BIO 202 Immune Defenses – Part I Innate Defenses Viruses and Bacteria: An Introduction ∙ Viruses are nonliving biological agents that invade cells ∙ Viruses consist of o A nucleic acid core ∙ Either DNA or RNA o A coat of protein molecules called the capsid o Some may have an additional lipid envelope ∙ Many bacteria perform useful functions, while others cause serious diseases (E. coli) ∙ Bacteria are classified living organisms that reproduce on their own without taking over host cells ∙ Bacteria are prokaryotes and do not have a nucleus ∙ Genetic material consists of a single DNA chromosome o May contain extra‐chromosomal “plasmid” DNA ∙ Bacteria typically do not invade cells ∙ Bacteria may produce toxins ∙ Bacteria respond to antibiotics; viruses do not Immunity ∙ Resistance to disease ∙ Immune system has two intrinsic systems o Innate (nonspecific) defense system o Adaptive (specific) defense system Antigens ∙ Substances that trigger an immune response ∙ Antigens are antibody‐generating substances ∙ Large molecular weight molecules, particularly proteins and polysaccharides ∙ Small molecules generally do not generate an immune response o Some molecules (penicillin, formaldehyde, poison ivy toxin) will bind with cellular proteins forming unique (foreign) protein complexes ∙ Microbes and viruses contain foreign proteins and sugars on their surfaces act as antigens ∙ Cells transplanted from one person to another elicit immune responses due to the individual “cellular fingerprint” of each cell ∙ Cancer cells produce slightly different “cellular fingerprints”, causing an immune response