The immune system Chapter 43

Several elements of an innate immune response You must know Several elements of an innate immune response The differences between B and T cells relative to their activation and actions How antigens are recognized by immune system cells The differences in humoral and cell-mediated immunity Why helper T cells are central to immune response https://www.youtube.com/watch?v=CeVtPDjJBPU

Doesn’t matter whether body has encountered pathogen before or not I. Innate immunity Doesn’t matter whether body has encountered pathogen before or not Barrier defenses – skin, mucous membranes, act as physical barrier and produce secretions that lower pH 1. Lysozyme – breaks down bacterial cell walls, found in saliva, mucus, tears

Cellular innate defenses – 1. Phagocytic white blood cells (leukocytes) a. Toll-like receptors (TLRs) – recognize fragments of molecules characteristic of pathogens (ex. polysaccharides found on many bacterial cell surfaces) https://www.youtube.com/watch?v=iVMIZy-Y3f8

Neutrophils – ingest and destroy microbes Monocytes – go into tissues then develop into macrophages Eosinophils – defend against parasites, release hydrolytic enzymes near parasite Dendritic cells – found in areas that are in contact with the environment Natural killer (NK) cells – recognize and remove diseased cells https://www.youtube.com/watch?v=HNP1EAYLhOs

g. Macrophage Recognizes ‘self’ (normal cells) from ‘not-self’ (invading cells) https://www.youtube.com/watch?v=JnlULOjUhSQ Based on protein molecules of surface of all cells/viruses ‘Self’ cells not ingested, ‘not-self’ cells are ingested via phagocytosis Contain lots of lysosomes to chemically digest foreign bodies Non-specific response – identity of pathogen not known

2. Antimicrobial proteins a 2. Antimicrobial proteins a. Interferons – cause cells next to cells infected by viruses to produce substances that inhibit viral replication b. Complement system – consists of about 30 proteins that enhance immune response

Inflammatory Response a. Triggered by tissue damage b. Release of chemical signals c. Ex. Histamines released by mast cells trigger dilation of nearby capillaries to help deliver clotting agents and phagocytes to the area d. Includes fever and increased production of WBCs

II. Adaptive/Acquired Immunity Pathogen-specific Develops after exposure to agents such as microbes, toxins, or other foreign substances

C. Lymphocytes – like all blood cells, produced in the bone marrow 1 C. Lymphocytes – like all blood cells, produced in the bone marrow 1. B cells - mature in the bone marrow 2. T cells – mature in the thymus

D. Antigens – foreign particles that elicit a response from lymphocytes (B cells and T cells have receptors on their membranes that recognize antigens) E. Antibodies – proteins secreted by B cells during response

Antibody Production Macrophage (phagocytic cell) – first to encounter an antigen Macrophage engulfs ‘not-self’ antigen by phagocytosis and only partially digests it Molecular pieces of antigen are displayed on cell membrane of macrophage (antigen presentation) Helper T-cells chemically recognize antigen and become activated Helper T-cells turn immune response from non-specific (‘not-self’) to antigen-specific (identity of pathogen) Helper T-cells chemically communicate with (activate) specific B cell type (able to produce the antibody needed)

F. Activation – occurs when an antigen binds to a B or T cell Induces lymphocytes to divide quickly (make more) B cells activation enhanced by cytokines (cell signals secreted by macrophages)

3. Clonal Selection b. The lymphocyte forms two clones of cells which results in thousands of cells, all specific to this antigen i. Effector (plasma) cells – produce antibodies, short-lived ii. Memory cells - bear receptors for the same antigen, thus allowing them to quickly mount an immune response in subsequent infections

G. Major Histocompatibility Complex (MHC) molecules 1. Protein products of a group of genes that help define “self” Class I MHCs – found on almost all cells except RBCs Class II MHCs – made by some cells of the immune system (macrophages, B cells) 2. MHC molecules bind and transport antigen fragments to the cell surface, antigen presentation 3. T cells bind antigens displayed by antigen-presenting cells 4. Responsible for tissue/organ rejections

H. Specificity of B & T cells results from shuffling and recombination of Ig (immunoglobulin) genes (over 1 million different B cells and 10 million T cells) 1. Each one responds to only one antigen

I. Immune Response 1. Primary – first exposure to an antigen 2 I. Immune Response 1. Primary – first exposure to an antigen 2. Secondary – same antigen encountered again later, faster and greater a. Starts with memory B cells

Secondary Immune Response A specific antigen type is identified A specific plasma cell is identified that can produce an antibody which will bind to the antigen Plasma cells clone themselves (mitosis) to rapidly increase the number of identical B cells Begin antibody production Released antibodies circulate in bloodstream, find their antigen match Various mechanisms, antibodies helps eliminate pathogen Some plasma cells remain in bloodstream for second infection (memory cells).

III. Two Branches of adaptive immunity Humoral Immune Response – activation of clonal selection of effector B cells to produce antibodies to circulate in the blood Cell-mediated Immune Response – activation of clonal selection of cytotoxic T cells to identify and destroy infected cells

Types of cells involved 1. Helper T cells – activated by class II MHCs of an APC to secrete cytokines to activate B cells and cytotoxic T cells 2. Cytotoxic T cells – bind to class I MHCs that are displaying antigenic fragments on the surface and destroy infected cells 3. Plasma cells – secrete antibodies

IV. Active & Passive immunity Active Immunity – develops from being exposing to a pathogen (infection, vaccination) Passive Immunity – receive antibodies without being infected (from mother to baby)

V. Blood types RBCs may present antigens and the immune system 1. Transfusion of wrong blood type causes an immune response that results in agglutination

VI. Immune system disruptions Allergies – exaggerated responses to antigens, subsequent exposures cause release of histamine from mast cells which causes inflammation, and in extreme cases, anaphylactic shock

Autoimmune diseases 1. Immune system thinks certain self cells are non- self, cytotoxic T cells attack and damage the body’s own cells (Ex. lupus, rheumatoid arthritis, type I diabetes, multiple sclerosis, celiac)

C. HIV – infects helper T cells – antibody production doesn’t occur resulting in large number of symptoms, collectively called AIDS