31. Peripheral (secondary) lymphoid organs. Immune response in peripheral lymphoid organs.

Question 1

Peripheral Lymphoid organs - lymph nodes

Answer 1

Peripheral Lymphoid Organs: Lymph nodes are the most important.
- Function: Antigens from injured tissue pass into the lymph and are transported to the nearest lymph node, causing swelling during infection.

Structure: Lymph nodes are surrounded by a connective tissue capsule and consist of three parts:
1. Cortex:
- Predominantly B-lymphocyte region.
- Contains cell clusters called follicles.
-> Primary Follicle: Not involved in immune response, small and dense.
-> Secondary Follicle: Participates in humoral immune response, enlarges, and forms a germinal center for intense B-lymphocyte division and differentiation.

2. Paracortex: T cell region where T-lymphocytes proliferate in a cellular immune response.

3.Medulla: Rich in plasma cells that secrete antibodies.

Question 2

Peripheral lymphoid organs - Spleen

Answer 2

Spleen Function: Reacts against antigens in circulation.

Structure: Surrounded by a connective tissue capsule and consists of two parts:
1.Red Pulp:
- Made up of sinuses and venous sinuses.
- Serves as a blood depot.
2. White Pulp:
- Lymphoid organ.
- Clustered around arterial branches (arterioles).

Lymphoid Tissue in White Pulp:
- Periarteriolar Zone: Closest to arterioles, T-cell zone.
- Marginal Zone: Border with red pulp, B-cell zone.
- Primary Follicles: In the marginal zone, similar in structure and function to those in the lymph node cortex.

Question 3

Peripheral lymphoid organs - MALT

Answer 3

Function: Reacts with antigens on the surface of mucous membranes in the respiratory, digestive, and genitourinary systems.

Structure: Non-encapsulated lymphoid tissue (organs lack connective tissue capsules).

Components: Includes tonsils, adenoids (third tonsil), Peyer’s patches, appendix, and smaller lymphoid masses.

Support: MALT and nearby lymph nodes support each other.

Humoral Immunity:
- Encapsulated lymphoid organs primarily use IgG.
- Mucosa-associated immunity primarily uses IgA.

Question 4

Mechanism of the immune response in the
peripheral lymphoid organs. Clonal selection

Answer 4

1. Antigen Presentation:
   An antigen is presented to lymphocytes in peripheral lymphoid organs until a match is found.

2.Activation:
A successful match between an antigen receptor and an epitope activates the lymphocyte.

3.Proliferation:
The activated B or T cell undergoes multiple divisions, creating a clone of identical cells.

4.Effector Cells:
- Some cells from the clone become effector cells, which are short-lived and act immediately against the antigen and its pathogens.
- B Cells: Effector forms are plasma cells that secrete antibodies.
- T Cells: Effector forms are helper T cells and cytotoxic T cells.

5. Memory Cells:
   Remaining cells become memory cells, long-lived and capable of producing effector cells if the same antigen is encountered later.
6. Clonal Selection:
   The process is called clonal selection because the encounter with an antigen selects which lymphocyte will divide to produce a clonal population specific to the particular epitope.

Question 4

Immune response in the peripheral
lymphoid organs. Clonal selection

Answer 4

B and T Cell:
- Most die in central lymphoid organs shortly after development without functioning.
- Some mature and migrate via the blood to peripheral (secondary) lymphoid organs.
- In peripheral lymphoid organs, foreign antigens activate B and T cells.

Question 5

Differentiation of T-lymphocytes in the peripheral lymphoid organs

Answer 5

Unstimulated (Native) T-Cells:
- Mature T-lymphocytes that haven’t participated in an immune response.
- Includes helper T cells and cytotoxic T cells.

Activation and Differentiation:
-Native T cells proliferate and differentiate into effector and memory cells upon encountering their specific antigen on an activated dendritic cell in a peripheral lymphoid organ.
- Activated dendritic cells display the antigen with MHC proteins and co-stimulatory proteins.

Dendritic Cells:
- Most numerous under the skin and mucous membranes where pathogens enter.
- Known as Langerhans cells in the skin.
- After phagocytosis, dendritic cells travel to the nearest peripheral lymphoid organ to present the antigen.

Question 6

Differentiation of B-lymphocytes in the peripheral lymphoid organs

Answer 6

B Cell Clones and Receptors:
- Each B cell clone makes Ig molecules with a unique antigen-binding site.
- Initially, Ig molecules are inserted into the plasma membrane as B cell receptors (BCRs).

Activation and Differentiation:
- Antigen binding to BCRs, along with co-stimulatory signals from helper T cells, activates B cells.
- Activated B cells proliferate and differentiate into memory cells or antibody-secreting effector cells.

Antibody Secretion:
-Effector cells secrete antibodies with the same antigen-binding site as BCRs.

IgM and IgD Expression:
- After leaving the bone marrow, developing B cells express both IgM and IgD BCRs with the same antigen-binding sites.

• Primary Antibody Response:
  Stimulation by antigen and helper T cells activates B cells to become IgM-secreting effector cells, dominating the primary antibody response.

Class Switching:
- During somatic hypermutation, antigen and helper-T-cell-derived cytokines stimulate B cells to switch from making IgM and IgD to making IgG, IgE, or IgA.
- Some cells become memory cells expressing the corresponding class of Ig as BCRs.
- Others become effector cells that secrete Ig molecules as antibodies.

Secondary Classes of Immunoglobulins:
- IgG, IgE, and IgA retain their original antigen-binding site.
- Referred to as secondary classes of immunoglobulins, they dominate secondary antibody responses and make up the secondary Ig response.

Question 7

Primary and secondary immune response

Answer 7

Primary Immune Response: Occurs on first encounter with antigen.

Characterized by Four Periods:
1. Latent (Incubation) Period:
- Duration: 3-4 days.
- Antigen recognition and interaction with immunocompetent cells until emergence of the corresponding cell clone.

2. Exponential Phase:
   - Duration: 4-5 days.
   - Logarithmic rise in antibody titer.
3. Stationary Phase (Peak Period):
   - Duration: About 10 days.
4. Downtime Period:
   - Duration: 10-15 days.

Secondary Immune Response: Occurs upon re-encounter with the same antigen.

Characterized by Five Periods:
1. Latent Period:
- Much shorter: a few hours.

2. Logarithmic Growth Period:
   - Rapid progression.
3. Expedited Phase:
   - Duration: 1-2 days.
4. Higher Antibody Peak:
   - Significantly increased antibody levels.
5. Longer Attenuation Period:
   - Prolonged duration.

Due to Immune Memory B and T Cells:
- Appear during primary immune response.
- Can circulate in the blood for years.
- Upon encountering antigen, divide and become effector cells:
–> T-effector cells.
–> B-effector cells (plasma cells).