IMMUNE SYSTEM

Immune system = defense against infectious organisms, and cancerous body cells

Potential infectious agents (pathogens) = bacteria, protists, fungi, animals (parasites), viruses

(Note: Most micro-organisms are harmless, including many that live in and on us. Only a few are harmful.)

Barriers to infectious agents - structural features and secretions that help prevent access of pathogens

Question: List as many barriers to infection that you can think of.

NON-SPECIFIC IMMUNE RESPONSE

Means of destroying novel invaders - no prior exposure necessary for this response

Inflammatory response

• damaged tissues release histamine
• histamine causes localized vasodilation, bringing more leukocytes (macrophages)
• increases capillary permeability

Question: How do the effects listed above contribute to defense against pathogens?

Question: During an allergic response, the immune system responds inappropriately to an antigen and the inflammatory response is triggered. What sort of drug could help reduce the allergy symptoms?

Question: During severe allergic response, there may be massive, widespread release of histamine. What would happen to the blood pressure in this circumstance?

Fever

• chemicals released by pathogen and/or leukocytes initiate increase in body temperature
• body regulates at a higher set point - above optimal temperature for pathogen growth
• enhances several immune responses

Question: Many experiments have been done in which an animal is infected with a pathogen, and then prevented by various means from raising its body temperature. Based on the information above, predict the results of these experiments on recovery time and survival.

Several other non-specific immune responses occur but will not be covered.

SPECIFIC IMMUNE RESPONSE

Response to particular type of foreign cell; fastest and most effective response occurs after a prior exposure

• Primary immune response = what happens on first exposure; takes 10 - 14+ days to peak
• Secondary immune response = what happens on subsequent exposures; takes 2-5+ days to peak, stronger response, more prolonged; symptoms reduced or absent

Antigen - material that stimulates an immune response

• generally a protein or complex carbohydrate on a foreign cell

Antibody - protein produced by B cells in response to antigen

• binds to antigen and inactivates the pathogen
• very specific for a particular antigen

Antibody-mediated immune response - mostly effective against viruses, bacteria

B-cells

• formed in bone marrow; found in blood, in all tissues, in spleen and lymph nodes
• millions of clonal lines of B cells, each line with specific antibodies on surface
• initially inactive, become activated when encounter antigen specific to its antibody
• undergo rapid cell division; great increase in cell number of that specific clonal line

plasma cells - fate of some activated B cells

• produce more antibody at high rates
• fight current infection
• live a few days

memory cells - fate of other activated B cells

• produce only small amount of antibody during current infection
• live many years - up to a lifetime
• fight future infections by same pathogen
• on subsequent exposure, immediately respond by producing more plasma and memory cells

What do antibodies do?

• agglutination - cause pathogens to clump together, easier for macrophages to destroy
• neutralization - block action of pathogens by binding
• membrane attack complex - make holes in cell wall of bacteria - cell bursts

Question: Suppose a person did not have the clonal line of B cells specific for disease X. What would happen when the person was exposed to disease X?

Types of immunity

Active immunity

• results from prior infection with particular pathogen (process described above)
• can also be achieved by injection with modified pathogen

Passive immunity

• results from being given already-formed antibodies
• injection of antibody
• through colostrum (early milk)

Question: Which type of immunity would last longer? Which would be short-lived? Why?

Question: In what way would a pathogen be modified to use in a vaccine?

Cell-mediated immune response

Cytotoxic T cells

• effective against infected body cells and cancer cells
• formed in bone marrow, mature in thymus
• millions of clonal lines of T cells, each line specific to a particular pathogen
• during maturation develop ability to recognize "self" and "non-self" cells
• self - determined by MHC genes (major histocompatibility complex); code for specific proteins found on surface of all nucleated cells of body; unique combination in each individual

Activation of T cells

• specific clonal line of T cells activated when encounter infected cell with non-self antigens on its surface
• undergo rapid cell division
• produce active cells and memory cells
• active cells destroy infected cell and pathogen
• memory cells available when same antigen encountered again

NOTE:

Similarity in basic pattern of response via antibody-mediated and cell-mediated pathways

Helper T cells

• produce interleukins
• stimulates proliferation of B cells and cytotoxic T cells
• crucial for activation of B cells and cytotoxic T cells

Suppressor T cells - involved in stopping immune response after infection has been eliminated

Question: What are the key differences between B cells and cytotoxic T cells?

Question: Explain in your own words why you get sick when first exposed to a pathogen (e.g. measles), then recover, and then don't get sick from the same pathogen again. Include the details of how this works.

Question: Which type of cell would be responsible for rejecting an organ transplant? Why would donor organs be targeted? How could donor organs be chosen to reduce the likelihood of rejection?

Question: Immunosuppressant drugs are given to help prevent rejection of donor organs. How should these drugs be designed to reduce rejection, but not completely compromise immune function?

Question: Which type of cell would be responsible for destroying cancerous cells? Why would cancerous cells be targeted?

Question: HIV infects helper T-cells. Why would this compromise the person's ability to fight other infectious organisms?