cells and tissues of the immune system

granulocytes

neutrophils

eosinophils

basophils

mononuclear cells

monocytes

lymphocytes

phagocytosis of bacteria

• neutrophls

• monocytes

destruction of parasites

• eosinophils

• basophils (tissue mast cells)

immunity to viruses

lymphocytes

primary lymphoid organs

• bone marrow

• thymus

lymphocyte development

secondary lymphoid organs

• lymph nodes

• spleen

• malt (eg peyers patches)

lymphocyte response to infection

lymphocyte development

• the bone marrow is the source of all lymphocytes

• thesedifferentiate from lymphoid precursors into B or T cells

• B cells remain in B

• Tcells must leave B and goto thymus

thymus

cranial to the heart

lobulated

each lobule has a darker cortex and lighter medulla

T cell development in the thymus

• must generate a unique antigen receptor (TCR)

• mus decide whetehr to become a CD4 or CD8

• immature t cells in the thymus are initially double positive (CD4 and CD8)

• if the tcr perfers mhc class 1 they become cd t cell

• if the tcr prefers mhc class ii they become cd4 t cell

T cell selection in the thymus

• some tcr fail to interact with mhc molecules and are therefore useless

• some tcr recognise peptides from self antigens and are potentially dangerous

• the thymus assesses t cells and disposes of those that ae unreactive or autoreactive

thymic lymphoma

• neoplasia of thymic kymphocytes

• associated with FeLV infection

• anorexia, weight loss, dyspnoea

bursa of fabricius

• found in birds

• cloacal tonsil

• primary lymphoid organ for maturation and differentiation of B cells

• bone marrow in mammals

• bursa in birds

• reaches maximum size around 2 weeks after hatching and then undergoes gradual involution

where are t cells and b cells in the lymph node

• t cells in paracortex

• b cells in follicles

lymph node function

• filters tissue fliod (lymph) and traps foreign organisms

• lymphocytes enter via blood vessels to inspect th anitgens present

• the LN provides an env for recognition of antigens by lymphocytes

• also provide the right conditions for lymphocytes activationa nd differentiation

what do the efferent lymphatics join to form

• thoracic and tracheal duct

where do the thoracic and tracheal ducts empty

cranial vena cava

what are the 2 routes

• free antigen is swept into LN in affernt lymph and is trapped

• specialist antigen presenting cells in hte tissues eat antigen then migrate via the afferent lymph to the LN paracortex

dendritic cells

• differentiate from circulating monocytes

• immature dc usually located at epithelial surfaces where they eat antigen

• migrate via afferent lymphatics to LN during which times they process antigen

• take up residency in the LN paracortex where they present antigen to naive T cells

lymphadenopathy

• enlargement of lymph node

• infection- reactive lymph node

• neoplasia- lymphoma

what can be taken to see what is happening inside the LN

aspirate

lymphoblastic lymphoma

what is this

spleen

in the abdominal cavity

spleen structure

white pulp for haemotological functions

white pulp for immunological functions

• periarteriolar lymphoid sheaths

spleen function

• haematopoietic organ in foetal life

• sore of rbc and platelets

• removal of particulate material from bloodstream

  • foreign organisms

  • old rbc

• immune responses to blood borne pathogens often transmitted by arthropod vectors

what can the spleen be affected by

• haemangiosarcoma in german sheperd dogs

• neoplasia

muscosal associated lymphoid tissue

• respiratory tract- bronchial associated

• git- gut associated

• tonsils are aggregates of lymphoid tissue in the pharynx that initiate immune responses entering the respiratory or alimentary tracts

• initiate immune responses to organisms at muscosal surfaces

peyers patches

found predominantlyin the ileum at the ileocaecal junction

large aggregates of lymphoid tissue found in the walls of the small intestine particularly the ileum and ileocaecal junction

consist of lymphoid follicles of densely packed lymphocytes predo B cells

overlying epithelium containsmicrofold cells

• important in transferring antigen from the intestinal lumen to the lymphoid tissue underneath

B cells which are stimulated with antigen differentiate into plasma cells which secrete IgA which is exported across the mucosal surface into the lumen to prevent pathogens from attaching to mucosal epithelial cells

reactive lymph node

• in a healty animal lymph nodes contain primary follicles

• when there is an infection lymph become activated and multiply and enlarge

• B cells begin to proliferate and differentiate into plasma cells

  • this can be seen histologically as a central lighter region called the germinal centre

  • this is the secondary follicle

• following activation the mature plasma cells migrate to the medullar cords where they begin secreting antibody into efferent lymph which reaches the blood

what are lymph nodes surrounded by

a capsule that is penetrated by many afferent lymphatic vessels

path of lymph

• lymph is filtered as it moved through the lymph node then leaves through a single efferent lymphatic vessel located at the hilius

• lymph travels through a network of lymphatic vessles returning to the circulation via the trachea and thoracic duct

• blood vessels also enter and leave the LN from the hilius

• within the paracortex blood vessels form high endothelial venules that function to allow lymphocytes to exit the bloodstream and enter the lymph tissue to look for antigen

structure of the lymp

• divided into a peripheral cortex and centra medulla

• paracortex in between

• b cells are located in the cortex and arranged into follicles

• t cells are found scattered loosely in the paracotex

• medulla contains many macrophages and plasna cells

• in pgs the lymph node structure is inverted with the medulla surrounded by a central cortex

what are the antigen presenting cells in the paracortex called

• dendritic cells

• they present the antigen to t cell

where is the spleen located and what is the structure of it

• located in anterior left quadrant of the abdomen

• surrounded by a fibrous or fibromuscular capsule

• red pulp functions as teh erythrocyte and platelet store

• white pulp is rich in lymphocytes and is where immune responses occur

  • located around blood vessels forming peri arteriolar lymphnatic sheaths

• marginal zone between red and white is rich in macrophages for trapping particulate material and antigens

• neutrophil

• polymorphonuclear leukocytes

• important in the acute inflammatory response to infection and play a key role in phagocytosis

• eosinophils

• play a major role in inflammation

• selectively recruited to tissue during parasite infections

• desgined to degranulate and release their biologically active molecules onto the surface of pathogens

• cell numbers often raised in parasite infection and allergic disease

• basophils

• contribute to inflammation

• mast cells are widespread throughout the tissues and are prominent around the epithelial surfaces

• cytoplasmic granules contain inflammatory mediators including histamine

• their degranulation results in a potent inflammatory response.

• important in parasite defence

• monocyte

• produced in the bone marrow and circulate before leaving the blood to differenitate into ither tissue macrophages or dendritic cells

• function primarily as phagocytic scavenger cells and are important for bacterial infections

• professional antigen presenting cells

• pick up antigen in infected tissues then move to LN to alert T cell

• lymphocyte

• adaptive immunity cells

• B and T

• circulate in blood and lymph and go to different secondary lymphoid tissues as part of immune surveillance

• important for virus

how to evaluate wbc in a sample

• use a tube containing anti coagulant eg EDTA, heparin

• if you spin the sa,ple the cells are in the pellet and the clear liquid is plasma

• if you dont use anti coagulant and allow blood to clot the clear liquid is serum which can be used to evaluate the soluble mediators of immunity

thymus

• located in the anteiror mediastinum of the thoracic cavity

• size varies with age

  • increases in size from birth to puberty

  • then it undergoes involution and is replaced by adipose tissue

• arranged in lobules with an outer more densely cellular cortex and an inner less cellular medulla

• immature t cells leave the bone marrow and enter the subcapsualr region where they proliferate before migrating through the cortex and medulla where they are assessed for suitability ebfore they are allowed to exit into the ciruclation as functional naive t cells

problems with TCR

• they are made through random re arrangement of variable genes

• some express receptors that fail to engage with MHC which is useless to the immune system and these cells are eliminated

• some receptors are created tha inadvertently recognise self antigen, and these autoreactive T cells must also be eliminated otherwise they would attack healthy tissues after leaving the thymus