Immunology Dr Shoaib Raza

Immune System Defense system Fights against bacterial and other infections Rejects the foreign tissue or organ Necessary for life Can cause disease Too low immunity - Immune deficiency Too high immunity - Autoimmune disorders

Types of Immunity Innate Immunity Acquired Immunity Since birth Nonspecific Exposure independent Similar response Means Skin Mucus membranes Gastric acid secretion Tears (Lysozyme) Inflammation Acquired Immunity Acquired during life Specific Exposure dependant Variable magnitude Means T- Cells B- Cells

Types of Immunity Active Immunity: Passive Immunity: Can be induced by Infection or subclinical infections Antigen administration (Immunization) Passive Immunity: Can be : Transferred from mother to fetus Induced by administration of antibodies

Active versus Passive Immunity Active Immunity Active involvement of immune system Infections, subclinical infection Vaccination Delayed onset Longer lasting Memory cells formation Passive Immunity Passive involvement of immune system Passive transfer of antibodies Preformed antibodies Readily available Transient No memory cell formation

Types of Immunity Cell Mediated Immunity Humoral Immunity Mediated by T-Cells (CD4+ or CD8+ T Lymphocytes) Humoral Immunity Mediated by antibodies formed from plasma cells Plasma cells are formed after activation of B- Cells

Immune System Organs Cells Primary lymphoid organs Bone marrow Secondary lymphoid organs Lymph node, tonsils, spleen etc Tertiary lymphoid organs Formed in follicular inflammation Cells Lymphocytes Macrophages Polymorph cells Other cells like platelets, endothelial cells etc

Lymphocyte 30-45% of all WBC Roughly 2100 cells/mm3 of blood T- Cells (60%) B- Cells (30%) NK Cells (10%)

T- Cells Formed in primitive yolk sac in embryo Migrate to bone marrow Preprocessed and differentiates into Thymus Recognized cell bound antigens Present in the paracortical areas of lymph node Two types: CD4+ T cells – Regulatory cells (2/3rd of all T cells) CD8+ T Cells – Effector cells (1/3rd of all T Cells)

Lymph Node Histology

Thymic Education Preprocessing, processing, differentiation and clonal expansion of T Cells occurs in thymus T Cells enter from primitive yolk sac into thymus TCR + lymphocytes Develop two proteins, CD4 and CD8 Double positive T Cells

Thymic Education In the thymic cortex they come in contact with either MHC I or MHC II Lose one of the protein, i.e. either CD4 or CD8 (Single positive T Cells) Those who do not bind with either MHC I or MHC II are negatively selected for apoptosis Those who bind firmly are selected positively for apoptosis

Thymic Education Thus either CD4+ or CD8+ T cells (single + T Cells) leave thymus and enter peripheral circulation. T-Cells thus educated have following characteristics: Can only recognized antigen presented to them in association with either MHC I or MHC II Do not recognize self proteins as antigens (Tolerance)

T-Cell Activation T-Cell activates through two signals Antigen binds to TCR MHC II or I binds to either CD4 or CD8 Co-stimulatory signal binds to CD3 group of proteins Clonal expansion (proliferation) Memory cell formation

T-Cell Activation

Types of CD4+ T-Cells Th-1 cells: Th-2 cells: Secretes IL-1, IL-2, γ-IFN etc and activates other T Cells (CD8+, CD4+ and induce cell mediated immunity), macrophages etc Th-2 cells: Secretes IL-4 or IL-5, (BCGF & BCDF) and activated B Cells to proliferate and synthesize antibodies (inducing humoral immunity)

B Cells Are present in lymphoid follicles Can recognize circulating antigen (do not require antigen presentation) Gets differentiated into plasma cell in the germinal centre of lymphoid follicle IL-4 and IL-5 from T Cells help in growth and differentiation of B Cells Secretes antibodies

Antibodies These are gamma globulins Secreted by plasma cells Also called as immunoglobulin Five classes Immunoglobulin G Immunoglobulin M Immunoglobulin A Immunoglobulin D Immunoglobulin E

Plasma Cell

To be continued……