Chapter 24: The Immune System – Anatomy & Physiology Study Notes

Chapter 24: The Immune System

24.1 Overview of Immunity

The immune system is the body's defense mechanism against pathogens and abnormal cells. It distinguishes between self (the body's own cells) and non-self (foreign invaders such as bacteria, viruses, and parasites). The immune response involves several steps to detect, communicate, recruit, and eliminate threats.

• Immunity: The body's ability to protect itself from disease-causing agents.

• Self vs. Non-self: Essential for targeting pathogens while sparing normal cells.

• First Line of Defense: Physical, chemical, and mechanical barriers prevent pathogen entry.

• Four-Step Response if Barriers Are Breached:

  1. Detection & identification of the pathogen

  2. Communication with other immune cells

  3. Recruitment and coordination of response

  4. Destruction or suppression of the pathogen

Innate vs. Adaptive Immunity

• Innate Immunity: Immediate, non-specific, rapid (minutes to hours), not remembered by the immune system. Involves inflammation and antigen-presenting cells (APCs).

• Adaptive Immunity: Specific, slower (days to weeks), remembered by the immune system. Includes cell-mediated (T cells) and antibody-mediated (B cells/antibodies) responses.

Major Functions of the Immune System:

• Recognize and remove abnormal "self" cells

• Remove dead or damaged cells

• Protect the body from pathogens

24.2 Anatomy of the Immune System

Lymphoid tissues are distributed throughout the body and are classified as primary or secondary based on their function in immune cell development and activation.

• Primary Lymphoid Tissues: Thymus gland and bone marrow (sites of immune cell development).

• Secondary Lymphoid Tissues:

  • Encapsulated: Spleen and lymph nodes

  • Diffuse: Tonsils, mucosa-associated lymphoid tissue (MALT), gut-associated lymphoid tissue (GALT)

Leukocytes: The Immune Cells

Leukocytes (white blood cells) are the primary cells of the immune system, with various types specialized for different functions.

• Granulocytes, Phagocytes, APCs: General categories of immune cells.

• Basophils & Mast Cells: Mediate inflammation and innate responses. Basophils circulate; mast cells are tissue-resident.

• Eosinophils: Combat parasites and mediate allergic reactions.

• Neutrophils: Phagocytic, release cytokines and inflammatory mediators.

• Monocytes & Macrophages: Monocytes are precursors to macrophages (Kupffer cells in liver, microglia in brain, osteoclasts in bone).

• Dendritic Cells: Antigen-presenting cells in skin and organs.

• Lymphocytes: Include B cells (produce antibodies), T cells, and natural killer (NK) cells.

24.3 Development of Immune Cells

Immune cells originate from hematopoietic stem cells in the bone marrow. Their development is regulated by cytokines such as colony-stimulating factors and interleukins.

• T Lymphocytes (T cells): Mature in the thymus.

• B Lymphocytes (B cells): Mature in the bone marrow; differentiate into plasma cells that secrete antibodies (immunoglobulins).

• Natural Killer (NK) Cells: Develop in bone marrow and other tissues.

• Clonal Selection: Each B and T cell binds a specific antigen; clones are groups of lymphocytes with the same specificity.

The Immune System Must Recognize "Self"

Self-tolerance is the ability to distinguish self from non-self, preventing autoimmune reactions. This process begins during embryogenesis and involves negative selection and clonal deletion of self-reactive lymphocytes.

• Negative Selection: Destruction of lymphocytes that recognize self-antigens.

• Hygiene Hypothesis: Early exposure to pathogens strengthens immunity.

24.4 Molecules of the Innate Immune Response

Several molecules mediate the innate immune response, many of which are always present in the body.

• Chemotaxins: Attract phagocytes to infection sites.

• Opsonins: Coat pathogens to enhance phagocytosis.

• Pyrogens: Induce fever.

• Acute-Phase Proteins: (e.g., C-reactive protein) Enhance the inflammatory response.

• Histamine: Vasodilator released by mast cells and basophils.

• Complement Proteins: Mediate the complement cascade, leading to pathogen lysis via the membrane attack complex.

Class	Function
Chemotaxins	Attract phagocytes to infection
Opsonins	Coat pathogens for phagocytosis
Pyrogens	Induce fever
Acute-phase proteins	Enhance inflammation
Complement	Opsonization, cell lysis, inflammation
Histamine	Vasodilation, bronchoconstriction
Interferons	Inhibit viral replication
Interleukins	Mediate immune cell communication
Lysosome	Enzyme that digests bacteria
Perforin	Forms pores in target cells
Granzymes	Induce apoptosis in target cells

24.5 Antigen Presentation and Recognition Molecules

Antigen presentation is essential for activating adaptive immunity. Major histocompatibility complexes (MHC) display antigen fragments on cell surfaces for recognition by T cells.

• MHC Class I: Present on all nucleated cells and platelets.

• MHC Class II: Present on antigen-presenting cells (APCs).

• Antigen-Recognition Molecules: Antibodies (B lymphocytes), T cell receptors (T lymphocytes).

Molecule	Function
MHC	Display antigen fragments for T cell recognition
Antibodies	Bind and neutralize antigens
T-cell receptors	Recognize antigens presented by MHC

B Lymphocytes Produce Antibodies

Antibodies (immunoglobulins) are Y-shaped proteins composed of two light and two heavy chains. The stem (Fc region) determines the antibody class, while the arms (Fab region) bind antigens.

• Five Classes of Antibodies:

  • IgG: Most abundant, crosses placenta, secondary response

  • IgA: Found in external secretions

  • IgE: Targets parasites, involved in allergies

  • IgM: Primary response, strong complement activation

  • IgD: Surface of B cells, function unclear

24.7 The Immune Response

The immune system employs multiple barriers and mechanisms to prevent and eliminate infections.

• Physical Barriers: Skin, mucous membranes, ciliated epithelium

• Mechanical Barriers: Removal of pathogens (e.g., coughing, sneezing)

• Chemical Barriers: Enzymes like lysozyme

Innate Immunity Provides Nonspecific Responses

Innate immunity is triggered by pathogen-associated molecular patterns (PAMPs) and involves phagocytosis, inflammation, and the action of natural killer (NK) cells.

• Pattern Recognition Receptors: Detect PAMPs

• Phagocytosis: Neutrophils, macrophages, dendritic cells engulf pathogens

• Inflammation: Attracts immune cells, creates barriers, promotes repair (signs: redness, heat, swelling, pain)

• Antigen-Presenting Cells: Bridge innate and adaptive immunity

Adaptive Immunity Creates Antigen-Specific Responses

Adaptive immunity involves clonal expansion and differentiation of lymphocytes upon first exposure to an antigen, resulting in effector and memory cells.

• Clonal Expansion: Increases the number of antigen-specific lymphocytes

• Effector Cells: Short-lived, actively combat pathogens

• Memory Cells: Long-lived, enable faster secondary responses

• B Cells: Differentiate into plasma cells (antibody production)

• T Cells: Destroy infected cells, regulate immune responses

Primary vs. Secondary Immune Response

• Primary Response: First exposure, slower, mainly IgM antibodies

• Secondary Response: Subsequent exposures, faster and stronger, mainly IgG antibodies

Antibody Functions

• Clumping (agglutination) of antigens

• Neutralization of bacterial toxins

• Opsonization (enhancing phagocytosis)

• Triggering degranulation of immune cells

• Activating complement proteins

• Antibody-dependent cell-mediated cytotoxicity

• Activating B lymphocytes

Active and Passive Immunity

• Active Immunity: Body produces its own antibodies (natural: infection; artificial: vaccination)

• Passive Immunity: Antibodies acquired from another source (natural: placenta, breast milk; artificial: injection of gamma globulins)

T Lymphocytes and Contact-Dependent Signaling

• Cytotoxic T Cells (Tc): Destroy infected cells displaying antigen-MHC complexes

• Helper T Cells (Th): Activate other immune cells via cytokine secretion

• Regulatory T Cells (Treg): Suppress excessive immune responses

Integrated Immune Responses

• Bacterial Invasion: Triggers complement activation, phagocytosis, adaptive response, and tissue repair

• Viral Infections: Require intracellular defense; antibodies prevent entry, cytotoxic T cells destroy infected cells

• Allergic Responses: Hypersensitivity to non-pathogenic antigens (allergens); can be immediate (antibody-mediated) or delayed (T cell-mediated)