Introduction of Immunology

Outline Immunology Immune system & immunity Types of immunity Details of innate immunity Details on phagocytosis Natural killer cell Immune response Details of immune response

Immunology : A science that studies the structure,function and mechanism of immune system which protects body from infections. The term “Immunology” originated from the Latin word- immunitas. Marcus Annaeus Lucanus first applied the word “immunes”. In 14 th century,Colle used this word while describing plague epidemic. In 19 th century,the word “Immunology” attained currency due to Edward Jenner,who discovered vaccine. In late 19 th century,the discoveries of Koch and Pasteur gave a new turn to immunology.

Immune system : System of biological processes of cells,tissues and molecules that mediates resistance to infections. Immunity : Ability of a host to protect itself against pathogen and their toxic effects.

Types of immunity Innate immunity Natural resistance with which a person born. 1 st and 2 nd line of defense. Relies on mechanisms that exist before infection. Handled by epithelial barrier,enzyme,phagocytosis etc. Adaptive immunity Activate against pathogens that overcomes innate immunity. 3 rd line of defense. Relies on mechanisms that are adapted after infection. Handled by B- lymphocyte and T-lymphocyte.

Body defenses Body defenses

Principle mechanism of innate & adaptive immunity

Details of innate immunity Details of innate immunity Components of innate immunity  Mechanical and chemical barrier  Phagocytosis  Fever  Inflammation  Acute-phase proteins

Details on phagocytosis Phagocytosis : A process of engulfing and ingestion of harmful particles by phagocytes. Cells that participate in phagocytosis are called phagocytes. Cells of phagocytosis are- macrophage,eosinophil and neutrophil etc.

Major steps in phagocytosis Adherence of the particle to the cell membrane receptor of phagocyte. Engulfment of the particle by phagocyte. Formation of phagosome- a large intracellular vesicle. Fusion of phagosome with lysosome forms phagolysosome. Various enzymes get activated and kills or digests the ingested particles. Release of debris or undigested remnants.

Phagocytosis process Phagocytosis process

Role of phagocytes in innate immunity Entry of pathogen Recognition of pathogen Induction of inflammation Attachment of cells to infection site Phagocytosis and killing Pathogen elimination Tissue repair and remodeling

Inflammatory response

0vercoming phagocytic defenses Micro-organisms get killed in phagocytosis process but some can successfully overcome this phagocytic defenses.Like- By producing cell bound coagulase on bacterial surface- Staphytococcus aureus. Antigen containing bacteria- E. coli. By inhibiting chemotaxis containing- Streptococcal streptolysin. By preventing the fusion of phagosomes with lysosomes- M. tuberculosis. By escaping into cytoplasm- Rickettsia. Due to the ability to survive inside phagolysosomes- Bacillus anthracis.

Natural killer cells NK cells or null cells: NK cells are large granular lymphocytes Produced in the bone marrow Found throughout the body,mainly in blood Participate in both innate and adaptive immunity Lack B-cell and T- cell receptors hence called null cells

Mechanism of NK cells 1)Recognition of infected cells: -All nucleated cells in the body express molecules on their surface called Major Histocompatibility Complex (MHC) proteins. -Cells lacking the normal pattern of expression of MHC molecules are recognized and killed.

2)Killing strategy Mode of NK cells mediated killing involves following two pathways- a)Death receptor dependent pathway -Involves an attack on the target cell’s membrane. -Involves an attack on the target cell’s membrane. -Death receptors include receptors for Fas,TNF,TRAIL etc. -Death receptors include receptors for Fas,TNF,TRAIL etc. -Ligation of receptor with membrane bound ligand on NK cells can result in death of the cell bearing the receptor. -Ligation of receptor with membrane bound ligand on NK cells can result in death of the cell bearing the receptor.

b) Cytotoxic granule dependent pathway -Binding of NK receptors to the surface of the infected cell triggers the release of cytotoxic protein- perforin and granzymes. -Binding of NK receptors to the surface of the infected cell triggers the release of cytotoxic protein- perforin and granzymes. -Perforin forms a pore in the target cell’s membrane for delivery of granzymes. -Perforin forms a pore in the target cell’s membrane for delivery of granzymes. -Granzymes induce apoptic cell death. -Granzymes induce apoptic cell death.

Figure of mechanism Receptor mediated pathway Cytotoxic granule pathway

Details of immune response Mechanism of immune response-  Humoral or antibody mediated: -Involves the production of antibody -Involves the production of antibody -Mediated by B-lymphocytes -Mediated by B-lymphocytes  Cellular or cell mediated: -Involves the production of cytotoxic T-cell -Involves the production of cytotoxic T-cell -Mediated by T-lymphocytes -Mediated by T-lymphocytes