Specific Immunity
Destroy specific antigens that invade the body
Specific Immune Responses
Take several days to activate
Have immunological memory
2 types: Cell-mediated immunity Antibody-mediated immunity
Meet the players…
MHC
APCs
Lymphocytes
Major Histocompatibility Complex
Group of closely linked genes – polymorphic
Distinguish self from non-self
Genes encode MHC antigens Differ in chemical structure, function and
tissue distribution
3 Types of MHC genes:
Class I – encode glycoproteins found on surface of most cells of body
Class II - glycoproteins expressed on APCs Combine with foreign antigens
Class III – secrete proteins involved in immune response
Complement system
Antigen-Presenting Cells (APCs)
Include: Macrophages Dendritic cells B cells
Inactive until their receptors recognize PAMPs on pathogen
Display foreign antigens + own on cell surface
APCs cont.
APCs ingest pathogen Break down most of it Remaining pieces of antigens displayed on
cell surface along with self-molecules
Signaling molecules are released
Present antigen to T cells
Lymphocytes
3 main types: Natural Killer Cells T cells B cells
All 3 develop from stem cells in bone marrow
Large #s of lymphocytes found in lymph organs
Memory
T cell
B
Lymphocytes, cont.
T cells Travel to site of infection Attack foreign cells & own infected cells
B cells Mature into plasma cells Plasma cells produce antibodies
T cells
Mature in the thymus gland Become immunocompetent Develop surface proteins w/ specific receptor
sites Must recognize self-antigens Must bind with foreign antigens If can’t distinguish between self & non-self,
undergo apoptosis http://www.bio.davidson.edu/courses/Immuno
logy/Flash/Main.html
Types of T cells
Cytotoxic T cells (TC cells) Recognize and destroy foreign antigens
including Grafts Virus-infected cells Cancer cells
Helper T cells (TH cells) Regulatory Secrete cytokines to activate B cells and
macrophages
Cell-Mediated Immunity
APCs
T cells
T cells
TC cells Have CD8 glycoproteins 50,000+ receptors that bind to 1 specific
antigen Recognize antigen-class I MHC complex
TH cells Recognize antigen-class II MHC complex
Less than 1 in 10,000 T cells have same antigen specificity
T cell activation
Cells must be activated before they can respond to pathogen
Activation requires: Properly presented antigen
Co-stimulatory signal Released by APC
Interleukin signal Released from macrophages and lymphocytes
After activation…
TC cells:
Enlarge and clone many TC cells specific for that antigen
Leave lymph nodes and go to infection
Combine with antigen and destroy cells within seconds
Secrete perforins and granzymes
Release and find new antigens to destroy
After activation, cont…
TH cells:
Clone
Release cytokines to attract macrophages and TC cells
Also play a role in antibody-mediated immunity
Both TC cells & TH cells produce memory cells for secondary immunity
Summing up Cell-Mediated Immunity:
Pathogen infects body cell
Cell breaks up pathogen and displays foreign antigen along with MHC-class I or II antigen on surface
Specific T cells activated and clone
TC cells leave lymph, go to infection, and destroy cell
TH cells release cytokines to stimulate TC cells and macrophages; also trigger antibody-mediated immunity
Antibody-Mediated Immunity
B cells
B cell activation
APC presents antigen to TH cell: Multiple chemical signals sent back & forth TH cell becomes activated **B cell can be an APC
Activated TH cell releases interleukins which, along with antigen, activates B cell
http://bcs.whfreeman.com/thelifewire/content/chp18/1802004.html
After activation…
B cell divides by mitosis Clone of identical cells Specific for the activating antigen
B cells mature into plasma cells Secrete specific antibodies
Plasma cells remain in lymph node but secrete antibodies into lymph/blood
Antibody (aka Immunoglobulin) Structure:
Types of Immunoglobulins:
5 classes of antibodies based on amino acid sequences
IgG 75% of human circulating antibodies Along with IgM, interacts with macrophages
and activates complement system Defend against blood pathogens – viruses,
bacteria, some fungi
Immunoglobulins,cont.
IgA – mucus, tears, saliva, breast milk Prevents bacteria and viruses from attaching to
epithelium Defends against inhaled or ingested pathogens
IgD Found on surface of B cells Helps activate B cells Aids in maturation of B cells
IgE Plays a role in allergic response Immune response to parasitic worms
How do antibodies work??
Highly specific
Binds to specific part of antigen Epitope
Activates processes that destroy antigen
Labels antigen for destruction, it doesn’t destroy antigen itself
Usu. Several antibodies bind with several antigens creating an antigen-antibody complex May inactivate pathogen or toxin
Prevent binding to host
Antigen-antibody complex stimulates phagocytosis
IgG and IgM antibodies work mainly through complement system Coat surface of pathogen to increase
phagocytosis Stimulate release of histamine Attracts phagocytes Complement proteins destroy the pathogen
Summing Up Specific Immunity
http://www.learner.org/channel/courses/biology/archive/animations/hires/a_hiv1_h.html
Memory
Responsible for long-term immunity Memory T cells Memory B cells
Primary immune response Occurs when 1st exposed to antigen Antibodies appear in 3-14 days After recognition, antibodies build up for
several days then decline
Secondary immune response Memory B cells persist for many years When exposed to antigen again, secondary
response is much more rapid Often don’t “get sick” because response is so
quick Booster shots given to trigger secondary
immune response
Active vs. Passive Immunity
Active – body develops after exposure to antigen Natural – pathogen enters naturally Artificial – pathogen introduced ex. vaccine
Passive – antibodies given; borrowed immunity Natural – mother to baby Artificial –injection with gamma globulin