Topic 14: Adaptive immune response: structure of the antigens and immunoglobulins

Question 1

What should be mentioned in this topic?

Answer 1

• Basics about specific (adaptive) immune response
• Antigens
• Antigen classification
• Immungolbulins
• Structure of antibodies
• Function

Question 2

Basics about specific (adaptive) immune response

Answer 2

The two major forms of specific (acquired) immune response are the humoral and cellular immunity The specific immune response is usually induced when the individual gets in contact with a non-self antigen. This kind of immunity is also called active immunity because the immune system of the individual actively responds to the antigen. An individual may specifically be made immune by recieving the cells or immunglobulins of an already immunized individua l. This is called passive immunity.

Question 3

Antigens

Answer 3

“antigen is a substance, which is recognized by T cell and/or B cell receptors, and it is able to induce active immune response or tolerance according to the host MHC haplotype.”
- The acquired immune response is specific for different kinds of antigens. The immune response can specifically recognize the structural components of complex proteins, polysaccharides and other antigens and then it reacts to each one.
- This precise specificity is present since on the surface of the B- and T-lymphocytes there are membrane receptors that can recognize tiny differences in the structure of antigens.
- Antigen specific lymphocytes develop without any antigen stimulus, namely the cells that can recognizeacertainantigenarepresentinnon-immunized individuals aswell.
- The antigen is a non-self matter that elicits the immune response. Normally, it does not belong to the body. Generally, it belongs to the body of the pathogenic agent.
- It is usually a protein with large molecular weight (polysaccharide etc.).
- The whole antigen molecule is responsible for triggering the immune response.
- Only part of the antigen molecule (epitop) takes part in the binding of the antibody produced
against it.

Question 4

Antigen classification

Answer 4

• Antigenicity

- Immunogenicity

Question 5

Antigen classification

Antigenicity

Answer 5

• Antigens can be auto-antigens (self-matter), allo-antigens (matter of the same species), or xeno-antigens (matter of another species), depending on their origin.
• Antigenicity is determined by the epitop:
  
  • Binding force:
    
    • Affinity (one epitop) = antigen-antibody complex concentration multiplied bytheproductofthefreeantigenandthefreeantibody
    • Avidity (more than one epitop)
  • Specificity: Means that the antibody is specialized to bind one well defined
    chemical structure. The antibody can often be so specific that it can
    differentiate one stereoisomer of the antigen from another.

Question 6

Antigen classification

Immunogenicity

Answer 6

• The antigen can react (bind) with the antibody produced against it.
• Determined by the whole molecule:
  
  • Grade of foreigness:
    - Autoantigen
    - Alloantigen
    - Xenoantigen
  • Molecular weight
    
    • Under 4000 – weak
    • Over 100 000 - strong
  • Complexity of the chemical structure: immunogenicity increases with the
    complexity.

Question 7

Immunoglobulins

Answer 7

• Large glycoproteins that are secreted by plasma cells and that function as antibodies in the
  immune response by binding with specific antigens.
• Production:
  
  • Synthesized by plasma cells (which are differentiated from B-lymphocytes)
  • During humoral immune response a selected antigen specifically activates the B-
    cells, which will produce the immunoglobulins.
  • B-lymphocytes both recognize and produce antibodies in the different phases of their
    maturation.
  • B-lymphocytes with cell surface immunoglobulins are also called the “recognizing
    cells” in the humoral immune response as they specifically bind to a given antigen. Antigen binding triggers the cell differentiation which leads to the transformation of the B-lymphocyte into plasma cells secreting antibodies

Question 8

Structure of antibodies:

Answer 8

• FAB
• FC
• Variable region
• Joint region (hinge)
• H-chains (heavy chains)
• L-chains (light chains)
• Amino acids in the variable (V) domain.
• Constant (C) chains have constant
  amino acid components.

Question 9

Structure of antibodies:

FAB

Answer 9

FAB (fragment antigen binding)

- The part that is responsible for binding the antigen.

Question 10

Structure of antibodies:

FC

Answer 10

FC (fraction crystallisable)
- The part that is responsible
for the biological effect.

Question 11

Structure of antibodies:

Variable region

Answer 11

Variable region

- Two hypervariable antigen binding sites.Eachofthese pairs of subunits binds one antigen.

Question 12

Structure of antibodies:

Joint region

Answer 12

Joint region (hinge)
- Makes the molecule more
flexible while binding the
antigen.
- Disulfide bond.

Question 13

Structure of antibodies:

H-chains

Answer 13

H-chains (heavy chains): two

Question 14

Structure of antibodies:

L-chains

Answer 14

L-chains (light chains): two

o Two different types: lambda and kappa.

Question 15

Function:

Answer 15

• Non-specific cellular immunity can recognize and immediately kill some antigens with the
  help of natural killer cells.
• Only when the dissolved antibodies as a result of the humoral immune response are already
  present, can some lymphocyte populations bind the antibodies at their FC region, and with these can they specifically recognize and immediately kill the invader. They them selves cannot do all these without the antibodies.