Chapter 43 Biology – Campbell • Reece The Immune System Chapter 43 Biology – Campbell • Reece

Nonspecific Defenses First line of defense: Unbroken skin (minute cuts will allow the bacteria or virus through) Mucous membranes (line the digestive, respiratory and genitourinary tracts) Chemical secretions of skin and mucous membranes Lysozyme – digests the cell walls of many bacteria

Nonspecific Defenses Second line of defense: Phagocytic white blood cells (neutrophils, macrophages, eosinophils) – ingest the invading organism Antimicrobial proteins – either attack directly or inhibit reproduction Inflammatory Response – blood vessels dilate to increase blood flow to the area, histamine is released, blood clotting begins, phagocytes are sent

Inflammatory Response

Specific Immunity Third line of defense Lymphocytes are the key cells of the immune system B lymphocytes (B cells) – develops in the bone marrow T lymphocytes (T cells) – differentiate under the influence of the thymus Primary immune response – attack by the lymphocytes the first time the body is exposed

Specificity of Lymphocytes Antigen – a foreign molecule that elicits a specific response Antibodies – proteins that recognize specific antigens Antigen receptors – receptors on the B and T cells that bind certain antigens

Clonal Selection When the antigen is recognized, the proper lymphocyte is selected and cloned Two forms of clones are produced: Effector (plasma) cells – short-lived cells that secrete the specific antibody Memory cells – long-lived cells bearing receptors for the same antigen

Clonal Selection

Secondary Immune Response When the individual is exposed to the same antigen later, the response is much faster, of greater magnitude, and more prolonged The memory cells recognize the antigen and produce a response more quickly

Distinguishing self from nonself The body will destroy any lymphocytes that bear receptors for molecules already present in the body Failure to do this leads to autoimmune diseases (such as multiple sclerosis) Cells have special glycoproteins on their surface called MHC (major histocompatibility complex) or HLA (human leukocyte antigens) which mark body cells as “self”

Distinguishing self from nonself Two types – Class I MHC molecules and Class II MHC molecules Two types of T cells that responds to each kind of MHC molecule Cytotoxic T cells – bind to cells with class I MHC molecules Helper T cells – bind to cells with class II MHC molecules Triggers an immune response

Cytotoxic and Helper T cells

Cell-mediated Response Class I MHC molecules capture fragments of the infecting agent and transport them to the surface of the cell These foreign particles are “presented” and a cytotoxic T cell is activated The cytotoxic T cell kills the infected cell The pathogen is now exposed to the antibodies that will destroy it

Humoral Response Initiated when B cells bind with specific antigens B cells are activated by Interleukin-2 and other cytokines secreted from the Helper T cells The B cells then generate the plasma cells that produce antibodies to destroy the pathogen

Antibody Structure & Function The antigen has several different epitopes or locations where an antibody can bind A single antigen may bind several antibodies

Antibody Structure & Function Antibodies are also called immunoglobulins Each molecule has four polypeptide chains: two identical heavy chains and two identical light chains, joined by disulfide bridges to form a Y-shaped molecule Variable regions at the ends of the light and heavy chains provide the specific binding site

Antibody Structure & Function

Immunoglobulins

Antibody-Mediated Disposal of Antigen Neutralization – the antibody binds to and blocks the activity of the antigen Opsonization – the bound antibodies enhance macrophage attachment to the pathogen Agglutination – clumping of the pathogen Complement fixation – activation of the complement system that leads to cell lysis

Antibody-Mediated Disposal of Antigen