Cells and Tissues of the Immune System

Blood cells

-Leukocyte

-Lymphocyte

-Granulocyte

-Mononuclear cells = lymphocyte or moncytes

Leukocyte

Any white blood cell (WBC)

Lymphocyte

a type of WBC (T cell, B cell and NK cell)

Granulocyte =

Polymorphonuclear cells = neutrophils, eosinophils, basophils

Platelets

Important in blood clotting

RBCs =

Red blood cells

Basophils make up

0.5% of WBCs in circulation

Basophils contain

granules (which stain basophilic) filled with inflammatory mediators (histamine, serotonin etc.).

Basophils are important in

allergy and parasitic infections (secondary to eosinophils).

Basophils may or may not

become tissue mast cells

Basophils half life =

1-2 days

Eosinophil make up

1 - 3 % of WBCs in circulation; half-time of 30 minutes

Eosinophils contain

granules (which stain eosinophilic) filled with potent mediators (major basic protein and eosinophilic cationic protein) capable of killing parasites

Eosinophils are important in

control of extracellular parasites + allergic reactions

Eosinophils occur in

some parasitic infections and allergies

Eosinophils half life

in the blood stream for about 30 min, then go to tissues and are mostly found under epithelial surfaces; they live a couple of weeks in the tissues and then replaced by new cells.

Monocyte makes up

3 - 7% in circulation

Monocyte half life

circulate 1 - 2 days then migrate to tissue and differentiate into a macrophage

Monocytes/Macrophages arrive at the site of infection

after neutrophils.

Accumulation of Monocytes/Macrophages at site of inflammation =

sign of chronic infection.

Macrophages are found

in most tissues and are extremely important in immune response

Neutrophils =

Cluster responders

Neutrophils make up

55 - 90% of WBCs in circulation

Neutrophils life span

Short-lived, survive about 1-2 days at most

Neutrophils half-life in blood

is 8-10 hr; about 2.5 times a day all the neutrophils are replaced by new ones

Bone marrow spends a lot of energy

making neutrophils

Bone marrow increases production of neutrophils in response to

bacterial infections resulting in neutrophilia (left shift)

Neutrophilia

elevated neutrophil count in the blood, is commonly seen in animals with bacterial infections

Neutropenia

is lower than normal numbers of neutrophils in the blood stream; can be seen in animals with viral infection.

Neutrophilia job

Attack and destroy pathogens, especially bacteria and fungi, and die in the process which results in formation of pus

Role of neutrophils in immune response

First responders- arrive in substantial numbers within 4 hrs

Neutrophils get to infection by

Exit blood stream at the site of infection and accumulate in large numbers to ingest and kill the pathogens

Lymphocyte makes up

Between 20 - 35% in most animals

Lymphocytes =

B cells, T cells and NK cells

Lymphocytes half-life

Circulate for about 4 months (120 days) between blood and lymphoid tissues searching for antigens. This reentering in blood is unique as other WBCs once exit blood stay in tissues.

Both B and T cells

are part of adaptive immunity

Lymphocytes circulate

-looking for the one antigen they recognize; if do not come in contact with their antigen, they die

-If they meet their antigen, they get activated and some differentiate into memory cells

Endothelial Cells Line Blood and Lymph Vessels

Important for regulating leukocytic traffic

Endothelial Cells Line Blood and Lymph Vessels contain

adhesion molecules, called addressins, that allow circulating leukocyte to know where they are in the body

Addressins

are upregulated during infection to facilitate binding of neutrophils to the endothelial cells and their subsequent exit to the site of infection

All immune cells originate

from bone marrow

Three lineages of immune cells

• Erythroid

• Myeloid

• Lymphoid

Erythroid

RBCs, platelets

Myeloid

Monocytes, neutrophils, eosinophils and basophils, Some dendritic cells – important in initiation of immune response, Mast cells

Lymphoid

B cells, T cells, NK cells, some dendritic cells

Myeloid cells =

• Granulocytes

• Dendritic cells

• Mast cell

Granulocytes

are released in a mature state

Dendritic cells

migrate to tissues and mature. These are important sentinel cells and antigen presenting cells. Key in initiation of adaptive immunity.

Mast cell

precursors leave bone marrow and mature in tissues. Live from weeks to months and important in parasitic infection and allergies.

Lymphoid cells: T lymphocytes

are released immature from the bone marrow as pre-T cells, which go to the thymus to mature.

In the thymus,

a T-cell receptor is generated by DNA rearrangement.

If the newly generated receptor recognizes self-antigen in thymus,

it is eliminated.

If the newly generated receptor recognizes MHC molecule but not antigen

then the T cell matures and goes to secondary lymphoid tissue

Maturation & education means cells are not

Self reactive

Lymphoid cells: B lymphocytes

B lymphocytes are released immature from the bone marrow as pre-B cells, which mature in primary lymphoid tissue

Bird - B lymphocytes =

Bursa of Fabricius

Mammals - B Lymphocytes =

-Bone marrow (many species)

-Peyer’s patch (part of Gut-associated lymphoid tissue, GALT) at ileocecum in some species (ruminants)

In the primary lymphoid tissue,

the B cell develops its receptor (BCR).

If the BCR recognizes an antigen during development,

it is eliminated. If it does not recognize an antigen, it matures and goes to the secondary lymphoid tissues.

Lymphoid cells: NK cells released mature from

the bone marrow, ready to function.

NK cells are not

antigen specific, unlike B and T cells.

NK cells have

no memory and are part of innate immunity.

Differentiation in secondary lymphoid organ occurs when a mature,

naïve B or T cell meets its antigen

B cells differentiate

into effector plasma cells that secrete antibody, or memory cells which are long lived clones of the cell

T cells, in response to their antigen, differentiate

into effector cells or memory cells

clonal expansion of lymphocytes

which is mitosis of lymphocytes, to make many copies of itself.

Sites of lymphocyte development (primary)

• Thymus (T)

• Bursa (B)

• Peyer’s patch (B)

• Bone marrow (B)

Sites where lymphocytes respond to antigens (Secondary)

• Tonsils

• Spleen

• Lymph nodes

• Peyer’s patches

• Bone marrow

Primary Lymphoid Tissues:

Maturation of lymphocytes takes place in primary lymphoid tissues- lymphocytes from bone marrow travel to these tissues to mature.

Primary lymphoid organs T-cells

Thymus

Primary lymphoid organs B-cells

• Birds: Bursa of Fabricius

• Primates, rabbits, rodents: Bone marrow

• Ruminants, pigs, and dogs: Peyer’s patches

Secondary Lymphoid Tissues:

Large number of lymphocytes are located in secondary lymphoid tissues waiting to encounter their antigen. SLT increase chances of a lymphocyte meeting its antigen.

Lymph nodes:

lymphocyte rich tissue connected to lymphatic system, where adaptive immune response to lymph-borne antigen is initiated

Spleen:

site for adaptive immune response to blood-borne antigens

MALT:

Mucosal associated lymphoid tissue, where adaptive immune response to antigens invading from the mucosal surfaces is initiated.