Specific Host Defenses

A collection of key terms and definitions related to specific host defenses in immunology.

Difference between nonspecific and specific defenses

Nonspecific defenses are innate immune responses that are general and not tailored to a specific pathogen, while specific defenses are adaptive or acquired responses that target specific pathogens.

Important characteristics of the immune system

Specificity and memory.

Functions of dendritic cells

1) Phagocytosis 2) Antigen presentation (Ag-MHC marker complex, a 'wanted poster').

What are MHC markers?

Surface proteins on your own cells that are usually ignored by the immune system.

What does MHC stand for?

Major Histocompatibility Complex.

Classes of MHC markers

Class I – found on all cells; involved in cell-mediated immune response. Class II – found on antigen-presenting cells (APCs: dendritic cells, macrophages, B lymphocytes); involved in antibody-mediated response.

What is an antigen?

A foreign marker that triggers an immune response; can be proteins or large polysaccharides.

What is an epitope?

The part of an antigen recognized by the immune system.

What is a paratope?

The part of an antibody that binds to the epitope.

True or False: All foreign compounds are antigens.

False; not all foreign compounds trigger an immune response.

What is a primary immune response?

The first-time encounter with a specific antigen.

Which leukocyte produces antibodies?

B lymphocytes.

What type of molecule is an antibody?

Protein.

Paratopes in antibody types

Monomeric – 2; Dimeric – 4; Pentameric – 10.

True or False: An antibody can have two different shaped paratopes.

False; all paratopes on a particular antibody are identical.

True or False: A B cell can produce antibodies with different paratopes.

False; all antibodies produced by a single B cell have the same paratope.

What determines the shape of an antibody?

Its protein structure, which is determined by DNA (genes).

What makes an antibody specific for a particular antigen?

The structure of its paratope, which fits the antigen's epitope.

How do B cells produce millions of different antibodies?

Recombination randomly deletes and rearranges gene sections during B cell maturation.

Can one B cell produce antibodies with different paratopes?

No; a single B cell produces antibodies with identical paratopes.

Does the immune system ever make B cells that react with self-protein?

Yes; random gene rearrangement can produce self-reactive B cells.

How does the immune system prevent B cells that react to self-proteins from surviving?

In the bone marrow, B cells are tested against self-proteins; those that react are eliminated.

What does Ig stand for?

Immunoglobulin (antibody).

Define antiserum.

Serum that contains antibodies.

Define serology.

The study of antigen-antibody reactions.

What is a primary immune response?

The first encounter with a specific pathogen.

If you get sick with the flu this winter, is it a primary or secondary immune response? Why?

Primary, because influenza mutates each year, requiring a new response.

Main targets of antibody-mediated immune response?

Extracellular pathogens (bacteria, viruses).

Main targets of cell-mediated immune response?

Intracellular pathogens (bacteria, viruses).

Functions of dendritic cells in antibody-mediated immunity

Phagocytosis & antigen presentation.

What does APC stand for?

Antigen Presenting Cell.

Which cells are APCs?

Dendritic cells, macrophages, B lymphocytes.

What is displayed on the surface of dendritic cells after phagocytosis?

Ag-MHC II marker complex ('wanted poster').

Why can’t dendritic cells just display antigen on their surface?

They would be recognized as foreign and attacked.

How does a virgin T cell interact with an APC?

Its receptor binds specifically to the Ag-MHC complex on the APC.

What induces a virgin T cell to divide?

Interleukins produced by the dendritic cell.

APCs induce virgin Helper T cells to divide into what?

A clonal population of activated Helper T cells.

Main function of activated Helper T cells in antibody-mediated immunity?

Produce interleukins that stimulate B cells to divide.

What activates a virgin B cell?

Antigen binding to membrane-bound antibodies.

Activated B cells divide into which 2 populations?

Plasma B cells & memory B cells.

What induces activated B cells to divide?

Interleukins from Helper T cells.

How are plasma cells different from activated B cells?

Plasma cells secrete antibodies; activated B cells have membrane-bound antibodies.

Memory B cells participate in what kind of immune response later?

Secondary immune response (anamnestic response).

Functions of antibodies (NIAAO)

Neutralization, Immobilization, Agglutination, Activation of complement, Opsonization.

What can complement do after activation by antibodies (COLA)?

Chemotaxis, Opsonization, Lysis, Activation of inflammatory response.

Which antibody stimulates release of histamine?

IgE

Which antibody is found in body secretions (saliva, milk, mucous, tears)?

IgA

Which antibody can cross the placenta?

IgG

Which antibody is involved in inflammation and allergic responses?

IgE

Which antibody predominates in secondary immune responses?

IgG

Which antibody includes antitoxins?

IgG

Which antibody protects mucosal surfaces?

IgA

Which antibody is called 'early antibody'?

IgM

Which antibody is the main type on the surface of B cells?

IgD

Which antibody is large; pentameric (made of 5 Y’s)?

IgM

Which antibody is produced by memory B cells?

IgG

Which antibody is fixed to the surface of basophils?

IgE

True or False: A particular plasma cell can switch classes of antibody produced; however, all of these antibodies will still have the same paratope as the original activated B cell.

True.

What specific cell displays monomeric IgM?

Virgin B lymphocyte.

What specific cell displays monomeric IgM and IgD?

Activated B lymphocyte.

What specific cell secretes pentameric IgM?

Plasma cell.

Explain how a memory B cell will function in a secondary (anamnestic) response.

As soon as the membrane-bound antibody on a memory B cell binds antigen, the cell immediately divides into plasma cells secreting antibody. It bypasses the need for helper T cell activation, acting as a 'fast pass' to antibody secretion.

What kind of pathogen is targeted by the cell-mediated immune response?

Intracellular pathogens and viruses.

What is the major lymphocyte in the cell-mediated immune response?

Cytotoxic T lymphocyte.

What induces a virgin cytotoxic T cell to divide into a clonal population of activated and memory cytotoxic T cells?

Interleukin produced by an APC presenting an Ag-MHC II marker complex.

Explain the interaction between a viral-infected cell and an activated cytotoxic T cell.

The complex receptor of the cytotoxic T cell binds specifically to the Ag-MHC I complex on the surface of the infected host cell.

Why do infected cells display viral antigen with an MHC I marker on their surfaces?

Cells randomly display some proteins they are making with MHC I markers to alert cytotoxic T cells in case the proteins are viral.

Describe apoptosis of an infected cell by a cytotoxic T cell.

Nucleases fragment the host cell DNA, causing the cell to collapse/implode.

Why does the cytotoxic T cell induce apoptosis rather than lysis of the infected host cell?

Apoptosis contains the pathogen inside the host cell, preventing it from spreading to other cells.

How does the cytotoxic T cell kill the viral-infected host cell?

1) Perforins create pores in the plasma membrane. 2) Proteases enter the cell through pores and activate a cascade producing nucleases. 3) Nucleases fragment DNA, causing apoptosis.

Ag-MHC I complexes bind to receptors on what cells?

Cytotoxic T cells.

Ag-MHC II complexes bind to receptors on what cells?

Helper T cells.

Why would HIV affect the antibody-mediated immune response even though B cells produce antibodies?

HIV infects Helper T cells, which are necessary to induce activated B cells to divide into plasma cells that secrete antibodies.

A person receives an oral polio vaccine made from weakened polio viruses.

Artificially-acquired active immunity.

A person gets the chicken pox and is protected from future infections.

Naturally-acquired active immunity.

A person receives a toxoid vaccine to protect them from tetanus.

Artificially-acquired active immunity.

Antibodies transferred from a mother across the placenta to her fetus.

Naturally-acquired passive immunity.

An AIDS patient who has never contracted chicken pox is given a dose of chicken pox immunoglobulin to prevent infection.

Artificially-acquired passive immunity.

A patient neglected getting a tetanus booster and got tetanus from a soil-contaminated wound. He received TIG as a treatment.

Artificially-acquired passive immunity.

What are disadvantages of artificially-acquired passive immunity?

Antibodies are short-lived and serum sickness can occur if antibodies come from an animal and are administered more than once.

A person suffering from tetanus is given tetanus immunoglobulin (antitoxin). What are they getting and why?

They are being given antibodies against the tetanus toxin to neutralize it; it acts as a 'boost' to their own immune response.

What kind of immune response is elicited by a first-time vaccination?

Primary immune response.

After you are vaccinated and then infected by the bacteria you were vaccinated against, what kind of immune response occurs?

Secondary immune response.

After you receive a booster vaccine, what kind of immune response occurs?

Secondary immune response.

What is an antitoxin/antivenom?

Antibody against a toxin or venom.

From which animals is antivenom commonly harvested?

Horse (equine) and sheep (ovine).

What is serum sickness?

A severe reaction to foreign antibody; the body produces antibodies against the foreign antibody, activating complement and causing inflammation (skin, joints, etc.).

What is herd immunity?

Occurs when a significant proportion of a population is immune (via vaccination or prior infection), providing protection for susceptible individuals.

What does ADCC stand for?

Antibody-Dependent Cell-mediated Cytotoxicity.

Why is ADCC necessary to kill helminths?

Helminths (parasitic worms) are too large to be phagocytized.

Classical ADCC is mediated by what type of leukocyte?

Natural Killer (NK) cells, which produce perforins.

What type of leukocyte can kill some helminths via ADCC?

Eosinophils.