Adaptive acquired Immune System In adaptive immunity receptors

Adaptive (acquired) Immune System

In adaptive immunity, receptors provide pathogen-specific recognition Lymphocytes: Originate from stem cells in bone marrow 1. B lymphocytes (B cells): Mature in bone marrow 2. T lymphocytes (T cells): Mature in thymus

• Lymphocytes recognize specific antigens: Large molecules on surface of foreign cells or viruses • B & T cells bind to antigens via proteins: Antigen receptors • Countless different antigen receptors but those made by single B or T cell are identical

B Cell Receptors • Y-shaped molecule with 4 polypeptide chains (2 heavy, 2 light) linked with disulfide bridges • One end anchored into cell’s membrane • Variable (V) regions • Constant (C) regions

B cell response: • Epitope (antigenic determinant): Small portion of antigen that binds to antigen receptor (lock & key) • Binding of receptor to antigen triggers secretion of soluble receptor: Antibody (immunoglobin, Ig)

T Cell Receptors • 2 polypeptide chains: a chain & b chain • Variable (V) regions at tips (1 antigen binding site) • Anchored in cell membrane

T Cell Response: • When a pathogen infects cell, enzymes inside cell cut antigen into smaller peptides (antigen fragments) • Fragments bind to MHC (major histocompatibility) molecules • Antigen presentation: MHC & bound antigen move to cell surface • T cell receptor binds to MHC/Antigen

1. 4 Characteristics of Adaptive Immunity Diversity: Large diversity of lymphocytes and receptors can detect new pathogens never encountered before

1. 4 Characteristics of Adaptive Immunity Diversity: Large diversity of lymphocytes and receptors can detect new pathogens never encountered before 2. Self tolerant: Not reactive against own cells

1. 4 Characteristics of Adaptive Immunity Diversity: Large diversity of lymphocytes and receptors can detect new pathogens never encountered before 2. Self tolerant: Not reactive against own cells 3. Clonal Selection: B & T cells multiple after being activated: Clonal selection creates clones (effector cells) • • B effector cells: Plasma cells that make antibodies T effector cells: Helper T cells and Cytotoxic T cells

Clonal Selection

1. 4 Characteristics of Adaptive Immunity Diversity: Large diversity of lymphocytes and receptors can detect new pathogens never encountered before 2. Self tolerant: Not reactive against own cells 3. Clonal Selection: B & T cells multiple after being activated: Clonal selection creates clones (effector cells) • • B effector cells: Plasma cells that make antibodies T effector cells: Helper T cells and Cytotoxic T cells 4. Immunological memory: Stronger and more rapid response to antigen already seen

Immunological Memory

Adaptive immunity defends against infection of body fluids and body cells 2 Types of Immune Response: 1. Humoral: Occurs in blood, lymph • Antibodies (immunoglobins, Ig) neutralize pathogens 2. Cell-mediated: Cytotoxic T cells destroy infected host cells

Helper T Cells • Antigen-presenting cell engulfs a pathogen, degrades it & displays antigen fragments complexed with class II MHC molecules (only on antigen-presenting cells) on the cell surface • Specific Helper T cell binds to this complex via its antigen receptor & an accessory protein, CD 4

Helper T Cells • Binding of helper T cell promotes cytokine secretion by antigen-presenting cell • Cytokines activate helper T cell and stimulates it’s proliferation

Helper T Cells • Cell proliferation produce clone of activated helper T cells • All cells in the clone have receptors for the same antigen fragment complex • Cells secrete cytokines which activate B cells and cytotoxic T cells

Humoral B Cell Activation

Humoral B Cell Activation

Humoral B Cell Activation

Antibodies Effect of Pathogens

Antibodies Effect of Pathogens

Antibodies Effect of Pathogens

Cell-mediated: Cytotoxic T Cells CD 8

Cell-mediated: Cytotoxic T Cells CD 8

Cell-mediated: Cytotoxic T Cells CD 8

Overview of Adaptive Immune Response

Disruptions in immune system function can elicit or exacerbate disease. • Allergies: Hypersensitive response to environmental antigens (allergens) • Anaphylactic shock: Extreme response that can lead to death

Autoimmune disease: When immune response active against molecules of own body, e. g. lupus, rheumatoid arthritis

Immunodeficiency: Reduced immune response to pathogens, e. g. Severe combined immunodeficiency (SCID) caused by lack of lymphocytes

Immunity • Active immunity: Clones of memory cells form when particular pathogen infects body • Passive immunity: Antibodies transferred from pregnant female to fetus (nursing continues protection) • Immunization (Vaccines): Deliberate introduction of antigens to stimulate immunological memory