Lymphoid Tissue

Question 1

What are lymphoid tissues?

Answer 1

Organs in which lymphocyte production, maturation (primary), and differentiation occur (often within secondary).

Reactions to antigens and proliferation of immune cells take place.

Question 2

Name primary lymphoid tissue

Answer 2

• where lymphocytes are generated and matured

1. Bone marrow
2. Thymus
3. Bursa of fabricus
4. Ileal Peyers patch
5. Appendix

Question 3

Talk about the primary lymphoid tissue

Answer 3

• where lymphocytes are generated and matured

1. Bone marrow
2. Thymus
3. Bursa of fabricus (in birds) - B cells educated
4. Ileal Peyers patch - maturation/ education T cells in sheep, cattle, pigs, dogs and horses)
5. Appendix (caecal patch in rabbits)

Question 4

Name secondary lymphoid tissue

Answer 4

• where differentiation occurs
  Where lymphocytes interact with Antigen Presenting Cells (APC):
  Or innate cells react with pathogens or portions of pathogens.

1. Lymph Nodes
2. Mucosal-associated lymphoid tissue (MALT) (tonsils, gastrointestinal tract, nasopharynx, thyroid, breast, lung, salivary glands)
3. Spleen

Question 5

What is the difference between primary and secondary lymphoid tissue and infection

Answer 5

infections within in primary lymphoid tissue, prognosis for animal is often quite poor, pretty severe. Infections within secondary LT is pretty standard and prognosis is much better

Question 6

Name examples of sources of lymphocytes, sites of development and sites where lymphocytes respond to antigens

Answer 6

1. sources = yolk sac, fetal liver, bone marrow
2. Development: Primary LT = Thymus, Bursa. Peyer’s patches, bone marrow
3. Respond to antigens (2nd LY) = MALT (tonsils etc), spleen, lymph nodes

Question 7

Thymus about
what occurs?
when most active?
What it contains?

Answer 7

1. small thoracic organ near heart

ROLE:
• T must eliminate T cells that can cause autoimmunity
• Produced T cells that recognise antigen

If fail these (cortex are broken down by macrophages)
If pass, into medulla and become specialised CD4 (MH2) or CD8 (MH1)

2. lymphocytes, produced by BM transported to T in blood, mature and selection occurs here (primary LT)
3. T lymphocyte maturation and selection occurs here
4. Most active in young, shrinks and atrophies, producing fewer T cells when animal ages
5. • Contains: immature T cells, mature T cells, epithelial cells lining T & dendritic cells. Hassall corpuscles (how T cells gain access into T. Specialised vessel that eventually splits off from standard blood flow)

Immature T cells = thymocytes, can say immature

Question 8

Structure of thymus

Answer 8

1. outer fibrous capsule
2. Lobules, within each lobule you have:
   a) CORTEX - looks darker, densely populated with lymphocytes immature and mature, occasional macrophage which phagocytose thymocytes which have undergone apoptosis
   b) Thymocytes mature to adult T cells in the cortex and migrate to MEdulla

c) MEDULLA - lighter in colour, Meet antigen presented to them, Thymic selection (majority die) – survivors become functional T-cells., Survivors go into circulation and move through 2° lymphoid tissues

Question 9

Passage through thymus

Answer 9

• Immature thymocytes enter via cortex and migrate through medulla.
• Various cell contacts in this process result in 95% of T cells being eliminated on the journey
• Process thymic maturation removes most self-reactive T cells. (T cells that would recognise proteins, molecules or complexes from host animal as being pathogenic and would ATTACK in an immune response, triggering auto immunity