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The Immune System, Viruses, and Disease. Virus essentials. Viruses are not cells Replicate by invading host cell Use host organelles and enzymes to make more viruses Called obligate intracellular parasites
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Virus essentials • Viruses are not cells • Replicate by invading host cell • Use host organelles and enzymes to make more viruses • Called obligate intracellular parasites • Outside cell, virus is lifeless – spread by wind, in water, in food, or via blood or other body fluids
Viral structure Essential features: • nucleic acid – RNA or DNA • capsid – protein coat • surface protein receptors • some have an envelope made of lipids taken from a host cell – influenza, chickenpox, herpes simplex, HIV • Are viruses alive?
Herpes simplex virus emerges from host nucleus into cytoplasm
There are many different species of mosquito, which can carry some of the world's most common and significant infectious diseases, including West Nile, Malaria, yellow fever, viral encephalitis, and dengue fever. (Image courtesy of the Centers for Disease Control and Prevention.)
Defenses Against Viruses Non-specific defenses •skin – with its tough keratin shield •mucous membranes – line respiratory and digestive systems, urethra, vagina; secrete mucus that traps pathogens • cilia – in respiratory tract; sweep mucus and pathogens to pharynx for swallowing •inflammatory response -- includes phagocytes like macrophages; natural killer cells (NK’s) •Interferon -- protein that inhibits virus production; signals healthy neighbor cells to defend themselves •Fever -- when moderate, stimulates bodies defenses; goes beyond best temp for virus reproduction
Defenses cont’d. Specific defenses – Adaptive immunity • Organs -- bone marrow, thymus, lymph nodes, tonsils, adenoids, spleen • Bone marrow – soft, inside long bones; makes billions of new wbc’s every day • Lymph nodes – along lymph vessels; filter viruses and expose them to lymphocytes • Spleen – just behind stomach; filters viruses; stocked with wbc’s
Specific defenses (cont’d.) • Lymphocytes – leukocytes (white blood cells) found in these organs and in blood and lymph; accumulate in lymph and lymph nodes, but are also in spleen and blood • B cells – one of two main types of lymphocytes; produced in bone marrow and mature there • T cells – also produced in bone marrow, but go to thymus to mature
B Cells and T Cells Lymphocytes are one of the five kinds of white blood cells or leukocytes, circulating in the blood. Although mature lymphocytes all look pretty much alike, they are extraordinarily diverse in their functions. The most abundant lymphocytes are: • B lymphocytes (often simply called B cells) and • T lymphocytes (likewise called T cells). B cells are not only produced in the bone marrow but also mature there. However, the precursors of T cells leave the bone marrow and mature in the thymus (which accounts for their designation). Each B cell and T cell is specific for a particular antigen. What this means is that each is able to bind to a particular molecular structure. The specificity of binding resides in a receptor for antigen: • the B cell receptor (BCR) for antigen and • the T cell receptor (TCR) respectively.
How do lymphocytes recognize viruses as foreign invaders? Antigen – any substance that the immune system recognizes as a foreign invader and provokes an immune response Receptor proteins – on surface of every lymphocyte; recognize and bind to antigens that match their their 3-D shape It’s all in the fit! Variety – theimmune system makes millions of different kinds of lymphocytes with different shaped receptors Specificity – due to specificity of antigen receptors on lymphocytes ; some have receptors for a cold virus (more than 100 viruses can cause a cold), some have receptors for a flu virus, etc.
Immune Response A two-pronged attack • Cell-mediated immune response – involves T cells • Humoral immune response – involves mainly B cells • Both controlled by helper T cells
