Immune System Flashcards

Immune System and Body Defense

• The immune system protects us from infectious agents and harmful substances.
• It is composed of numerous cellular and molecular structures.
• These structures function together to provide immunity.
• Function is dependent on the specific type of infectious agent.

Types of Immunity

• Nonspecific or Innate Immunity
  • Present at birth
  • Acts against all microbes the same way (nonspecific)
  • First line of defense
  • Second line of defense
• Specific or Adaptive Immunity
  • Involves activation of specific lymphocytes that combat a particular pathogen or other foreign substance
  • T cells and B cells

Innate Immunity

• Protects against numerous different substances.
• Present from birth.
• Does not require previous exposure to a foreign substance.
• Responds immediately to potentially harmful agents.

Innate Immunity: First Line of Defense

1. Skin/Epidermis
2. Mucous Membranes
   • Mucus
   • Lacrimal fluid
   • Saliva
   • Urine
   • Vaginal secretions
   • Defecation
   • Vomiting
3. Chemicals
   • Sebum
   • Perspiration
   • Gastric juice

Innate Immunity: Second Line of Defense

1. Antimicrobial substances
2. Phagocytes
3. Natural killer cells (NK cells)
4. Inflammation
5. Fever

Phagocytosis

1. Chemotaxis: Phagocytes are attracted to microbes.
2. Adherence: Phagocyte attaches to microbe.
3. Ingestion: Phagocyte engulfs microbe.
4. Digestion: Microbe is broken down by digestive enzymes in phagolysosome.
5. Killing: Digested microbe is expelled or becomes residual body.

Innate Immunity: Second Line of Defense - Inflammation

• Nonspecific, defensive response of the body to tissue damage.
  • Redness: Increased blood flow.
  • Heat: Increased blood flow and increased metabolic activity.
  • Swelling: Increase in fluid loss from capillaries to interstitial space.
  • Pain: Stimulation of pain receptors.
  • Loss of function: May occur in severe cases.

Stages of Inflammation

1. Vasodilation and increased permeability of blood vessels
   • Increased diameter of arterioles and increased permeability of capillaries
   • More blood flow to the area
2. Emigration of phagocytes from the blood into interstitial fluid
   • Phagocytes appear within an hour after the inflammatory process begins.
   • WBCs squeeze out of blood vessels into damaged area, with more WBCs following.
3. Tissue repair
   • Within a few days, a collection of dead cells and fluid form pus.

Effects of Inflammation

• Net movement of fluid from blood into infected/injured area and then into lymph system, washing out the area.
• Within 72 hours, the inflammatory response slows down.

Innate Immunity: Second Line of Defense - Fever

• Abnormal elevation of body temperature by hypothalamus, at least $1^\circ C$ from normal $(37^\circ C)$.
• May accompany inflammatory response.
• Results from:
  • Release of pyrogens
  • Toxins from infectious agents
  • Drug reactions
  • Trauma
  • Brain tumors

Benefits of Fever

• Inhibits reproduction of bacteria and viruses.
• Increases activity of adaptive immunity.
• Accelerates tissue repair.
• It is recommended to leave a low fever untreated.

Adaptive Immunity

• Adaptive immunity is the ability of the body to defend itself against specific invading agents.
• Antigens are substances recognized as foreign that provoke immune responses.
  • Antigens are not always bad.
• Immunology is the branch of science that deals with body responses.
• Adaptive immunity has both:
  • Specificity
  • Memory

B Cells and T Cells

• Both are lymphocytes.
• Both develop in primary lymphatic organs.
  • B cells complete their development in red bone marrow.
  • T cells develop from pre-T cells that migrated from red bone marrow into the thymus, where they mature.
• Two types of immunity:
  • Cell-mediated: Cytotoxic T cells directly attack invading antigens.
  • Antibody-mediated: B cells transform into plasma cells, which synthesize and secrete specific proteins called antibodies or immunoglobulins.

Adaptive Immunity - T cells & B cells

• T cells
  • Pre-T cells migrate to Thymus to become mature T cells
  • Mature T cells located in secondary lymphatic organs and tissues
  • Cytotoxic T cells use CD8 protein
  • Helper T cells use CD4 proteins
• B cells
  • Mature B cells located in red bone marrow.
  • Mature B cells located in secondary lymphatic organs and tissues

Antigens vs. Antibodies

Antigens

• Immunogenicity: Ability to provoke an immune response by the production of antibodies, T cells, or both.
• Reactivity: Ability of an antigen to react specifically with antibodies or cells.
• Entire microbes or just parts of microbes may act as antigens.
• Typically, only certain small parts of a large antigen molecule act as the triggers for immune responses.
  • These small parts are called epitopes.

Epitopes

• The small parts of antigens that trigger immune responses are called epitopes.
• The human immune system can recognize and bind to at least a billion different epitopes so the body can recognize nearly everything.

MHC (Major Histocompatibility Complex)

• In the plasma membrane of body cells are "self-antigens" known as major histocompatibility complex (MHC) antigens, also known as human leukocyte antigens.
• Thousands of MHC molecules mark the surface of your body cells as YOU.
• Enable the ability to recognize an antigen as foreign and not self.

How Adaptive Immunity Works

1. Antigens enter the body.
2. A small group of lymphocytes with the correct antigen receptor responds to the antigen and recognizes it.
3. Antigen gets inside cell – T Cells
4. Antigen in ECF – B cells
5. Clonal selection: Lymphocyte proliferates and differentiates in response to a specific antigen, forming many lymphocytes capable of fighting off infection.

Pathways of Antigen Processing

• For an immune response to occur, B cells and T cells must recognize that a foreign antigen is present.
  • B cells can recognize and bind to antigens in lymph, interstitial fluid, or blood plasma.
  • T cells only recognize fragments of antigenic proteins that are processed and presented in a certain way.

Antibody-Mediated Immunity – B Cells

• In antibody-mediated immunity:
  • An antigen is recognized and bound.
  • Helper T cells costimulate the B cell so the B cell can proliferate and differentiate into a clone of effector cells that produce millions of antibodies for about 4 or 5 days.
  • Antigen is marked by antibody for destruction.
  • Memory B cells can quickly proliferate and differentiate into more plasma cells and more memory B cells should the antigen reappear.

Antibodies/Immunoglobulins

• $5$ different classes biologically and distinct chemical structures
• Variable region recognizes antigens

Classes of Immunoglobulins (Igs)

• IgG
  • Most abundant, about $80\%$ of all antibodies in blood; found in blood, lymph, and intestines; monomer (one-unit) structure.
  • Protects against bacteria and viruses by enhancing phagocytosis, neutralizing toxins, and triggering the complement system.
  • Is the only class of antibody to cross the placenta from mother to fetus, conferring considerable immune protection in newborns.
• IgA
  • Found mainly in sweat, tears, saliva, mucus, breast milk, and gastrointestinal secretions. Smaller quantities are present in blood and lymph.
  • Makes up $10-15\%$ of all antibodies in blood; occurs as monomers and dimers (two units).
  • Levels decrease during stress, lowering resistance to infection.
  • Provides localized protection of mucous membranes against bacteria and viruses.
• IgM
  • About $5-10\%$ of all antibodies in blood; also found in lymph. Occurs as pentamers (five units); first antibody class to be secreted by plasma cells after initial exposure to any antigen.
  • Activates complement and causes agglutination and lysis of microbes. Also present as monomers on surfaces of B cells, where they serve as antigen receptors.
  • In blood plasma, anti-A and anti-B antibodies of ABO blood group, which bind to A and B antigens during incompatible blood transfusions, are also IgM antibodies.
• IgD
  • Mainly found on surfaces of B cells as antigen receptors, where it occurs as monomers; involved in activation of B cells.
  • About $0.2\%$ of all antibodies in blood.
• IgE
  • Less than $0.1\%$ of all antibodies in blood; occurs as monomers; located on mast cells and basophils.
  • Involved in allergic and hypersensitivity reactions; provides protection against parasitic worms.

Cell-Mediated Immunity – T Cells

• In cell-mediated immunity:
  • An antigen is recognized and bound by an inactive cytotoxic T cell.
  • It becomes an activated cytotoxic T cell.
• Clonal selection of active cytotoxic T cells:
  • Recognize antigen and deliver a “lethal hit” that kills it.
  • Memory cytotoxic T cells can quickly proliferate and differentiate into more active cytotoxic T cells should the antigen reappear.

Immunologic Memory

• Immunological memory is due to the presence of long-lasting antibodies and memory B and T cells.
• Initial exposure:
  • Can be active infection or vaccine.
• Primary response:
  • Measurable response of antibody production to first exposure.
• Lag or latent phase:
  • Initial period of no detectable antibody – $3$ to $6$ days.
• Production of antibody:
  • Within $1$ to $2$ weeks – levels peaking and generally decreasing over time.

Measure of Immunologic Memory

• Subsequent exposures/secondary response:
  • Can occur after varying lengths of time.
  • Measurable response termed the secondary response.
• Lag or latent phase:
  • Much shorter with subsequent exposures.
  • Due to memory lymphocytes.
• Production of antibody:
  • Levels rising more rapidly.

Immunologic Memory (cont.)

• With subsequent antigen exposure:
  • Many memory cells make contact with antigen more rapidly.
  • Produce more powerful response.
• Pathogen typically eliminated before disease symptoms develop.
  • Ex. person who develops measles will not have it again.
• Vaccines are typically effective in developing memory.

Clinical Connection: Vaccinations

• Weakened or dead microorganism (attenuated).
• Stimulate immune system to develop memory B-lymphocytes.
• Provide a relatively safe means for initial exposure to microorganism.
• If later exposed, secondary response triggered.
• May provide lifelong immunity or require a booster shot.
• Vaccination Schedule

Clinical Connection: Pus and Abscesses

• Pus:
  • Contains destroyed pathogens, dead leukocytes, macrophages, and cellular debris.
  • Removed by the lymphatic system or through the skin.
• If not completely cleared, may form abscess:
  • Pus walled off with collagen fibers.
  • Usually requires surgical intervention to remove.

Clinical Connection: Hypersensitivities

• Abnormal and exaggerated response of the immune system to an antigen.
• Allergies occur when a person is overly reactive or hypersensitive to a substance that is well-tolerated by most others.
  • Acute hypersensitivities occur within seconds.
  • Subacute hypersensitivities occur within 1-3 hours.
  • Delayed hypersensitivities occur within 1 to 3 days.

Clinical Connection: Autoimmune Disease

• An autoimmune disease occurs when the immune system fails to display self-tolerance and, instead, attacks the person’s own body tissue(s).
  • Type 1 diabetes
  • MS
  • Rheumatoid arthritis
  • Lupus
  • Addison’s disease
  • Ulcerative colitis

HIV and AIDS

• Acquired immunodeficiency syndrome (AIDS) is a condition in which a person experiences an assortment of infections due to the progressive destruction of immune system cells by the human immunodeficiency virus (HIV).

HIV and AIDS (cont.)

• HIV – human immunodeficiency virus
• AIDS – acquired immunodeficiency virus
• HIV:
  • Transmitted via exchange of bodily fluids.
  • Resides in body fluids.
  • Early 1980s appearance in the US.
  • $60$ million infections.
  • Epidemic in Africa and Asia.

HIV and AIDS - How HIV Works

• Helper T-cells are destroyed by HIV infection and become dangerously low over the course of time.
• Loss of normal immune function.
• Commonly hear “T-cell count”.
• Experience flulike symptoms.

HIV Transitions into AIDS

• Persons T-cell count falls below $200$ cells per cubic milliliter.
• Opportunistic infections occur.
  • Don’t have an immune system to fight off infections.
  • $80\%$ of all AIDS-related deaths are from opportunistic infections.