Immune System :Immune System


Immune System :Immune System • Keeps animals alive by fighting foreign materials and organisms • 3 levels of defense: – Physical barriers – Innate immune system – Adaptive (Acquired) immune system


1) Physical Barriers :1) Physical Barriers Skin, scales, and other tough barriers that are difficult to penetrate • Acids, enzymes (present on the outside of the skin and in glands) kill some intruders • Physiological conditions (such as temperature, pH level) do not provide for a good breeding/living environment for many foreign organisms


2) Innate Immune System :2) Innate Immune System • Attack foreign objects that get into the organism • Usually by engulfing the intruders • Phagocytotic cells (roaming scavengers; e.g. macrophages) ingest foreign material they find, digest them, and expel the byproducts into the environment


3) Adaptive Immune System :3) Adaptive Immune System • Adaptively acquired throughout the organism’s life • Main workhorses: B & T Lymphocytes, specific types of white blood cells (leukocytes) • White blood cells: ~25% lymphocytes, ~60% neutrophils (similar to phagocytes), ~15% others


Adaptive Immune System :Adaptive Immune System • Must be activated by presence of a foreign substance (called an antigen) • Antigens: molecules on surface of viruses, bacteria, mold spores, cancel cells, pollen, dust, etc. • Antigen elicits an immune response • Antibody (protein) attaches to a specific type of antigen


lymphocytes :lymphocytes White blood cells normally found in the lymphatic system • Originate from stem cells in bone marrow • Specialize into B cells (in the bone marrow) and T cells (in the thymus)


B-lymphocytes :B-lymphocytes • B cells produce antibodies (immunoglobulins) that have parts called antigen-binding sites • Antigen-binding sites can bind to localized regions of antigen molecules (key-lock mechanism) • Thus, an antibody can recognize a specific antigen • At maturity, B cells shuffle gene segments to produce millions of variants that are present in the organism


Clonal Selection (B cells) :Clonal Selection (B cells) Relies on the millions of different B cell variants (varying in the structure of the antigen receptors) • When an antigen in the body is recognized by a B cell, the cell receives a signal and starts to massively divide • Thus, clones of the antigen-specific B cell provide basis for the immune response


Result of Clonal Selection :Result of Clonal Selection • Result: – Pool of plasma cells (B-cells made for massive production of antibodies); an antibody being a secreted version of the antigen receptor – Pool of memory cells (copies of the original B cell that was activated by the antigen); due to memory cells in the system, secondary response to the antigen is much faster


Antibody Purpose :Antibody Purpose • Neutralization of antigens • Agglutination of microbes • Precipitation of dissolved antigens • > facilitates phagocytosis • Activation of complement • > leads to cell lysis


Antigen-Presenting Cell :Antigen-Presenting Cell • Macrophages ingest foreign substances and break them down into fragments (antigens) • Proteins bind the antigens and display them on the cell surface forming an antigen-presenting cell (APC) • Helper T cells bind to the exposed antigen/protein combination


T-lymphocytes :T-lymphocytes Helper T cells binding to APCs stimulates cell division of the T cell and activates other T cells (such as cytotoxic T cells) and B cells. • Cytotoxic T cells kill infected cells (exposing an antigen) by cell lysis


References :References • Course notes: http://pages.cpsc.ucalgary.ca/~jacob/Courses/Winter2005/CPSC565-607/Slides/11- ImmuneSystem.pdf • Web sites: http://www.bio.miami.edu/tom/bil255/bil255goods/22_immune.html http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/ClonalSelection.html