Chapter 21: The Immune System – Innate and Adaptive Body Defenses

The Immune System: Overview

Introduction to Immunity

The immune system is the body's defense mechanism against disease-causing agents. It is divided into two main branches: innate (nonspecific) defenses and adaptive (specific) defenses. These systems work together to protect the body from pathogens and maintain health.

• Innate defenses: Provide immediate, general protection against pathogens.

• Adaptive defenses: Provide specific, targeted responses and develop memory for future protection.

Innate (Nonspecific) Defense System

Surface Barriers

Surface barriers are the body's first line of defense, preventing the entry of pathogens through physical and chemical means.

• Skin: Acts as a mechanical barrier to block pathogen entry.

• Mucous membranes: Trap microbes and contain chemicals like mucus, acidity, and lysozyme that inhibit pathogen growth.

Internal Defenses

If pathogens breach surface barriers, internal defenses are activated. These include cellular and chemical mechanisms.

• Phagocytes: Engulf and destroy pathogens.

• Natural Killer (NK) cells: Attack abnormal cells lacking "self" markers.

• Inflammation: Local response to injury or infection.

• Antimicrobial proteins: Interferons and complement proteins that attack microbes or hinder their reproduction.

• Fever: Systemic response that increases body temperature to inhibit pathogen growth.

Phagocytes

Phagocytes are immune cells that ingest and digest foreign invaders. The main types are:

• Macrophages: Derived from monocytes; chief phagocytic cells in tissues.

• Neutrophils: Become phagocytic upon encountering infectious material.

Natural Killer (NK) Cells

NK cells are large granular lymphocytes that target cells lacking "self" markers, such as virus-infected or cancerous cells. They induce apoptosis and secrete chemicals to enhance inflammation.

Antimicrobial Proteins

These proteins attack microbes or hinder their ability to reproduce.

• Interferons: Released by virus-infected cells to warn neighboring cells and activate immune responses.

• Complement proteins: A group of ~20 plasma proteins that enhance inflammation, promote phagocytosis, and cause cell lysis.

Inflammation

Inflammation is triggered by tissue injury or infection. It prevents the spread of damaging agents, disposes of debris, and sets the stage for repair.

• Cardinal signs: Redness, heat, swelling, pain.

• Mechanisms: Vasodilation, increased vascular permeability, and recruitment of immune cells.

Fever

Fever is a systemic response to infection. Pyrogens reset the hypothalamic thermostat, raising body temperature.

• Benefits: Sequesters iron and zinc, increases metabolic rate, and speeds up tissue repair.

• Risks: High fever can denature enzymes and be dangerous.

Adaptive (Specific) Defense System

Overview of Adaptive Immunity

The adaptive immune system is specific, systemic, and has memory. It consists of two main arms:

• Humoral (antibody-mediated) immunity: Involves B cells and antibodies targeting extracellular pathogens.

• Cellular (cell-mediated) immunity: Involves T cells targeting infected or abnormal cells.

Cells of the Adaptive Immune System

• Lymphocytes: Originate in red bone marrow. B cells mature in bone marrow; T cells mature in the thymus.

• Immunocompetence: Ability to recognize specific antigens and tolerate self-antigens.

Development of Self-Tolerance

T cells undergo positive and negative selection in the thymus to ensure they recognize self-MHC but do not react to self-antigens, preventing autoimmunity.

Humoral Immunity

Humoral immunity is mediated by B cells, which produce antibodies to neutralize extracellular antigens.

• B cells: Differentiate into plasma cells (antibody-secreting) and memory B cells.

• Antibodies: Proteins that bind specifically to antigens.

Antigen Challenge and Clonal Selection

The first encounter between a B cell and an antigen triggers clonal selection, producing plasma cells and memory cells.

Immunological Memory

• Primary response: First exposure, slower and lower antibody production.

• Secondary response: Faster and stronger due to memory cells.

Types of Humoral Immunity

Type	How Acquired	Example
Active, Naturally Acquired	Infection; contact with pathogen	Chickenpox
Active, Artificially Acquired	Vaccine; dead or attenuated pathogens	Flu shot
Passive, Naturally Acquired	Antibodies from mother to fetus/infant	Placenta, breast milk
Passive, Artificially Acquired	Injection of immune serum (gamma globulin)	Antivenom

Antibody Structure and Classes

Antibodies (immunoglobulins) have two heavy and two light chains, with variable regions forming antigen-binding sites. The constant region determines the class and function.

Class	Structure	Main Function
IgM	Pentamer	First antibody released; agglutination; complement activation
IgA	Monomer/dimer	In mucus/secretions; prevents pathogen entry
IgD	Monomer	B cell receptor
IgG	Monomer	Main antibody in plasma; crosses placenta
IgE	Monomer	Allergies, parasitic infections; triggers histamine release

Actions of Antibodies

• Neutralization: Block harmful effects of toxins/viruses.

• Agglutination: Clump cell-bound antigens.

• Precipitation: Cross-link soluble antigens for phagocytosis.

• Complement fixation: Trigger cell lysis via complement system.

Cell-Mediated Immunity

Antigen-Presenting Cells (APCs)

APCs engulf antigens and present fragments to T cells. Major types include dendritic cells, macrophages, and B cells.

T Cells and Their Functions

• CD4 cells: Become helper T cells (TH) or regulatory T cells (TREG).

• CD8 cells: Become cytotoxic T cells (TC) that kill infected or abnormal cells.

• Memory T cells: Provide rapid response upon re-exposure.

T Cell Activation and Antigen Recognition

T cells are activated when their receptors bind to antigens presented by MHC proteins on APCs.

• Class I MHC: Present on all cells; activate CD8 cells.

• Class II MHC: Present on APCs; activate CD4 cells.

Actions of Helper T Cells

Helper T cells coordinate the immune response by activating B cells, T cells, and macrophages. Without them, the adaptive immune response is severely impaired.

Actions of Cytotoxic T Cells

Cytotoxic T cells directly attack and kill infected or abnormal cells by releasing perforin and granzymes, inducing apoptosis.

Actions of Regulatory T Cells

Regulatory T cells suppress immune responses to prevent autoimmunity.

Clinical Aspects and Homeostatic Imbalances

Organ Transplants and Rejection

• Autografts: Same person

• Isografts: Identical twins

• Allografts: Same species, different genetics

• Xenografts: Different species

Immunosuppressive therapy is required to prevent rejection, but increases risk of infection.

Immunodeficiencies

Conditions where immune function is impaired. Example: AIDS (caused by HIV) destroys helper T cells, leading to severe infections.

Autoimmune Diseases

The immune system attacks self-tissues. Examples: Multiple sclerosis, Type I diabetes, lupus, rheumatoid arthritis.

Hypersensitivities

Excessive immune responses to harmless substances (allergens). Acute reactions can cause anaphylactic shock, which is life-threatening.

Additional info: This guide covers the main concepts, cells, and mechanisms of the immune system as outlined in Chapter 21 of a typical Anatomy & Physiology II course. It includes both innate and adaptive defenses, the roles of various immune cells, antibody structure and function, and clinical relevance such as immunodeficiencies and hypersensitivities.