Chapter 34: Resistance of the Body to Infection: II. Immunity and Allergy Innate Immunity

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 34

To read more check out Chapter 17:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-17-local-and-humoral-control-of.html

Introduction to Immunity

The human body possesses a remarkable internal defense system designed to resist nearly all harmful organisms, toxins, and foreign agents that threaten tissue and organ function. This overall defense capability is broadly categorized into two major mechanisms: innate immunity and acquired (adaptive) immunity. While innate immunity provides non-specific physical, chemical, and cellular barriers right from birth, acquired immunity offers a highly specialized, targeted response that develops upon exposure to specific foreign substances.

How these immune pathways originate, differentiate, and interact is fundamental to human physiology and clinical medicine. The study of adaptive mechanisms encompasses humoral immunity (mediated by B-cell antibodies) and cell-mediated immunity (driven by functional T cells), as well as the regulation, tolerance, and clinical conditions like hypersensitivities and autoimmune disorders.

Innate Immunity

Innate immunity represents the body's general, non-specific defense mechanisms present before exposure to specific pathogens. It acts as the first line of defense against biological invaders and toxic substances.

• Resists broad types of damaging invaders.
• Does not require prior exposure.
• Involves white blood cell phagocytosis.
• Includes tissue macrophage system responses.
• Destroys swallowed pathogens using stomach acid.
• Relies on digestive enzyme activity.
• Uses skin as physical barrier.
• Contains blood chemical defense compounds.
• Utilizes lysozyme to dissolve bacteria.
• Uses basic polypeptides against gram-positive bacteria.
• Employs complement complex protein system.
• Uses natural killer cells against tumors.
• Protects humans from animal viral infections.

Acquired (Adaptive) Immunity

Acquired immunity is a highly specific defense mechanism that develops after the body encounters a pathogen or toxin for the first time. It forms targeted antibodies or activated lymphocytes to eliminate individual foreign agents.

• Develops extremely powerful specific protection.
• Initiates only after antigen invasion.
• Requires weeks or months to form.
• Protects against high toxin doses.
• Forms basis for clinical immunization.
• Produces specific target antibodies.
• Generates specifically activated lymphocytes.

Basic Types of Acquired Immunity: Humoral and Cell-Mediated

Acquired immunity consists of two distinct but complementary branches. Humoral immunity relies on plasma antibodies, while cell-mediated immunity depends on direct cellular action by activated T lymphocytes.

• Divided into two major branches.
• Humoral immunity uses circulating antibodies.
• B lymphocytes mediate humoral immunity.
• Antibodies are blood plasma globulins.
• Cell-mediated immunity uses T lymphocytes.
• T cells activate in lymph nodes.
• Both branches form in lymphoid tissues.

Initiation of Immunity by Antigens

Acquired immune responses are triggered when the body recognizes foreign molecular structures known as antigens. Specific molecular regions called epitopes determine antigenicity.

• Initiated exclusively by specific antigens.
• Antigens are foreign proteins or polysaccharides.
• Requires high molecular weight above 8000.
• Depends on surface epitope groups.
• Stereochemical structures trigger recognition.

Role and Preprocessing of Lymphocytes

Lymphocytes are essential cells responsible for acquired immunity. Derived from embryonic stem cells, they undergo preprocessing in specialized organs to become functional T cells or B cells.

• Lymphocytes are essential for survival.
• Concentrated heavily in lymph nodes.
• Located in spleen and gastrointestinal mucosa.
• Present in thymus and bone marrow.
• Derived from pluripotent hematopoietic stem cells.
• Forms common lymphoid progenitor cells.
• T cells preprocess in thymus gland.
• B cells preprocess in fetal liver.
• B cells preprocess in bone marrow.
• Named after avian bursa of Fabricius.

Detailed Preprocessing of T and B Lymphocytes

During preprocessing, lymphocytes develop antigen specificities and undergo rigorous selection to prevent self-reactivity and autoimmune destruction.

• T cells migrate to thymus gland.
• Rapidly divide inside thymus gland.
• Develop extreme diversity for specific antigens.
• Thousands of distinct T-cell types form.
• Thymus selects against self-reactive T cells.
• Destroys up to ninety percent self-reactive cells.
• Preprocessing completes around birth period.
• Preprocessed T cells populate lymphoid tissues.
• B cells secrete reactive antibody molecules.
• B cells exhibit vast antibody diversity.

Clonal Selection and Activation Mechanics

The body maintains stored collections of preformed lymphocytes called clones. Contact with matching antigens triggers rapid clonal expansion and specific immune activation.

• Lymphoid tissue stores preformed clones.
• Each clone recognizes one specific antigen.
• Gene segment combinations generate diversity.
• Gene mixing creates whole functional genes.
• B cells possess cell surface antibodies.
• T cells express surface receptor proteins.
• Antigens selectively bind complementary receptors.
• Macrophages phagocytize and digest invaders.
• Macrophages present antigens to lymphocytes.
• Macrophages secrete interleukin-1 to stimulate cells.
• Helper T cells release activating lymphokines.

B-Lymphocyte System and Humoral Immunity

Activated B lymphocytes differentiate into antibody-secreting plasma cells or long-lived memory cells that confer long-term protection against recurring infections.

• Antigen exposure activates dormant B clones.
• Activated B cells form lymphoblasts.
• Lymphoblasts differentiate into plasmablasts.
• Rough endoplasmic reticulum proliferates rapidly.
• Plasmablasts in divide every ten hours.
• Mature plasma cells secrete gamma globulins.
• Produces two thousand antibodies per second.
• Memory B cells remain dormant temporarily.
• Memory cells enhance secondary responses.
• Secondary response acts faster and stronger.

Structure and Action of Antibodies

Antibodies are specialized immunoglobulins composed of light and heavy polypeptide chains. They bind antigens tightly and execute protective functions directly or via the complement system.

• Immunoglobulins constitute major plasma proteins.
• Formed by light and heavy chains.
• Bound together by disulfide bonds.
• Contains variable antigen-binding portions.
• Contains constant functional portions.
• Variable regions bind via specific steric fit.
• Binding involves hydrogen and ionic bonds.
• Five classes: IgM, IgG, IgA, IgD, IgE.
• IgG forms primary circulating bivalent antibody.
• IgM dominates early primary responses.
• IgE participates in allergic reactions.
• Direct action causes antigen agglutination.
• Direct action induces molecular precipitation.
• Direct action causes toxin neutralization.
• Direct action induces membrane lysis.

The Complement System

The complement system is a cascade of plasma proteins that dramatically amplifies antibody-mediated defenses against cellular invaders.

• Composed of twenty circulating proteins.
• Main pathway triggered by antigen-antibody complex.
• Classic pathway begins with C1 activation.
• Sequential enzyme cascade amplifies reaction.
• C3b causes opsonization and phagocytosis.
• C5b6789 complex causes cell lysis.
• Complement factors promote antigen agglutination.
• Neutralizes viral structural integrity.
• C5a fragment induces white cell chemotaxis.
• C3a, C4a, C5a activate mast cells.
• Triggers localized inflammatory tissue responses.

T-Lymphocyte System and Cell-Mediated Immunity

Activated T lymphocytes provide direct cellular defenses. They recognize degraded antigen peptides displayed alongside major histocompatibility complex (MHC) proteins on antigen-presenting cells.

• Activated T cells enter circulation continuously.
• Recirculate between blood and tissue fluids.
• Memory T cells store clone specificity.
• Requires antigen-presenting cells for activation.
• Dendritic cells present antigens most potently.
• Macrophages and B cells present antigens.
• MHC I presents to cytotoxic T cells.
• MHC II presents to helper T cells.
• T cells express specific surface receptors.

Types of T Cells and Regulatory Functions

T lymphocytes consist of distinct functional subtypes: helper T cells, cytotoxic T cells, and suppressor (regulatory) T cells.

• Helper T cells constitute major population.
• Helper T cells regulate overall immunity.
• Helper cells secrete functional lymphokine mediators.
• Interleukins stimulate B and T growth.
• AIDS virus destroys helper T cells.
• Cytotoxic T cells directly attack invaders.
• Cytotoxic cells release hole-forming perforins.
• Cytotoxic cells deliver lethal cell toxins.
• Cytotoxic cells destroy virus-infected cells.
• Cytotoxic cells kill foreign cancer cells.
• Suppressor T cells inhibit excessive responses.
• Suppressor cells maintain internal immune tolerance.

Immune Tolerance and Autoimmunity

Immune tolerance prevents the body from launching destructive immune attacks against its own tissues. Failure of tolerance mechanisms results in autoimmune pathology.

• Tolerance prevents immune self-destruction.
• Developed mainly during thymic preprocessing.
• Immature self-reactive cells undergo destruction.
• Loss of tolerance causes autoimmune disease.
• Rheumatic fever damages heart valves.
• Glomerulonephritis damages kidney membranes.
• Myasthenia gravis attacks acetylcholine receptors.
• Lupus erythematosus causes widespread tissue damage.

Artificial Immunization and Passive Immunity

Immunity can be induced artificially through active immunization with modified antigens or acquired temporarily via passive transfer of preformed antibodies or lymphocytes.

• Active immunity uses dead pathogenic organisms.
• Active immunity uses chemically detoxified toxins.
• Active immunity uses attenuated live viruses.
• Passive immunity infuses preformed antibodies.
• Passive immunity transfers sensitized T lymphocytes.
• Passive protection lasts several weeks.

Allergy and Hypersensitivity Reactions

Allergies represent harmful immune hypersensitivity reactions mediated by either activated T cells (delayed-type) or excessive IgE antibody activity (atopic/immediate-type).

• Delayed-reaction allergy mediated by T cells.
• Poison ivy causes delayed cell-mediated damage.
• Atopic allergies involve excess IgE antibodies.
• IgE binds strongly to mast cells.
• IgE binds strongly to basophils.
• Allergen binding causes cell rupture.
• Releases histamine and toxic leukotrienes.
• Anaphylaxis causes systemic circulatory shock.
• Slow-reacting substance causes bronchial spasm.
• Urticaria causes localized skin hives.
• Hay fever causes swollen nasal linings.
• Asthma involves bronchiolar smooth muscle spasm.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.