Immunity to Infection: Adaptive Immunity and Antibodies

Question 1

What is adaptive immunity?

Answer 1

It is the body’s second-line of defense provides specific response to an antigen and confers long-lasting protective immunity and immunologic memory.

Question 2

What is an antibody?

Answer 2

An antibody is a Y-shaped protein that is produced by the proliferation and differentiation of B lymphocytes into plasma cells, which can specifically bind to the corresponding antigen.

Question 3

Antibodies are only found in body fluids of vertebrates and on the B-cell surface.

True or False

Answer 3

True.

Question 4

After antibodies bind to different antigens, different responses are produced, so antibodies are given different names, such as:

Answer 4

• hemolysin
• lectin
• precipitin
• antitoxin
• lysozyme

Question 5

Describe the structure of an antibody.

Answer 5

An antibody is a globular blood protein macromolecule weighing about 150 kDa.
The monomer of an antibody is a Y-shaped molecule with a symmetrical structure of four polypeptide chains, of which two chains are longer heavy chains (H chains) with a relatively high molecular weight, and two chains are shorter light chains (L chains) with a relatively low molecular weight.

Question 6

Activated B cells can differentiate into two kinds of cells with different functions. They are:

Answer 6

i. Plasma cells: produce antibodies
ii. Memory cells memorise the antigens contacted

Question 7

Antibodies in a fetus and new born are provided by the mother as a kind of passive immunity.

True or False

Answer 7

True

Question 8

What is the main function of the antibody?

Answer 8

to combine with antigens (both foreign and self) to effectively remove foreign bodies, neutralise the toxins released by them, or to remove some autoantigens, maintaining the normal balance of the body

Question 9

Mention three types of antibodies that cause pathological damage to the body, leading to autoimmune disease.

Answer 9

i. Anti-nuclear antibodies
ii. Anti-double-stranded-DNA antibodies
iii. Anti-thyroglobulin antibodies

Question 10

Highlight the biological activity of the antibody.

Answer 10

1. They bind specific antigens: antibodies by to antigens by relying on the specific binding sites on the molecule.
2. They activate complement: after binding to the antigen, the complement system is activated using the exposed complement-binding sites
3. They bind cells: they bind to different cells involved in immune response
4. IgG can pass from mother to fetus, providing passive immunity. IgA provides local mucosal immune response.
5. Antigenicity: they also have the ability to stimulate the body to produce an immune response.
6. Antibody resistance to physical and chemical factors is different from general globulins:
   i. they are thermolabile (destroyed at 60-70 degree-celcius),
   ii. susceptible to destruction by enzymes and substances that cause protein coagulation.
   iii. precipitated by neutral salts e.g. ammonium sulfate or sodium sulfate

Question 11

Highlight the Production Law of Natural Antibodies.

Answer 11

1. Production of antibodies in primary response: When the antigen enters the body for the first time, antibodies are produced after a certain incubation period, but very little is produced; it is also maintained in vivo for a shorter time.
   Its main features are as follows:
   i. the latency of primary immune response is long
   ii. the concentration of antibody produced is low
   iii. and the antibody–antigen affinity is also low, mainly IgM.
   iv. Short in vivo retention time.
2. Production of antibodies in secondary response: Upon exposure to the same antigen, some of the original antibodies initially bind to the antigen, causing a reduction in the original amount of the antibody.
   Subsequently, there is a rapid increase to several times more than the primary response, and its in vivo retention time is also longer.
   Its main features are as follows:
   i. the latency is usually as short as 1–3 days
   ii. high antibody concentration
   iii. and the antibody-antigen affinity is high, mainly IgG.
   iv. Longer in vivo retention time
3. Production of antibodies in anamnesis reaction (memory): The antibodies produced by the antigens gradually disappear after a certain period. At this point, if the antigen is contacted again, there’ll be a rapid increase in the antibody.
   If the antigen stimulating the body again is the same as that at the first time, it is known as the specific anamnesis reaction; if it is different from that in the primary response, it is known as a nonspecific anamnesis reaction.
   The increase of antibodies caused by a nonspecific anamnesis reaction is temporary and will decline rapidly in a short time.

Question 12

Classify antibodies according to physical and chemical properties and biological functions.

Answer 12

i. IgG
ii. IgA
iii. IgM
iv. IgE
v. IgD

GAMED

Question 13

Briefly describe IgG.

Answer 13

• Accounts for 75% total serum immunoglobin, with normal values of 9.5-12.5mg/ml.
• 40-50% of IgG is in the serum, while the rest is in the tissues.
• Molecular weight is 150 kDa
• It’s mainly a monomer and has 4 subtypes IgG1, IgG2, IgG3 and IgG4
• It’s the only antibody that can cross the placenta and protects the fetus.
• It is also passed from breast milk to the newborn
• Most antibacterial, antiviral and antitoxin antibodies produced by antigens are IgG
• Many antibodies such as LE factor and anti-thyroglobulin antibodies are also IgG
• Synthesis starts from 3 months of age, peaking at age 3-5 years and declining at age 40.
• Most of IgG passed to fetus depletes at 6 months after birth.

Question 14

Briefly describe IgA.

Answer 14

• Second highest immunoglobin amount after IgG (10-20%)
• Divided into two types: serum IgA and secretory IgA
• Serum IgA does not show any important immune function in serum’
• Secretory IgA is present in saliva, lacrimal fluid amd other exocrine fluids
• Present in mucosal surfaces and produced by mucosa-associated lymphoid tissues
• It’s an important factor for mucosal immunity
• They cannot pass the placenta
• They can be passed to infant via breast milk.
• Synthesised 4-6 months after birth, peaking at age 4-12 years

Question 15

Briefly describe IgM.

Answer 15

• 5-10% of total serum immunoglobin
• Natural blood group antibody
• Cannot cross the placenta
• It is the first antibody synthesised in individual development
• It is the first antibody secreted during immune response
• Initiates complement cascade and causes agglutination of intruders to be engulfed by macrophages
• Of clinical diagnostic value for some infectious diseases
• Peaks at age 1 year

Question 16

Briefly describe IgE.

Answer 16

• Lowest amount of immunoglobulin in serum
• Produced by plasma cells in nasopharynx, tonsil, bronchial and other mucosal lining
• Molecular weight of 188 kDa
• Secretion occurs in response to allergen invasion and type 1 allergy

Question 17

Briefly describe IgD.

Answer 17

• Appears on surface of mature B cells
• Specific role in humans is unknown

Question 18

What are the two types/phases of antibody response?

Answer 18

i. Primary response: when an individual encounters an antigen for the first time, the antibody produced in response to that antigen is detectable in the serum within days or weeks. This time can vary depending on the nature and dose of the antigen and the route of administration (eg, oral, parenteral). The serum antibody concentration continues to rise for several weeks and then declines. The first antibodies produced are IgM. Then, IgG, IgA.
IgM levels decline sooner than IgG levels.

ii. Secondary response: when the same antigen is encountered months or years after the primary response, the second antibody response is more rapid and generates higher levels than during the primary response. This change in response is attributed to the memory cells that were generated during the primary immune response.
In the secondary response, the amount of IgM produced is qualitatively similar to that produced after the first contact with the antigen. However, more IgG is produced, and the level of IgG persists much longer than in the primary response. Furthermore, such antibody has higher affinity, and thus to dissociate less easily.

Question 19

Highlight the protective functions of antibodies.

Answer 19

1. Enhanced phagocytosis: Antibodies produce resistance by opsonizing organisms, making them more readily ingested by phagocytes. This engulfment leads to pathogen destruction.
2. Virus neutralisation: Antibodies can bind to the virus and block the ability of the virus particle to attach to its cellular receptor, preventing its invasion, and thus effectively preventing its replication.
3. Neutralization of toxins: Antibodies can neutralise toxins of microorganisms (eg, diphtheria, tetanus, and botulism) and inactivate their harmful effects.
4. Complement-mediated lysis: The attachment of antibodies to viral proteins on virus-infected cells, tumor cells, or to a microbial cell wall can activate the complement system leading to cell lysis.
5. Antibody-dependent cell cytotoxicity (ADCC): The attachment of viral-specific antibodies to viral proteins on a virus-infected cell can lead to the lytic destruction of the infected cell. This lysis is mediated by a killer cell (NK, macrophage, neutrophil) that binds to the Fc proton of that bound antibody.
   ADCC by eosinophils is an important defense mechanism against helminthes. IgE coats the worms and eosinophils attach to the Fc portion of IgE triggering eosinophil degranulation.

Question 20

Antibody-mediated immunity against the pathogen is most effective when directed against microbial infections in which virulence is related to polysaccharide capsules (eg, Pneumococcus, Haemophilus spp., Neisseria spp.)

True or False?

Answer 20

True.
In these infections, antibodies complex with the capsular antigens and make the organisms susceptible to ingestion by phagocytic cells.