7 TOB Lymph I

prophase of Meiosis I from a few months before birth until just before or during ovulation

• masses of lymphocytes and associated cells required to mount an immune response
• provide an environment that promotes immune cell-antigen interaction

• 1. as discrete organs covered by either an epithelium or connective capsule
• 2. may exist as isolated masses of cells withIN
• various organs in close proximity to the outside world (lumen)

diagnostic features: highly cellular, only a little CT, lots of cells per area, LARGE nucleus, small cytoplasm, mostly euchromatin (dark) present versus heterochromatin (light)

• 1. bone marrow - development of immunocompetent B-cells
• 

2. thymus - development of immunocompetent T-cells

lymph nodes, spleen, mucosa associated lymphoid tissue

• representative primary lymphoid organ where T-cells develop
• 

cells derived from the thymus and differentiate into several sub types (cytotoxic, helper, regulatory, etc)

• type of lymphocyte is specialized to recognize foreign antigens and, once activated, to produce antibodies specific to those antigens
• activation of B-cells requires helper T lymphocytes
• in order to produce antibodies, activated B cells must move to connective tissues where they undergo terminal differentiation into plasma cells

• T-cell: cell-mediated immunity
• B-cell: humoral immunity

when antigens are marked for an immune response by molecule that enhances phagocytosis

• the cytoreticulum (thymic epithelial cells) gets infiltrated by THYMOCYTES
• cytoreticulum acts as the sole physical support for the developing T-cells

• the thymus
• "starry sky" appearance
• CT capsule
• continuous brached medulla
• septa - CT that separates lobules

• mechanically support ‘nests’ of developing thymocytes
• produce thymic hormones that to promote T cell maturation
• stimulate ectopic expression of “self” proteins so thymocytes learn to distinguish self from non-self
• contribute to the formation of the blood-thymus barrier

• the thymic cortex contains developing lymphocytes (dark pink) and macrophages (white gaps, "stars")
• *up to 95% of thymocytes fail to become immunocompetent and die by apoptosis
• that's why the medulla is lighter than the cortex, because fewer mature T-cells are allowed to enter there, where they can potentially be exposed to the rest of the body

• YES only connective tissue it has
• separated from thymus by epithelial layer

• epithelioreticular cells
• support the lymphocytes in the medulla & cortex
• in the medulla they can form Hassall’s corpuscles
• also function in the weeding out of T-cells that aren't immunocompetent
• (white = macrophages, dark purple dots = lymphocytes)
• in general stains: appear lighter than lymphocytes

• prevents foreign antigens from getting into the compartment where thymocytes are developing and becoming immunocompetent
• composed of an inner endothelial layer and an outer epithelioreticular layer with a thick basement membrane in between that serves as a barrier for foreign material

• without the barrier a foreign protein could enter the thymus, it will be treated as derived from 'self', and all T-cells will be killed off that bind to it
• if you later come in contact with this foreign antigen IT WON'T BE RECOGNIZED AS FOREIGN

• thymic medulla
• has fewer cells (appears lighter), contains numerous blood vessels, many macrophages, epithelial cells that promote expression of self-peptides, Hassall’s corpuscles, & a cortico-medullary boundary characterized by post-capillary, high endothelial venules where immunocompetent T cells exit the thymus to enter the general circulation

• Hassall's corpuscles
• concentric layers of epithelioreticular cells that secrete thymic stromal lymphopoietin (TSLP) which stimulates differentiation of regulatory T-cells

• Hassall’s or thymic corpuscles
• also has adipocytes present
• 

• encapsulated: covered by CT
• unencapsulated: covered by epithelium, associated w/ mucosa

• nodulues (follicles): B-cells & in the center a germinal center
• diffuse: T-cells found between nodules and in the connective tissue underlying most epithelia (NO germinal centers)

• affinity maturation
• cells with the strongest affinity for antigens due to mutations will be selected over those that bind less strongly
• selected cells must migrate to a connective tissue to fully differentiate as antibody producing plasma cells

NEVER: B cells terminally differentiate in plasma cells only in CT

• 1. lymph nodes
• 2. spleen

• Lymphoid Nodules (lymph nodes)
• has a CT capsule (encapsulated)
• filter particulate matter & promote the interaction between antigens and cells of the immune system

• these encompass the UNENCAPSULATED secondary lymphoid tissue
• 1. Peyer's Patches (gastrointestinal)
• 2. Appendix (gastrointestinal)
• 3. Tonsils (gastrointestinal & respiratory tract)
• (other unencapsulated types of secondary lymph tissue = Bronchus ALT, GutALT, SALT)

• epithelial lining
• lamina propria
• muscularis mucosa

• masses of nodular & diffuse lymphoid tissue lying underneath the oral or pharyngeal epithelium
• epithelium frequently has CRYPTS (deep invaginations) which increase surface area and maximize the antigen-lymphocyte interactions
• lymphocytes will often migrate into the epithelium and sample antigen at the surface
• 
• have mast cells, plasma cells and neutrophils are usually present as well as lymphatics to drain the tissue.

• 1) palatine (paired), covered by stratified squamous epithelium
• 

• 2) pharyngeal (single), covered by pseudostratified ciliated columnar epithelium
• 3) lingual (multiple), covered by stratified squamous epithelium
• (MALTs can be distinguished by their epithelial covering)

• lingual tonsil
• stratified squamous non-keratinizing epithelium (SSNKE)

has an undulating basement membrane

• pharyngeal tonsil - pharynx is digestive & respiratory
• it has a pseudostratified, ciliated, columnar outer epithelial layer

• things sticking up = cilia
• pseudostratified
• respiratory epithelia overlying lymphocytes --> has to be pharyngeal tonsil

• Peyer's patch
• nodules (with intervening diffuse lymphoid tissue) in the intestinal mucosa and/or submucosa
• frequently found in the ileum

• cells that sample antigen; have a deep basal invagination that forms a pocket of immune cells
• found in the epithelium overlying the Peyer's patch
• nodules
• (allows immune cells to get close to gut lumen)

• 
• weird shaped cells are M cells

• rich in both nodular and diffuse lymphoid tissue
• Appendix functions in
• immune surveillance (abundant M-cells)
• endocrine organ
• reserve of gut flora (following loss due to diarrhea, toxic compounds)

• MALT
• lymphoid tissue that isn't necessarily an organ can be anywhere pathogens can access the body
• just a clump of lymphocytes near a lumen should be a BALT or MALT indicator