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-cells primary enact ________ immunity while B-cells primarily participate in _________ immunity
T-cell: cell-mediated immunity
B-cell: humoral immunity
when antigens are marked for an immune response by molecule that enhances phagocytosis
thymic epithelial cells form a ____________ that becomes infiltrated by T-cell precursors called:
the cytoreticulum (thymic epithelial cells) gets infiltrated by THYMOCYTES
cytoreticulum acts as the sole physical support for the developing T-cells
What is this?
"starry sky" appearance
continuous brached medulla
septa - CT that separates lobules
thymic cortex contains epithelial cells that function to:
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
Is there connective tissue (from the CT capsule and running through the septa) in the thymic medulla?
YES only connective tissue it has
separated from thymus by epithelial layer
What are the brown stains?
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
What might happen with a faulty blood-thymus barrier?
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
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
concentric layers of epithelioreticular cells that secrete thymic stromal lymphopoietin (TSLP) which stimulates differentiation of regulatory T-cells
What persists in an involuted thymus?
Hassall’s or thymic corpuscles
also has adipocytes present
What is the difference between an encapsulated secondary lymphoid tissue and an unencapsulated secondary lymphoid tissue?
encapsulated: covered by CT
unencapsulated: covered by epithelium, associated w/ mucosa
ALL secondary lymph organs contain BOTH nodular and diffuse lymphoid tissue
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)
Why aren't all B cells daughter cells the same if they divide by mitosis?
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
Will plasma cells (differentiated B cells) appear in nodules?
NEVER: B cells terminally differentiate in plasma cells only in CT
examples of encapsulated secondary lymphoid tissue
1. lymph nodes
Lymphoid Nodules (lymph nodes)
has a CT capsule (encapsulated)
filter particulate matter & promote the interaction between antigens and cells of the immune system
MALT (mucosa Associated Lymphoid Tissue)
these encompass the UNENCAPSULATED secondary lymphoid tissue