2 Lymphoid System

Question 1

What is an antigen?

Answer 1

It is a specific marker protein on a cell surface

- for recognition

Question 2

What are the 3 main types of T cells?

Answer 2

• Cytotoxic (Killer) T cell
• Helper T cell
• Regulatory T cell

Question 3

What is the Major Histocompatibility Complex (MHC)?

Answer 3

MHC is a collection of genes coding for MHC molecules
- on the surface of all nucleated cells of the body

Have a role in adaptive immunity. Two types:

• MHC1 - Found on all nucleated cells
  > present normal self-antigens, abnormal/non-self pathogens to effector T cell
• MHCII - Only found on macrophages, dendritic cells, and B cells
  > they present abnormal/non-self pathogen antigens for initial activation of T cells

Question 4

What is an Antigen Presenting Cell (APC)?

Answer 4

APC

• heterogeneous group of immune cells
• classical APCs: macrophages, dendritic cells, B cells

They have the ability to present antigens specifically for the purpose of activating T cells
- they process + present antigens for recognition by T cells

Question 5

What is the immune system?

Answer 5

Defends the body against infectious organisms + pathogenic agents + non-infectious substances + transformed cells + products

Question 6

What is the lymphatic system?

Answer 6

The morphological counterpart of the immune system

- group of cells, tissues, organs that monitor body surfaces + internal fluid compartments

Question 7

What is lymphoid tissue?

Answer 7

It forms the basis of the immune system of the body

- it reflects the physiological role of the lymphatic system

Question 8

What are the primary lymphatic organs?

Answer 8

Bone marrow

• T + B cell differentiation
• B cell maturation

Thymus gland
- T cell maturation

Question 9

What are the secondary lymphatic organs?

+ what is its significance

Answer 9

Lymphocytes enter the blood or lymphatic vessels to colonise

• secondary lymphatic organs and tissues
• where they undergo final stages of antigen-dependent activation

Organs:

• Spleen
• Lymph nodes
• aggregation of lymphatic nodules

Question 10

Describe the circulation of lymph throughout the body

Answer 10

• Blood capillaries (blood)
• Interstitial space (fluid)
• Lymph capillaries
• Lymph vessels
• Lymph nodes
• Lymph vessels
• Lymph ducts
• Subclavian vein

AND back into the blood capillaries (blood)

Question 11

What is a lymph node?

Answer 11

It is a secondary lymphatic organ

• Lymph nodes filter lymph along the pathway of lymphatic vessels
• AND initiate an adaptive immune response to antigens

Question 12

Describe the movement of lymph into the lymph node

Answer 12

Enters through AFFERENT (in) lymphatic vessel

Exits through EFFERENT (out) lymphatic vessel

Lymphocytes can + do travel in lymph vessels, but majority (90%) travel in blood stream

• They can enter the lymph nodes at the sires of HIGH ENDOTHELIAL VENULES (HEV) in the paracortex
• (they have cuboidal endothelia)

Question 13

Describe the general architecture of the lymph node?

Answer 13

Outer layer - Cortex (2 parts)

• superficial (1st half)
• deep/paracortex

Inner layer - medulla

Question 14

What cells are in the Cortical zone? (superficial cortex - outer)

Answer 14

B cells

Question 15

What cells are in the Paracortical Zone (inner cortex?)

Answer 15

T cells

- this area contains the majority of APCs in the node

Question 16

What cells are in the Medulla (most inner layer)?

Answer 16

• HIgh concentration of plasma cells

- also macrophages for cleanup or holding undigestable material

Question 17

Describe the Connective tissue in the Lymph node?

Answer 17

• Capsule - outer layer - Dense CT
• Trabeculae - dense CT (invaginations)
• Reticular tissue - supportive mesh work:
  > Reticular cells + fibres
  > Dendritic cells
  > macrophages

Question 18

Where are the follicles located in the lymph node?

Answer 18

In the cortex

Question 19

Describe the primary follicles found in the lymph cortex

Answer 19

In the absence of immune stimulation, the cortical lymphoid follicles exist as PRIMARY FOLLICLES
- which are made of small B cells (virgin B cells or recirculation memory B cells)

There is also fine meshwork or dendritic reticulin cell cytoplasm

Question 20

Describe the formation of secondary follicles

structure, activation, function etc.

Answer 20

With antigenic stimulation

• antigen recognising B cells are stimulated to replicate and differentiate
• This converts the Primary Follicle to SECONDARY FOLLICLE or GERMINAL CENTRE (indicates B cell response)

The Inner layer is the germinal centre

The GC is surrounded by a mantle zone (MZ)

• of small lymphocytes
• and a control area with replicating follicular centre cells + differentiation progeny

Question 21

Describe the significance of the Germinal Centre in the secondary follicle

Answer 21

The inner layer is Germinal Centre

• GC shows areas of B cell maturation, where some activated B cells die off
• others differentiate to majority plasma cells, some into memory cells
• once zoomed in, paler and larger nuclei show Follicular Dendritic Cells, where antigens are held and presented during maturation

Question 22

Describe the events that occur in the lymph nodes’ follicles, once an antigen is being presented

Answer 22

Antigen presentation occurs in the paracortex (T cell area)
- leading to T cells activation and proliferation

Meanwhile, B cells have picked up the antigen in the follicle
- ingested it and move to the B/T cell zone interface (edge of 2 cortical layers)

The activated T and B cells meet at the B-T cell interface zone

• The T helper cell-specific for this antigen activates the B cell
• to proliferate and mature

The activated B cells move back into the cortex

• to proliferate and mature
• This results in the formation of the germinal centre