell Apoptosis-initiating cells: NK cell Parasite-destroying cells: eosinophils Complement system actions: opsonization, inflammation, cytolysis, elimination Antiviral action of interferon Step 1 of inflammation: release of inflammatory and chemotactic factors Step 2 of inflammation: vascular changes Step 3 of inflammation: recruitment of immune cells Step 4 of inflammation: delivery of plasma proteins

Adaptive Immune System

Overview and Key Features

The adaptive immune system provides a specific response to pathogens, developing memory for faster responses upon re-exposure. It consists of two main branches: cell-mediated immunity (T-cells) and humoral immunity (B-cells and antibodies).

Adaptive immunity: cell-mediated and humoral branches

Antigens and Lymphocyte Receptors

Antigen: A molecule capable of inducing an immune response. Pathogens have unique antigens, often proteins or polysaccharides.
Epitope: The specific part of an antigen recognized by a lymphocyte receptor.
Immunogen: An antigen that elicits an immune response.
T-cell Receptor (TCR): Found on T-cells, recognizes antigen fragments presented by MHC molecules.
B-cell Receptor (BCR): Found on B-cells, binds directly to antigens.

B-cell and T-cell receptors

Lymphocyte Diversity and Clonal Selection

Random DNA rearrangement in developing lymphocytes creates a vast pool of unique TCRs and BCRs, enabling recognition of diverse antigens. Upon activation by a specific antigen, lymphocytes undergo clonal selection and proliferation.

Gene rearrangement in B-cells for antibody diversity

T-Cells: Helper and Cytotoxic Subtypes

Helper T-cells (CD4+): Activate B-cells, cytotoxic T-cells, and innate immune cells by secreting interleukins.
Cytotoxic T-cells (CD8+): Kill virus-infected and cancer cells by releasing perforin and granzymes.

Helper and cytotoxic T-lymphocytes

Antigen Presentation and MHC Molecules

Antigen-presenting cells (APCs): Dendritic cells, macrophages, and B-cells present antigens to T-cells using MHC molecules.
MHC Class I: Found on all nucleated cells; presents to CD8+ T-cells.
MHC Class II: Found on APCs; presents to CD4+ T-cells.

Antigen presentation by MHC molecules to T-cells

B-Cells and Antibody Production

B-cells recognize antigens, present them to helper T-cells, and differentiate into plasma cells that secrete antibodies. Antibodies neutralize pathogens, opsonize for phagocytosis, and activate complement.

Plasma cells: Specialized B-cells that produce large quantities of antibodies.
Memory B-cells: Remain in the body for rapid response upon re-exposure to the antigen.

Classes of Antibodies (Immunoglobulins)

IgG: Most abundant, crosses placenta, found in blood and body fluids.
IgM: First antibody produced, found in blood and breast milk.
IgA: Found in external secretions (tears, saliva, breast milk).
IgD: Functions as a B-cell receptor.
IgE: Involved in allergic reactions and defense against parasites.

Immunological Memory and Vaccination

Memory B-cells and T-cells enable a faster and stronger response upon subsequent exposure to the same pathogen. Vaccines exploit this principle to provide long-term protection.

Types of Immunity

Active immunity: Results from direct exposure to antigen (infection or vaccination); long-lasting.
Passive immunity: Results from transfer of antibodies (e.g., maternal antibodies, antivenom); short-term.

T-Cell Maturation and Selection

Positive selection: T-cells that bind to MHC molecules survive.
Negative selection: T-cells that do not react to self-antigens survive, preventing autoimmunity.
Only about 2% of T-cells survive both selection processes.

Clinical Correlations

Anaphylaxis: Severe allergic reaction treated with antihistamines, steroids, and epinephrine.
HIV/AIDS: HIV infects and destroys T-cells, leading to immunodeficiency.
Cancer Immunotherapy: PD-1/PD-L1 pathway manipulation allows T-cells to target cancer cells more effectively.