Adaptive immunity (3rd line of defence)
Overview of adaptive immunity
Goals
Destruction of foreign organisms that are resistant to innate immunity and inflammation
Long term, highly effective protection against future exposure
Characteristics of the adaptive immune response
Inducible
Requires stimulus
Diverse and specific
End products:
Specific immunoglobulins
Active lymphocytes (T cells, B cells)
Memory cells 
Components
Humoral (B cell mediated), leading to Ig (antibodies)
– Bind to antigens on bacteria and viruses
Cellular (T cell mediated), leading to generation of effector
cells
– Kill cell directly
Types
Active:
– Exposure to antigen
– Immunization
Passive:
– Via pre-formed antibodies (Ig)
Antigen recognition, processing and presentation
Antigen processed, then presented by APCs (dendritic cells, macrophages)
Recognition occurs:
– By means of clusters of differentiation (CD)
– In conjunction with MHC 
Results: (B and T cell activation)
– B cells differentiate into plasma cells (Ig producing cells)
– T cells differentiate into effector cells; cytotoxic T cells (killer cells)
Antibodies:
Antibody (Ig) Structure
4 polypeptide chains
– Light chains (2) and heavy chains (2)
Antigen-binding fragment (Fab)
– Recognition sites (receptors) for antigenic determinants
Crystalline fragment (fc)
– Responsible for biologic function
Hinge region
Variable region (V)
– Variation in sequence of AA
– Supports endless diversity of Ig generation
Constant region
– Similar sequence of AA
– Responsible for common features of Ig class 
Antibody classes
• IgG- most abundant, crosses placenta, secondary response to infection
• IgM- pentamer, first response to infection (primary response)
• IgA- dimer, in body secretions
• IgD- on B cells, initiate immune response
• IgE- Allergic reactions, lysis of parasitic worms 
Antibody functions
Destroys antigens:
– Directly
o Neutralization
o Agglutination/precipitation
– Indirectly
o Stimulate phagocytosis
o Stimulate complement 
Cells of Immune Response
Phagocytic cells:
Macrophages, neutrophils, monocytes
Antigen presenting cells:
Dendritic cells, macrophages
Specific cells of immune response
B cells (originates in bone marrow)
T cells (originates in thymus) 
– T helper – CD4
o Require that antigens be
presented in conjunction with MHC class II molecules
o TH1: Activate cells related to cell-mediated immunity
o TH2: Activate B cells to produce Ab
– Cytotoxic T – CD8
o Require that antigens be presented in conjunction with MHC class I molecules
o Recognize Ag + MHC I
o Induce apoptosis in target cell
– T regulatory – CD25 (& some CD4)
o Regulate other T cells
o Regulate T cells response against self
o Previously named T supressor (Ts)
Primary and Secondary Immune responses
Primary response
Initial exposure
Lag phase
Mediated by IgM
IgM detected 5-7 days 
Secondary response
Second exposure
More rapid response and larger amounts of Ig produced
Facilitated by memory cells
IgM is produced in similar quantities to the primary response, but IgG is produced in considerably greater numbers.
Aging and Immune function
Decreased T cell activity differentiation
Decreased antibody response to antigens
Increase in circulating antigen-antibody complexes
Increase in circulating autoantibodies
Decrease in circulating memory B cellshives)