How do your body systems protect you from infection and disease?

11.1 Defense Against Infectious Disease

Learning Targets: Describe blood clotting, challenge and response, clonal selection, and memory cells. Explain the principle of

vaccination.

Nonspecific Defenses:

• External defenses – skin and mucus membranes – Act as a barrier, trap

foreign materials– Low pH (such as in

stomach) and secretions (ex. lysozyme in tears) kill some pathogens

• Internal defenses – phagocytic cells (white blood cells that eat invaders)– Phagocytes or

macrophages may circulate or stay in one tissue – they catch microbes with long pseudopodia, and engulf pathogens

• Blood Clotting– Use the video clip

to put the steps in the correct order

• Blood Clotting– Initiated by

chemical signals released by damaged cells

– Platelets adhere to the damaged area and begin to plug the opening

• Blood Clotting– Tissues and

platelets release chemicals that convert prothrombin to thrombin

– Thrombin converts fibrinogen into fibrin which creates a supportive mesh to stabilize the clot

Why are the defenses above called nonspecific?

• These defenses respond to microbes or “invaders” in general, not specific to any bacteria, virus, or fungus.

Lymphatic System – circulates fluid and white blood cells back to blood

• Organs of the lymphatic system include the lymph nodes and spleen, which trap “invaders”

Lymphatic System – circulates fluid and white blood cells back to blood

• The lymphatic system also includes the bone marrow and thymus, where lymphocytes mature

Why does the doctor feel your lymph nodes during a regular checkup?

• Lymph nodes swell when the body is fighting disease, as the white blood cells in lymph nodes multiply to catch disease-causing invaders (pathogens)

Specific Defenses:• Involve lymphocytes (other specialized white

blood cells) responding to antigens• Lymphocytes include B cells and T cells

– Antigens = “antibody generators,” (ex. bacteria, viruses, fungi, cancer) - anything that causes B cells to produce antibodies

– Antibodies are proteins that bind antigens to help the immune system destroy them

• Each B cell and T cell has membrane receptors for one specific antigen

• In your blood cells, a type of recombination leads to millions of unique B cells and T cells so your body can respond to millions of different antigens

• Challenge and Response– A pathogen challenges the immune system and

is eaten by a phagocyte

– Parts of the pathogen (antigens) are presented on the surface of the cell

– A helper T cell with receptors that match that antigen is activated to start the immune response

• Clonal Selection– Helper T cells and B cells

that match the specific antigen (invader) are cloned

– Activated B cells divide and produce plasma cells and memory cells

– Plasma cells produce antibodies that attach to and mark antigens

– Memory cells activate the immune system quickly next time the antigen appears

Why are the defenses above called specific?

• Each lymphocyte (B cell or T cell) recognizes one specific antigen, so responds to one specific pathogen (bacteria, virus, fungus, etc). Each B cell and T cell has unique receptors on its membranes to recognize an antigen.

Immune Response• Primary response – first exposure to an antigen

– Response is 10-17 days while your body goes through clonal selection

– You may get sick in the “lag time” while your body multiplies B and T effector cells to combat the disease-causing antigen

• Secondary response – subsequent exposures to an antigen– Response is 2-7 days; your body makes more

effectors cells than in a primary response, and antibodies/cells have a higher affinity for the antigen

– You won’t get sick because your body remembers how to fight effectively

How is the difference between primary and secondary response related to vaccines?

• We use vaccines – a dead or non-pathogenic version of a disease-causing microbe – to expose our bodies to the antigen. Our bodies go through a primary response and create memory cells. If we are exposed to the disease-causing pathogen, our bodies respond quickly (secondary response) and we don’t get sick.

• Active vs. Passive Immunity

Immunity due to antibodies produced by the organism itself after challenge and response to a pathogen

Immunity due to antibodies acquired from another organism through the placenta, colostrum, or injection

• Polyclonal vs Monoclonal Antibodies

Many B cells are activated and divide in response to various antigens on the surface or one pathogen. Many types of antibodies are produced.

A specific B cell is isolated in laboratory culture to divide and produce one type of antibody.

• Monoclonal antibody production

• Monoclonal antibody production– A lab animal is injected

with an antigen– The activated B cells are

removed from the spleen– The B cells are fused with

tumor cells (=hybridomas)– These hybrids are tested

for antibody production– The desired cells are

cultured to produce the antibody

• Monoclonal antibody uses– Pregnancy tests – antibodies to

hcg (pregnancy hormone) are fused to an enzyme that changes color if antibodies bind the hormone

– Cancer treatment – how do you think antibodies could be used to fight cancer cells?

• Monoclonal antibody uses– Pregnancy tests – antibodies to

hcg (pregnancy hormone) are fused to an enzyme that changes color if antibodies bind the hormone

– Cancer treatment – antibodies could potentially target cancer cell markers and carry toxins to tumors

Self-Recognition• Your immune system must recognize your

own cells so it won’t attack them

• MHC (major histocompatibility complex) molecules are important self markers– MHC molecules bind antigens inside white blood cells

and present them on the surface to activate immune responses

Self-Recognition

– MHC genes affect which antigens your MHC molecules bind, so which antigens your immune system can respond to

– Summarize the dirty T-shirt study and its evolutionary explanation.