patho 11

Immune Response

Definition

The immune response is the collective, coordinated reaction of the cells and molecules of the immune system to defend the body against pathogens and other foreign substances.

• Innate (nonspecific) immunity: The natural resistance present at birth, providing rapid, general protection against pathogens.

• Adaptive (specific) immunity: The second line of defense, slower to respond but highly effective and specific to particular antigens.

Principal Cells of the Immune System

Lymphocytes

• Originate from stem cells in the bone marrow.

• B cells: Differentiate into plasma cells (produce antibodies) and memory B cells.

• T cells: Include cytotoxic T cells (killer T cells), helper T cells, and regulatory T cells (suppressor T cells).

• Natural Killer (NK) cells: Part of innate immunity; kill infected or cancerous cells without prior activation.

Accessory Cells

• Macrophages and dendritic cells: Engulf and destroy pathogens; function as antigen-presenting cells (APCs) by processing antigens into epitopes and presenting them to T cells, initiating adaptive immune responses.

Mediators of the Immune System

Mediators facilitate communication and interaction between immune cells and other cells.

• Cytokines: Soluble proteins secreted by cells of both innate and adaptive immunity; regulate immune responses.

• Chemokines: A subset of cytokines that stimulate migration and activation of immune and inflammatory cells.

• Colony-stimulating factors: Stimulate growth and differentiation of bone marrow progenitors into specialized immune cells.

Innate Immunity

Components

• Epithelial barriers: First line of defense; not antigen-specific; includes skin and mucous membranes.

• Phagocytic cells: Neutrophils and macrophages that ingest and destroy invading organisms.

• Natural Killer (NK) cells: Kill infected or cancerous cells without prior activation.

• Plasma proteins: Complement system, acute-phase proteins, collectins, and pentraxins; recognize, neutralize, and eliminate threats.

• Opsonins, cytokines, and acute-phase proteins: Enhance phagocytosis and inflammatory response.

• Induction of inflammatory response: Pathogen breaches barriers, detected by pattern recognition receptors (PRRs) and pathogen-associated molecular patterns (PAMPs), triggering inflammation.

Soluble Mediators of Innate Immunity

Act in the extracellular environment to facilitate immune defense.

• Opsonins: Enhance phagocytosis (e.g., acute-phase reactants, lectins, complement proteins).

• Immunoglobulin G (IgG): Most abundant in blood/tissues; triggers complement system; present in both innate and adaptive immunity.

• Cytokines: TNF, interleukins, interferons; mediate inflammation and immune cell communication.

• Acute-phase proteins: Mannose-binding ligand, C-reactive protein; promote pathogen clearance and regulate inflammation.

• Complement system: Cascade of plasma proteins; crucial for innate immunity; leads to lysis of pathogens and enhances phagocytosis.

Key processes: Cytolysis, opsonization, chemotaxis, anaphylaxis.

Innate Recognition Systems

Distinguish self from non-self and identify microbial threats.

• Pattern recognition receptors (PRRs): Detect threats and initiate immune responses.

• Pathogen-associated molecular patterns (PAMPs): Molecular signatures of pathogens recognized by PRRs.

• Toll-like receptors (TLRs): Subfamily of PRRs; trigger signaling pathways to activate immune responses and produce cytokines/chemokines.

Adaptive Immunity

Provides specific, long-lasting protection against pathogens and other antigens.

• Antigen recognition: Ability to recognize and react to a wide variety of antigens.

• Memory: Ability to "remember" pathogens and mount a rapid, heightened response upon re-exposure.

• Lymphocytes and their products: Activated T and B cells can become memory cells, providing long-term immunity.

• Antigen identification: Enables recognition and long-term protection against specific threats.

Types of Adaptive Immune Responses

Humoral Immunity

• Mediated by B cells and antibodies in the blood.

• Principal defense against extracellular microbes and toxins.

Cell-Mediated Immunity (Cellular Immunity)

• Mediated by T lymphocytes (helper T cells and cytotoxic T cells).

• Defends against intracellular microbes (e.g., viruses).

• B and T lymphocytes are programmed to remove invading organisms and remember antigens.

Two Types of Immune Cells

• Regulatory Cells: Prevent overactivity of the immune system and protect healthy cells; prevent excess inflammation.

• Effector Cells: Activated upon encountering specific antigens; carry out functions to eliminate threats (includes T cells, B cells, NK cells, macrophages).

Antigens and Antibodies

• Antigens: Substances foreign to the host that stimulate an immune response.

• Antibodies: Proteins that recognize and bind antigens; can be membrane-bound (receptors) or secreted.

Types of Antigens

• Bacteria

• Fungi

• Viruses

• Protozoa

• Parasites

• Nonmicrobial agents

Properties of MHC Molecules

Major Histocompatibility Complex (MHC)

• Cell surface proteins that present antigen fragments to T cells, initiating immune responses.

• HLA Antigens: Human Leukocyte Antigens; highly diverse, important for pathogen recognition and tissue transplantation.

• Classes:

  • Class I: HLA-A, HLA-B, HLA-C (present on virtually all nucleated cells)

  • Class II: HLA-DR, HLA-DP, HLA-DQ (restricted to immune cells, APCs, B cells, macrophages)

Functions

• Class I: Present processed antigen to cytotoxic CD8+ T cells; destroy infected or cancerous cells.

• Class II: Present processed antigenic fragments to CD4+ T cells; necessary for effective interaction among immune cells.

Antigen Presentation

• Macrophages and dendritic cells: Process and present antigen peptides to CD4+ helper T cells, enhancing antibody production and activating other immune cells.

• Antigen-presenting cells (APCs): Capture and present foreign antigens to T cells, initiating the immune response.

• Adaptive immunity is initiated when innate immunity fails to clear infection; antigens and APCs are transported to lymph nodes.

Functions of T Lymphocytes

• Activation of other T cells and B cells

• Control of intracellular viral infections

• Rejection of foreign tissue grafts

• Delayed hypersensitivity reactions

Classes and Functions of Immunoglobulins

Immunoglobulin	Main Functions
IgG	Antiviral, antitoxin, antibacterial; crosses placenta (protects newborn); activates complement; most abundant in blood
IgA	Predominant in body secretions; protects mucous membranes
IgM	Forms natural antibodies; prominent in early immune responses; activates complement
IgD	Found on B lymphocytes; needed for B cell maturation
IgE	Binds to mast cells and basophils; involved in parasitic infections, allergic and hypersensitivity reactions

Functional Groups of Cytokines

• Inflammatory cytokines: (e.g., IL-1, IL-6, TNF) produce fever, acute-phase response, attract and activate phagocytes (e.g., IL-8, IFN-γ).

• Maturation factors: (e.g., IL-3, GM-CSF) stimulate hematopoiesis (formation of new blood cells).

Immune Response: Active and Passive Immunity

• Active Immunity: Acquired through exposure to antigens (pathogens or vaccines); body produces its own antibodies.

• Passive Immunity: Acquired through transfer of protective antibodies from another source (e.g., maternal IgG crossing placenta, IgA in colostrum during breastfeeding).

Development of an Immune Response

• Fetal immune system develops at 5–6 weeks gestation.

• Secondary lymphoid organs are well developed at birth.

• IgA and IgM reach adult levels by 1 year of age.

The Elderly Immune System

• Declining ability to adapt to environmental stresses.

• Decrease in immune responsiveness.

• Decrease in size of thymus gland.

• Reduced numbers/responsiveness of T lymphocytes due to biological aging.

• Altered responses of immune cells to antigen stimulation.