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

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.