The Immune System Part D Chapter 22 The Immune System Part D

Adaptive (Specific) Defenses Is antigen-specific, systemic, and has memory Has two separate but overlapping components: Humoral, or antibody-mediated immunity Cellular, or cell-mediated immunity

Are substances that can: Antigens Are substances that can: Mobilize the immune system Provoke an immune response Are the ultimate targets of all immune responses Are mostly large, complex molecules Not normally found in the body (nonself)

Adaptive (Specific) Defenses The adaptive (specific) defense system represents: The 3rd line of defense Works in conjunction with the innate system Recognizes & attack particular foreign substances Acts to: Immobilize Neutralize Destroy foreign substances

Complete Antigens Complete antigens have two important functional properties: Immunogenicity: Ability to stimulate: Proliferation of specific lymphocytes Antibody production Reactivity: Ability to react with: Activated lymphocytes Antibodies released in response to them

Complete Antigens, cont’d. Complete antigens include: Foreign proteins (strongest) Nucleic acids Some lipids Large polysaccharides

Incomplete Antigens (Haptens) Small molecules, such as: Peptides Nucleotides Many hormones Not immunogenic When attached to body proteins: Become reactive Recognized as foreign by the adaptive immune system Attacked by the system (resulting in allergy or hypersensitivity reactions) Found in: Poison ivy, animal dander, some detergents, cosmetics Some household & industrial products

Antigenic Determinants Only certain parts of an antigen are immunogenic These parts are called antigenic determinants Antibodies & activated lymphocytes bind to these antigenic determinants Most naturally occurring antigens have numerous antigenic determinants that a single antigen may: Mobilizes several different lymphocyte populations Forms different kinds of antibodies Large, chemically simple molecules (e.g., plastics) have little or no immunogenicity

Antigenic Determinants Figure 21.7

Self-Antigens: Major Histocompatibility Complex (MHC) Proteins Our cells are dotted with protein molecules These molecules are: Defined as “self antigens” Not antigenic to us, but Strongly antigenic to others One type (glycoproteins) is called: Major histocompatibility complex (MHC) proteins

Self-Antigens: MHC Proteins This group of glycoproteins marks a cell as self It is coded for by millions of gene combinations Unlikely two people (except identical twins) have the same MHC proteins There are two classes of MHC proteins: Class I MHC proteins: Found on virtually all body cells Class II MHC proteins: Found only on certain cells Such cells act in the immune response

Cells of the Adaptive Immune System There are three crucial cell types of this system: Two types of lymphocytes A 3rd type of antigen-presenting cells Lymphocyte types: B lymphocytes Oversee humoral immunity T lymphocytes: Non-antibody-producing cells Constitute the cell-mediated arm of immunity Antigen-presenting cells (APCs): Do not respond to specific antigens Play essential auxiliary roles in immunity

Lymphocyte “Education” Lymphocytes originate in red bone marrow During development lymphocytes are “educated” The aim of this education is twofold: Immunocompetence: Each lymphocyte must become able to recognize its one specific Ag by binding to it Self-tolerance: Each lymphocyte must be relatively unresponsive to self-antigens and doesn’t attack the body’s own cells

Lymphocyte Maturation Immature lymphocytes released from bone marrow are essentially identical Lymphocyte maturation “education” into B or T cells: Depends on location of competence development B cells become immunocompetent in bone marrow T cells migrate (in blood) to the thymus & mature there The thymus & bone marrow (competence locations) are called: The primary lymphoid organs All other lymphoid organs are called The secondary lymphoid organs

T cells mature in the thymus under two types of selection pressures: Negative selection: Eliminates T cells that are strongly anti-self Positive selection: Selects T cells with a weak response to self-antigens Thus positively selected T cells become both: Immunocompetent, and Self-tolerant

In bone marrow, B cells become: Immunocompetent, and Self-tolerant in bone marrow Self-reactive B cells are either: Eliminated by apoptosis Let undergo receptor editing (receptor rearrangement) Some do leave bone marrow & get inactivated peripherally

Antigen-Presenting Cells (APCs) Major roles of APCs in immunity: Engulf foreign particles (antigens) Present antigen fragments on their own surfaces Flag T cells to recognize antigens Major APCs are: Dendritic cells (DCs) Macrophages Activated B cells

Antigen-Presenting Cells (APCs) Dendritic cells (DCs): The major initiators of adaptive immunity Migrate to lymph nodes & other 2ndary lymphoid organs Present antigens to T and B cells

Macrophages and Dendritic Cells Secrete soluble proteins that activate T cells Activated T cells in turn release chemicals that: Speed up the maturation & mobilization of dendritic cells Prod macrophages to become activated macrophages Activated macrophages: Are insatiable phagocytes Secrete bactericidal chemicals

Humoral Immunity Response Antigen challenge: Refers to the 1st encounter between an antigen and a naive immunocompetent cell Takes place usually in the spleen or a lymph node May happen in any other 2ndry lymphoid organ If the challenged lymphocyte is a B cell: The antigen provokes a humoral immune response Antibodies are produced against the challenger

Proliferation to form a clone Antigen Primary Response (initial encounter with antigen) Antigen binding to a receptor on a specific B lymphocyte (B lymphocytes with non-complementary receptors remain inactive) Proliferation to form a clone B lymphoblasts Plasma cells Memory B cell Secreted antibody molecules Secondary Response (can be years later) Clone of cells identical to ancestral cells Subsequent challenge by same antigen Plasma cells Secreted antibody molecules Memory B cells Figure 21.10

Fate of the Clones Most clone cells become plasma cells Plasma cells are antibody-secreting cells They secrete specific antibody Secretion is at a rate of 2000 molecules per second

Immunological Memory: Primary & 2ndary immune response Primary immune response: Cellular differentiation & proliferation Occurs on the first exposure to a specific antigen A lag period of 3 - 6 days after antigen challenge Antibody plasma levels peak in 10 days Antibody levels then decline

Secondary immune response: Immunological Memory Secondary immune response: Re-exposure to the same antigen Sensitized memory cells respond within hours Antibody levels peak in 2 to 3 days Levels are much higher than in the primary response Antibodies bind with greater affinity Blood levels can remain high longer

Active Humoral Immunity Naturally acquired Abs: Response to a bacterial or viral infection Artificially acquired: Response to a vaccine By using dead or attenuated (weakened) pathogens Vaccines: Spare us the symptoms of disease Their weakened antigens provide: immunogenic & reactive antigenic determinants

Passive Humoral Immunity Differs from active one in Ab source & degree of protection: B cells are not challenged by antigens Immunological memory does not occur Protection ends when Ab naturally degrade in body Naturally acquired: From a mother to her fetus via the placenta Via milk to infants Artificially acquired: From serum injection Example: Gamma globulin

Types of Acquired Immunity Figure 21.12

Also called immunoglobulins Antibodies Also called immunoglobulins Constitute the gamma globulin portion of blood proteins Are soluble proteins secreted by plasma cells in response to an antigen Are capable of binding specifically with that antigen There are five classes of antibodies: IgG, IgM, IgA, IgD, IgE

Antibodies themselves: Antibody Targets Antibodies themselves: Do not destroy antigen They inactivate & tag it for destruction All form antigen-antibody (immune) complex Defensive mechanisms used by antibodies include: Neutralization, Agglutination Precipitation Complement fixation