Ch. 7 - Adaptive Immunity

Adaptive (acquired) immunity

• The third line of defense

• Develops more slowly than inflammatory response

• specific & has memory so it can provide permanent or long-term protection against specific microorganisms

  • 1 single strep, body might not mount response; larger quantities = response

Antigens

• Immune system challenged by these

• foreign substances located on the surface of every living cell

• Most antigens are immunogens (induce the immune response)

  • EVERYTHING has antigens: mold, pollen, etc. BUT viral ones are different

Immunogenic Antigens

• Induce the immune response

• Foreignness to the host

• Adequate size

• Adequate chemical complexity

• Present in sufficient quantities

The Immune Response

• how the body responds to the “antigen” challenge

• involves 2 types of lymphocytes

B Lymphocytes (B cells)

Produce antibodies that enter the blood and react with the antigen

T Lymphocytes (T cells)

Attack the antigen directly

Immune Response: Clonal Diversity

• Before birth, we produce T cells and B cells that recognize almost all foreign antigens in the environment

  • The sum of these is what makes diversity

• Each T and B cell specifically recognizes one particular antigen

• Released as mature cells having the ability to react with antigens

• Migrate to other lymphoid organs in prep for future antigen exposure (hang out there essentially, on standby)

• Thymus for T cells

• Bone marrow for B cells

Clonal Selection

• APC’s (antigen processing/presenting cells)

• T cells and B cells interact with the antigen

• B cells differentiate into plasma cells (antibody producing cells)

• T cells differentiate into effector cells, such as cytotoxic T (Tc) cells

• Both lines of cells also develop memory cells that react even more quickly the next time the antigen enters the body

  • Essentially copy over and over to handle scale of infection

Humoral Immunity (B-cells)

• Antibodies circulate and bind to the antigen on a pathogen

• Antibodies protect us against many bacteria and viruses

Cell-mediated Immunity (T-cells)

• T cells react directly with the antigen on the cell surface or infectious agent

• Helper T-cells (Th cells) - activate cytotoxic cells

• Cytotoxic T-cells (Tc cells) - Actually destroy antigens

• Regulatory T-cells - REGULATE & reduce immune response (“we’re done here”)

Natural Killer Cells

• Specialized lymphoid cell

• Similar to T cells except lack antigen specific receptors

• Kill target cell similar to Tc lymphocyte

• Effective against cancer cells that constantly change their antigen to avoid destruction

Antibodies

• Antibodies = immunoglobulins (Ig)

• Produced by mature B cells (plasma cells)

• Antigen-binding fragment - what attaches to antigen when cell recognizes it

• Crystalline fragment

Antibody Function – Direct

• Happens BEFORE invaders bind to host cells

• Neutralization – inactivating or blocking the binding of antigen to receptor - invader useless

• Agglutination – clumping insoluble particles that are in suspension

• Precipitation – making a soluble antigen into an insoluble precipitate - change chemical nature of invader, restricts ability to cause harm

Antibody Function – Indirect

• By activating components of innate resistance, like complement (C1 Pathway) and phagocytes

• Also forms the Membrane attack complex (MAC)

• antibodies can act as opsonin - chemical coating that enhances phagocytosis

IgG

• Most abundant immunoglobulin

• Accounts for most of the protective activity against infections

• Maternal IgG is the major class of antibody found in fetal blood and in newborn

• Transports across placenta, when baby is born they have adult levels of IgG in first 6 months thanks to mom!

IgM

• Largest immunoglobulin

• First antibody produced during the initial response to the antigen

• Synthesized early in neonatal life (First antibody/immunoglobulin that baby produces on their own!)

• Pentamer (5 identical molecules)

IgE

• Normally low concentrations in blood

• Specialized function as mediator of many common allergic responses and in the defense against parasitic infections

• Primary cause of common allergies (hay fever, dust allergies, bee stings)

IgD

• Found in low concentrations in blood

• Primary function is as an antigen receptor on the surface of early B cells

IgA

• Two classes

• IgA1 molecules are found predominantly in the blood

• IgA2 molecules are found predominantly in normal body secretions

• First line of defense, secretory immune system

Secretory Immune System

• Protects the external surfaces of body

• IgA dominant secretory immunoglobulin

  • Specifically IgA2 - protect baby against infections thru GI tract

  • Help identify pathogen before it breaks through first line of defense & causes inflammatory response

  • these are the reason why you can be around someone with a cold, but not get sick!

• IgM and IgG also present

• Antibodies in bodily secretions provide local protection

  • Tears

  • Sweat

  • Saliva

  • Mucus

  • Breast milk

Active Immunity

• “I MAKE my own immunity”

• Natural exposure to antigen

• After immunization

Passive Immunity

• “I can’t make my own immunity, so I need it from someone else”

• During pregnancy, antibodies cross placenta from mom → baby

• Pre-formed antibodies and/or t-cells are injected into the individual

• Individuals who are immunocompromised or have cancer (usually on immunosuppressants) can receive this form of treatment

Fetal and Neonatal Immunity

• Antibody function is deficient

• Capable of primary IgM response; unable to produce an IgG challenge

• Immunity provided by maternal antibody

• Trophoblastic cells transport maternal IgG across the placenta

• Newborn IgG levels are near adult levels (thanks mom)

  • Clonal selection & capacity to identify correct B or T cell & inflammatory processes have not yet matured and can be ineffective, hence why baby gets antibodies from mom

Primary Immune Response (Phase 1)

• Single initial response to most antigens

• Lag period with B cells differentiating and proliferating (copying)

• Then IgM and IgG

• No further exposure to the antigen → circulating antibody catabolized

• Measurable quantities fall BUT immune system is PRIMED

Secondary Immune Response (Phase 2)

• Second and subsequent challenge by same antigen

• More rapid production of larger amount of antibody than during primary (because of the role of memory cells)

• IgM level increases and IgG level markedly increases

• Upon reinfection, lag period gets smaller and immune response is faster

Aging and Immune Function

• Decreased T-cell activity

• Thymic size is 15% of its maximum size

• Thymic hormone production drops, as does the organ’s ability to mediate T-cell differentiation

• Decreased antibody response to antigens