Topic 15: Humoral immunity; B-lymphocytes

Question 1

What should be mentioned in this topic?

Answer 1

• Humoral immunity
• Plasma cells
• Memory cell
• Primary response
• Secondary response
• B-cells maturation
• ## Self restriction

Question 2

Humoral immunity

Answer 2

Humoral immunity = antibody-dependent specific immune defense (B-cell defense) Immunglobulins (antibodies) are synthetised only by B-lymphocytes(or rather by plasma cells;
differentiated form). During the humoral immune response a selected antigen specifically activates the B-cells, that will produce immunglobulins. B-lymphocytes both recognize antigens and produce antibodies in the different phases of their maturation.
B-lymphocytes with cell surface immunlgobulins 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
- The antigen-binding receptors located in the cell membrane of the B-lymphocytes are antibody molecules. A given antigen-binding receptor (antibody molecule) can only react with one particular antigen epitope.
- When an antigen enters the body it is immediately recognized by B-lymphocytes containing the proper antigen-binding receptor. The activated lymphocytes within this clone then begin to divide rapidly.
- The daughter cells differentiate into two directions and become plasma cells or memory cell.

Question 3

The daughter cells differentiate into two directions and become plasma cells or memory cell.

Plasma cells:

Answer 3

• Contain more cytoplasm and are larger than B-cells
• Produce and secrete antibodies identical to the receptors on the B-cells that
  recognized and bound the antigen epitope.
• Plasma cells die after a few days of intensive antibody production, since the
  antibodies are proteins (and the protein production of the plasma cells is
  directed towards the antibody production instead of its own needs).
• When a lymphocyte clone is activated by its antigen epitope, most of the cells
  are differentiated into plasma cells.

Question 4

The daughter cells differentiate into two directions and become plasma cells or memory cell.

Memory cells:

Answer 4

• Do not participate in the immediate attack on the antigen, but continue to live for months and years after the antigen has been eliminated, ready for stimulation if the antigen is encountered again.

Question 5

Primary response:

Answer 5

On first exposure to an antigen, several days elapse before formation of antibodies begins. The antibody formation is then moderate. This antibody production is the primary response.
- The delay is mainly due to the time required for the lymphocyte of the correct clone to come in contact with the antigen. The lymphocyte of this clone must also undergo a proliferation phase before a substantial antibody production can begin.

Question 6

Secondary response

Answer 6

When a previous encountered antigen reenters the body, the antibody formation changes character, resulting in the secondary response.

• Numerous memory cells, possessing the ability to recognize the antigen, will then be present as a result of the previous attack.
• The memory cells and their progeny produce a large amount of antibodies, often in a matter of a few hours.
• The attack on the antigen will therefore be quicker, more potent and longer lasting.

Question 7

Further activation:

Answer 7

For further activation to occur in the form of clonal expansion and substantial antibody synthesis, the B-cell In most cases needs the assistance of a T-cell.
- This implies that the antigen also must be recognized by a T-cell:
- The complex formed between the antigen and the membrane receptors of the B-cells is transported into the cell (endocytosis) –> broken down to antigenic
epitopes –> carried back to the surface of the cell, with the B-cell’s MHC
class II proteins.
- In this form the antigen will be recognized by helper T-cells, which binds to
this complex. Helper T-cell is activated –> produce cytokines that stimulate the B-cell to divide and differentiate into plasma cells.

Question 8

B-cells maturation:

Answer 8

Humoral acquired immunity develops in two phases.

• The first phase (antigen-independent)
• The second, antigen-dependent phase

Question 9

B-cells maturation:
The first (antigen-independent) phase

Answer 9

• pre-B-lymphocytes mature and become so-called virgin-B-lymphocytes. On
  the surface of each virgin-B-lymphocyte there will be an IgG-like molecule
  appearing against a certain possible antigen structure).

Question 10

B-cells maturation:
The second (antigen-dependent) phase

Answer 10

• begins when the cell get into the blood
  circulation. If there is an APC presented Ag against which the virgin B-Ly has an Ig structure, then this virgin-lymphocyte binds the antigen and it gets activated. Activation may be Th-independent or Th-dependent. Cell signals (interleukins) expressed during binding activateB-lymphocytes. The activated B-lymphocyte can settle in the B-dependent zones of the secondary
  lymphatic organs, where it goes through blastic transformation: the one and only activated cell starts then to reproduce very intensively and a clone of the original cell carrying the original Ig structure appears.Some cells transform into memorycells, but most of them get into the circulation where as plasma cells they produce free immunglobulins.

Question 11

Self Restriction

Answer 11

• Following Ag stimulus, the efficiency and potency of the immune respons will decrease due
  to:
  
  • Elimination of Ags, which reduces the Ag stimulus
  • The activated Lys will die or converted to memory cell
  • The immune response generates several costimulatory mechanisms(interactions of
    different membrane structures), lymphokines, FcRs, etc., which influence the immune response negatively or positively (Feedback