Secondary Lymphoid Organs and the Microbes They Sample

Question 1

Secondary Lymphoid Organs are the sites where what occurs?

Answer 1

Antigen presentation by DCs and MØs to lymphocytes

Question 2

Lymph Nodes (LNs)

Answer 2

Microbes that invade the skin or internal surfaces since most of our LNs are either subcutaneous or in the gut mucosa

Question 3

Lymph Node Structure and Associated Immune Cells

Answer 3

From the outside in:

• Capsule
• Outer cortex ⇒ B cells live here (Germinal Centers)
• Deep cortex (or paracortex) ⇒ T cells live here
• Medulla

Lymphatic vessels through out - DCs come in through afferent lymphatic vessels. Lymphocytes enter through these and through High Endothelial Vessels (HEVs) in the deep cortex.

Question 4

Germinal Center (GC) Structure

Answer 4

• Light center area of GC = maturing, proliferating B cells (centrocysts)
  
  • Plasma cells (mature B cells secreting antibody leave GC and enter medulla)
• Dark peripheral area of GC = immature B cells (centroblasts)
• Primary follicle = GC that has not been activated and is entirely dark
• DCs and MØs also live in GCs

Question 5

Spleen

Answer 5

The spleen samples blood-borne pathogens. For this reason, there are not afferent lymphatic vessels going to the spleen

Question 6

Spleen Structure

Answer 6

• Capsule (trabecular, vascular sinusoids)
• Red Pulp
  
  • Where old RBCs (are removed from circulated by MØs) go to live their last days
• White Pulp
  
  • Primary follicle & marginal zone ⇒ where B cells live
    
    • These B cells produce a lot of antibody - a lot of nonspecific IgM in response to the antigen
  • Periarteriolar Lypmhoid Sheath (PALS) ⇒ where T cells live
  • Lymphocytes enter spleen via central arterioles

Question 7

MALT

Answer 7

MALT samples microbes (or their products) that have breached mucosal surfaces

(MALT includes BronchialALT and GutALT)

Question 8

Mucosal Epithelial Cells that Contribute to Mucosal Health

Answer 8

• Goblet cells
  
  • In respiratory, GI, and urogenital epithelium
  • Secrete mucus
• Paneth cells
  
  • In GI tract
  • Secrete AntiMicrobial Peptides (AMPs), such as defensins
• Mucosal Epithelial cells
  
  • Transcytose secretory IgA (most abundant made, but not most measured) and sample antigens via TLRs
  • Microfold (M) cells
    
    • In respiratory and GI tract
    • Transcytose antigens from one side to the other (lumen to submucosa)

Question 9

3 types of GALT strutures

Answer 9

• Isolated lymphoid follicles
  
  • Everywhere in gut but ileum
  • Fewer M cells and less Ag sampling ∴ fewer GCs
• Diffuse Lymphoid Tissue
  
  • Are everywhere there are villi (are w/in it)
  • Have interdigitating DCs and intraepithelial lymphocytes
• Peyer’s Patches
  
  • Ileum only
  • Will see many GCs underneath M cells