Lecture 13 - Organs And Cells Of The Immnue System

ORGANS AND CELLS OF THE IMMNUE SYSTEM

ORGANS OF THE IMMUNE SYSTEM LYMPHOCYTE RECIRCULATION (TRAFICKING) CELLS OF THE IMMUNE SYSTEM Lymphoid Cells (T& B Cells) Mononuclear Phagocytes Polymorphonuclear Cells Mast cells, Dendritic cells

ORGANS OF THE IMMUNE SYSTEM (PRIMARY (Site of lymphopoiesis SECONDARY (Site of immune response generation)

HAEMATOPOIESIS Stem cell

lymphoid stem cellmyeloid stem cell

Pre-T cellPre-B cell

Thymus

mature T cell

Bone marrow fetal liver

mature B cell

CELLS OF THE IMMUNE SYSTEM • Are cells involved in the production of the immune response • Known as white blood cells (Leukocytes) • Haematopoiesis: Stem cells in bone marrow develop into RBCs and various types of WBCs • Regulatory role of growth factors and cytokines

Lymphocytes  One of many types of white blood cells  Originally produced in the bone marrow  Reside in lymphoid organs  Responsible for diversity, specificity, memory and self-non self recognition  Makes 20-40% of cells in WBC, circulate in blood, lymph & lymphoid organs

Subdivided according to function & cell membrane markers to T cells B cells Natural killer cells (NK)

T & B Lymphocytes are small, non-phagocytic agranular cells while NK cells are large granular lymphocytes.

At resting state, cells are described to be naive (unprimed)

Following Ag interaction, they differentiate into i. lymphoblasts & then into effector cells ii. memory cells

 Types of Lymphocytes can not be distinguished by morphology but by various specific surface markers or cluster of differentiation (CD)

 Different markers (CD) associate with certain cell population or activation state of lymphocytes

B lymphocytes  Mature in bone marrow and the fetal liver  Express on their surface I. membrane-bound Ig (IgM, IgD) as receptors for Ag ii. CD19, CD20 etc. iii. MHC-II molecules so could act as APC  Once activated, they develop into plasma cells (actively secreting Ab) & memory cells

T lymphocytes Mature in the Thymus Express cell surface antigen receptor (TCR &

-TCR1 on 5-10% TCR2 on 90-95%)

Natural Killer Cells  Large granular lymphocytes  Do not express TCR or Ig (i.e. has no receptor for Ag)  Act non-specifically against tumor cells & virusinfected cells (but no memory cells produced)  Express CD16 & CD56 markers

Antigen Presenting Cells (APC) Include macrophages, B cells and dendritic cells

APC express both MHC-I and MHC-II molecules on their surfaces

Function of APC: Antigen presentation MHC Proliferation

APC T cell

Cytokine production

Ag Cell-to-cell interaction

This leads to the activation of both humoral and cell-mediated Immunity

Mononuclear cells  Develop in bone marrow

 Found in blood as monocytes & in tissues as macrophages (e.g. kupffer cells, alveolar macrophages etc.)

Function: 1 . Phagocytosis a. Ingestion by formation of pseudopodia b. Digestion in phagolysosome 2 . Antigen presentation, in association with MHC molecules 3. Secretion of various cytokines

Dendritic cells  Unknown origin  Function as APC, express high levels of MHC-II molecules  Present in lymphoid & non-lymphoid organs

(Poly-morphonuclear cells (PMN  Develop in bone marrow, short-lived

 Are granulocytes with characteristic cytoplasmic staining & multi-lobed nuclie

Neutrophils Major granulocyte in circulation, Major phagocytic cells, produced massively during infection

Eosinophils Play a major role in defence against parasitic infections by secreting contents of their granules (Degranulation)

Basophils/Mast cells Play a major role in induction of allergic reactions by secretion of various mediators (e.g. histamine)