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
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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