Resistance and the Immune System: Acquired Immunity

Chapter 15

Resistance and the Immune System: Acquired Immunity

15.1 An Overview of the Acquired Immune Response

•  The Ability to Eliminate Pathogens Requires a Multifaceted Approach

•  Antigens are microbes or microbe parts that provoke an immune response

•  The immune system recognizes unique antigenic determinants (epitopes)

•  Immune deficiency is the loss of the body’s ability to respond to antigens and epitopes

•  Regulatory T cells prevent other T cells from attacking “self” cells

•  Autoimmune diseases occur when self-tolerance breaks down

•  If nonimmunogenic molecules (haptens) are linked to proteins, they may not be recognized as “self”

•  Thus they might provoke an immune response (allergies)

•  Immunological memory is the ability to “remember” past pathogen exposures

•  The body fights off any subsequent infections

•  Acquired Immunity Generates Two Complementary Responses to Most Pathogens

•  B lymphocytes (B cells) are involved in producing antibodies against epitopes

•  T lymphocytes (T cells) provide resistance through lysis of infected or abnormal cells

•  The humoral immune response involves:

•  activation of B cells

•  production of antibodies against the identified antigen

•  If the microbes enter cells, antibodies are useless

•  Then the cell mediated immune response is activated to eliminate “nonself” cells

•  Antigen exposure activates only T and B cells with receptors that recognize specific epitopes on that antigen

•  B and T cell clones contain lymphocytes that develop into:

•  Effector cells that target pathogens

•  Memory cells are long-lived B and T cells

–  They are capable of division on short notice

•  The Immune System Originates from Groups of Stem Cells

•  In the fetus, lymphocytes arise from hematopoietic stem cells in the yolk sac and bone marrow

•  They develop into:

–  Myeloid progenitors, which become:

»  red blood cells

»  most white blood cells

–  Lymphoid progenitors, which become lymphocytes

•  T lymphocytes are formed in the thymus

•  B cells are formed in the bone marrow

15.2 The Humoral Immune Response

•  Antibodies are of a class of proteins called immunoglobulins

•  Epitope recognition requires antibodies to have a special structure of:

•  2 identical heavy (H) chains

•  2 identical light (L) chains

•  Each light and heavy chain has:

•  A constant region, which determines the location and functional class of the antibody

•  A variable region, which contains different amino acids for the many antibodies produced

•  The variability allows formation of the specific antigen binding site

•  The Fab fragment of an antibody combines with the Epitope

•  The Fc fragment performs functions in:

•  opsonization

•  activation of the complement system

•  allergic reactions

•  There Are Five Immunoglobulin Classes

•  IgM is the first (but short-lived) Ig to appear in circulation after B cell stimulation

•  IgG (gamma globulin) is the major circulating antibody

•  It provides immunity to the fetus and newborn

•  IgA provides resistance in the respiratory and gastrointestinal tracts

•  It is found in colostrum

•  IgE plays a role in allergic reactions

•  IgD is a cell surface receptor on B cells

•  Antibody Responses to Pathogens Are of Two Types

•  A primary antibody response occurs the first time the body encounters a pathogen

•  A secondary antibody response is more powerful and sustained

•  It occurs with a subsequent infection by the same pathogen

•  Antibody Diversity Is a Result of Gene Arrangements

•  Somatic recombination is a random mix and match of gene segments

•  This accounts for the large number of unique antibodies encoded by immune system genes

•  Antibody Interactions Mediate the Disposal of Antigens (Pathogens)

•  Formation of antigen-antibody complexes result in the antigen:

•  death

•  inactivation

•  increased susceptibility

•  The membrane attack complex causes cell lysis

15.3 The Cell Mediated Immune Response

•  Cellular Immunity Relies on T-Lymphocyte Receptors and Recognition

•  Cytotoxic T cells have T-cell receptors (TCRs) and CD8 coreceptor proteins

•  Naïve T cells have TCRs and CD4 coreceptor proteins

•  Naïve T cells can help with both humoral and cell mediated immunity

•  HIV attaches to the CD4 receptor and infects the cell

•  TCRs and coreceptors allow T cells to recognize and bind to the major histocompatibility complex (MHC)

•  MHC proteins are unique for nearly all individuals

•  They mark the body’s cells as “self”

•  Class II MHC proteins on the surface of immune cells present antigen fragments to naïve T cells

•  They are called antigen-presenting cells (APCs)

•  Class I MHC proteins are found on the surface of nearly all the body’s cells

•  Naïve T Cells Mature into Effector T Cells

•  Cytotoxic T Cells Recognize MHC-1 Peptide Complexes

•  Host cells infected by viruses can:

•  degrade viral antigens

•  present peptide fragments with MHC-1 proteins on the cell surface

•  Activated cytotoxic T cells recognize and bind to the MHC-1/peptide complex on infected cells

•  They release toxic substances such as perforin and granzymes to:

•  cause cell death

•  expose pathogens to antibodies

•  T cells can also recognize and kill tumor cells

•  TH2 Cells Initiate the Cellular Response to Humoral Immunity