> See also: > [[The Lymphatic System]] # Lymph Nodes ## Connections to The Lymph Node Lymph **arrives** to the lymph node via **afferent lymphatic vessels** - Afferent (Arriving) Efferent (Exiting) [[Lymphocytes]] arriving at a lymph node **segregate to different areas** of the node. A **draining lymph node** is a node *closest to a site of infection*, where extracellular fluids containing antigens and immune cells from the site are transported. ## Lymph Node Structure ![[Pasted image 20220908160256.png]] Lymph nodes are bean-shaped structures th | Name | Relevant cells | Description | | --- | --- | --- | | Paracortex | T Cells | The inner cortex area of the lymph node. | | Medulla | Plasma Cells | | | Afferent Lymphatic Vessel | | | | Efferent Lymphatic Vessel | | | | Germinal Center | B Cells | B cells remain here until activation via T cell | Effector T cells and antibodies leave through the efferent lymphatic vessel - **Paracortex** *(T Cell Area)*: The inner cortex area of the lymph node. - **Medulla** (Plasma Cell Area): Lymph and dendritic cells carrying pathogens from the infected site arrive at a lymph node via the afferent (arriving) lymphati ![[Lymph Nodes Structure.png|300]] ### Secondary vs Primary Follicles B-Cells will clump together within lymph nodes in **primary follicles** If a B-Cell is activated within the primary follicle, it will begin to form a **secondary follicle** which contains a **germinal center** - Once a B-Cell successfully recognizes an antigen, it will begin expressing the CD69 receptor which will prevent the expression of the S1P receptor which allows them to leave the B-Cell area and enter the medulland X‑Gal that produces blue colonies  an LB agar plate w ### Germinal Centers (B-Cell Areas) At germinal centers, B-Cells will be activated when they come into contact with an antigen-presenting cell carrying the complementary antigen to that cell's receptor **The Dark Zone** - In the dark zone of the germinal center, *centroblasts* (recently activated B-Cells) will rapidly proliferate in an extremely high density. - While in this state, these B-Cells will be undergoing **somatic hypermutation** as well as [[Antibodies#Antibody Isotypes|isotype switching]] to increase their affinity for the present infection and to tailor the function of their antibodies to the type of enemy present - They will not be presenting their BCRs on their surface during this process **The Light Zone** - After proliferating in the dark zone, *centrocytes* (still B-Cells) will move to the less dense light zone where they will more easily encounter [[Dendritic Cells|follicular dendritic cells]] and [[Helper T-Cells|Tfh]] cells. - This process is known as **affinity maturation** - The newly proliferated B-Cells are running through yet another set of trials to see if the recent hypermutations made any receptors that did a gooder job of binding to the antigen than the original B-Cell had