ORGANS AND CELLS OF IMMUNE SYSTEM

By

Smt. Shilpa Bandrad

Dept of Microbiology

Smt V. G Degree College for Women

ORGANS OF IMMUNE SYSTEM

• The organs involved in

the development of

immune responses are

called lymphoid or

lymphatic organs.

• The lymphatic organs

are those organs in which lymphocyte maturation, differentiation & proliferation occurs.

• Functionally these can be categorised into primary & secondary lymphoid organs.

PRIMARY LYMPHOID ORGANS

• The primary or central lymphoid organs are those in which maturation of T & B lymphoytes occurs.
• There are two major primary lymphoid organs, one in which T- cells develops i.e, Thymus & the other in which B- cells develop i.e, the Bone marrow.

1. THYMUS

• FUNCTIONS-
• Production of thymic lymphocytes.
• Major site for lymphocyte proliferation in the body.
• Lymphocytes acquire new surface antigen.
• It play important role in the development of CMI.

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2. BONE MARROW

• FUNCTIONS-
• It gives rise to haemopoietic stem cells from which different types of blood cells and platelets develop.
• Majority of the lymphoid progenitors develop into B cells in the Bone marrow.
• During secondary immune response, it produces a large number of plasma cells.

SECONDARY LYMPHOID ORGANS

• The secondary or peripheral lymphoid organs are those organs in which antigen – driven proliferation & differentiation occurs.
• These are concerned with immune reactions & here lymphocytes are made functional.
• This group includes lymph nodes, spleen & mucosal associated lymphoid tissue.

1. LYMPH NODES

• FUNCTIONS-
• Carries nutrients & waste material between the body tissues & blood stream.
• Acts as a filter for the lymph and as an important centre of phagocytosis.
• Helps in the proliferation & circulation of T & B lymphocytes.

2. SPLEEN

• FUNCTIONS-
• Serves as a graveyard for damaged bloodcells.
• It is a reserve tank for blood.
• It acts as a filter for trapping blood borne foreign particles.

3. MUCOSA ASSOCIATED LYMHOID TISSUE (MALT)

CELLS OF IMMUNE SYSTEM

• All the cellular elements of blood arises from haematopoietic stem cells in the bone marrow by the process HAEMATOPOIESIS.
• These haematopoietic stem cells are maintained at stable levels throughout adult life due to their capacity of self renewal.
• The stem cells soon loose their capacity of self renewal and belong to either of the two types of progenitor cells.

MYELOID PROGENITOR CELLS

• Myeloid cells play important roles in engulfing & destroying m.o’s, presenting antigens & secreting cytokines.
• Myeloid progenitors generate progenitors of erythrocyte, mononuclear phagocytes (monocytes & macrophages), granulocytes neutrophils, eosinophils, basophils, mast cells & dendritic cells) & platelets.

1. ERYTHROCYTES

• Erythrocytes are cells that contains hemoglobin and can carry oxygen to the body.
• It is also called a red blood cell (RBC).
• The reddish color is due to the hemoglobin.
• Erythrocytes are biconcave in shape, which increases the cell's surface area and facilitates the diffusion of oxygen and carbon dioxide.

• The main physiological role of red blood cells (RBCs), or erythrocytes is to transport of gases (O₂, CO₂) from the lung to the tissues and to maintain systemic acid/base equilibria.

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2. MONONUCLEAR PHAGOCYTES (AGRANULOCYTES)

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• Monocyte circulating in the blood & Macrohages in the tissue constitute the mononuclear phagocytic system.
• In the bone marrow, during haematopoeisis granulocyte/ monocyte progenitors differentiate into promomonocytes & enter the blood where they further differentiate into mature monocytes.
• Monocytes circulate in blood stream for about 8 hours, increase in size & then migrate into the tissues differentiating into speific tissue macrophages or in dendritic cells.

• Macrophages are present throughout the body, where they may reside in particular tissues or remain motile.
• Wandering & fixed macrophages together form Reticuloendothelial (RE) system.
• Macrophages have a life span of weeks to months.
• Macrohages serve different functions in different tissues.

3. GRANULOCYTES

• Are a category of WBC.
• These cells are called polymorphonuclear leukocytes because of the varying shapes of nucleus, which is lobed in 3 segments in the bone marrow at the rate of 80 millions er minute & lie for 2-3 days.
• The predominant role of granulocytes is phagocytosis.

1. NEUTROPHILS

• They constitute about 45-60% of

total leucocyte count.

• They are actively phagocytic & are

the predominant cell type in

acute inflammation.

• They show antimicrobial activity by generating reactive oxygen intermediates & reactive nitrogen intermediates.
• Neutrophils express higher levels of defensive than macrophages do.

ii. EOSINOPHILS

• Eosinophil comprise 2-5% of

blood leucocytes.

• They have a bilobed nucleus&

cytoplasm show granules which

stain red with acidic stain like

esoin.

• Eosinophils are motile phagocytic cells & can migrate from blood into the tissues.
• They are attracted by products of anaphylaxis which are known as eosinophil chemotactic factor (ECF-A) released from T-cells, mast cells & basophils.

• Are capable of phagocytosing & killing ingested m.o’s.
• Can degranulate by certain stimuli such as parasitic infections.
• They bind with worm larvae (schistosomules) coated with IgG or IgE & degranulating releasing the toxin.
• They release histaminase & sulphatase which inactivate the mast cell products.

iii. BASOPHILS

• Are non-phagocytic cells.
• Comprise less than 0.2% of

leucocytes.

• These cells take deep violet

blue stain with dyes.

• Mature basophils have randomly arranged granules surrounded by membranes.
• They are found in blood & tissues.
• Are stimulated by an allergen.
• On degranulation release histamine which cause the adverse symptoms of allergry.
• Play a role in immunity against parasites.

iv. MAST CELLS

• Are differentiated from their

precursors only after they

leave the blood & enter the

tissues.

• They are of different types-
• The mucosal mast cells (MMC)
• The connective tissue mast cell (CTMC)
• Play an important role in development of allergies & have an important role in immunity against parasites.

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v. DENDRITIC CELLS

• Dendritic cells are

antigen-presenting cells (APCs)

which play a critical role in the

regulation of the adaptive

immune response.

• Dendritic cells are so called because, when they are mature, their cytoplasm extends into transient spiny dendrites and sheet-like veils.
• This provides a large surface area for their main

function of antigen presentation to T-lymphocytes.

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• The dendritic cells are of

4 types- Langerhans cells,

interstitial dendritic cells,

myeloid dendritic cells &

lymphoid dendritic cells.

• Dendritic cells are formed

from both myeloid &

lymphoid progenitors.

• Posses MHC class II antigen.
• Have a major function of presentation of antigen to T4 cells.

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vi. PLATELETS

• Are derived from

megakaryocytes in the bone

marrow & conatins granules

in their cytoplasm.

• Express class I MHC products

& various receptors.

• Play a major role in blood clotting mechanism.
• Are involved in immune responses especially in inflamation.
• The receptors & adhesion molecules present on platelets are important in activation of platelets which then adhere to the surface of damaged tissue.

LYMPHOD PROGENITOR CELLS

• The common lymphoid progenitor has the capacity to differentiate into either T-lymphocytes or B-lymphocytes depending upon the environment.
• Lymhocytes-
• These are the most important cells of the lymphoreticular system.
• These are white blood cells produced in the bone marrow by the process of Haematopoiesis.
• Lymphocytes are 20-40% of WBC, 99% of cells in

lymph.

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• Bridge between various parts of immune system
• Small, round 5-12 m diameter spherical densely

compact nucleus occupies almost entire cell & scanty cytoplasm.

• Lymphocytes are produced

at a very high rate of about

10⁹ per day.

• They occur in bone

marrow, central & peripheral

lymphoid organs.

• There are 3 types of lymphocytes.

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1. B- LYMPHOCYTES

• B lymphocytes represents 3 to 15% of circulating lymphoid cells and are primarily defined by surface immunoglobulins (Ig).
• The B lymphocytes are common

in areas of antibody production,

such as the germinal centers of

the lymph nodes and diffuse

lymphoid tissue of mucosal systems.

• They arise from the precursor cells from the yolk sac, fetal liver & bone marrow.

• During maturation the pre B-cells develop into B-lymphoblasts, which inturn change into immature B-cells with IgM molecule on the surface.
• B cells with IgM on the surface are known as virgin B cells.
• The main function of a B cell is to secrete soluble recognition molecules called antibodies which specifically bind to an antigen recognized by that B cell.
• Throughout the life, bone marrow remains the major repository of stem cells for B lymphocytes.
• A B cell will only produce antibodies when it has been activated by binding antigen; this activation process also usually requires help from T cells.

• The activated B cell undergoes multiple divisions and some of the resulting cells differentiate into antibody-secreting cells.
• These are known as plasma cells, and they possess copious rough endoplasmic reticulum involved in antibody synthesis.
• An important aspect of antigen recognition by B cells is that these lymphocytes, and the antibodies they produce, bind to antigens in their natural or native form, i.e. as they occur as constituents of pathogens

2. T - LYMPHOCYTES

• These cells mature in the thymus

hence called T-cells.

• About 70% of human blood

lymphocytes are T cells.

• T – cells can recognise the antigen

only when it is displayed together with MHC molecule on the surface of APCs or on virus infected cells, cancer cells or grafts.

• T-cell receptor which is a complex poypeptide is expressed by all the subpopulations of T-cells.

• The membrane molecules of T-cells include CD₃, CD₄, CD₈, CD₂₈ & CD₄₅ .
• Two major functional subpopulations of T-cells are T-helper (TH) (CD₄⁺ T-cells) & T-cytotoxic (TC) cells (CD₈⁺ T- cells).
• T-hepler cells-
• Helper T cells are the major driving force and the main regulators of the immune defense.
• Their primary task is to activate B cells and killer T cells.
• When the receptor of a helper T cell recognizes the antigen, the T cell is activated.
• Once activated, helper T cells start to divide and to produce proteins that activate B and T cells as well as other immune cells.

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• T- cytotoxic cells-
• These cells recognize certain histocompatibility antigens and are capable of killing foreign cells (i.e., virus) and altered self-cells (i.e. tumor antigens).
• T-cytotoxic cells are important in the cytotoxicity of graft reactions and graft-versus host reactions.

3. NATURAL KILLER (NK) CELLS

• Natural killer (NK) cells constitute

up to 15% of human blood

lymphocytes.

• Together with δ T cells and about

50% of CD8+ T cells they are known as large granula lymphocytes because, compared with most T and B lymphocytes, they have more cytoplasm and contain prominent granules.

• These are present naturally & are not produced due to antigenic stimulation.

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• They are present in the spleen & peripheral blood.
• In contrast to all T and B cells,

NK cells do not express

antigen-specific receptors and

do not possess the adaptive

property of memory cell

development; they are therefore

considered to form part of the innate immune system.

• Their activity does not require sensitization by prior antigenic contact & so it is ‘natural’ or ‘non-immune’.

• Their activity is increased by interferon.
• Their main function is to kill infected cells and tumor cells by inducing apoptosis of their targets.
• The cytotoxicity of the NK cells is due to the release of certain cytolytic factors like perforin, lymphotoxin & tumour necrosis factor.
• The NK cells are believed to play an important role in antiviral & antitumour immunity.

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