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6.3: Defense Against Infectious Disease

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Explain how the human body defends itself against pathogens.

Essential Idea: The human body has structures and processes that resist the continuous threat of invasion by pathogens.
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The first time of defense against pathogens is provided by skin and mucous membranes. Skin provides a physical barrier and mucus traps pathogens. Tears and mucus contain the enzyme lysozyme which destroy bacterial cell walls. Additionally, stomach, skin and vaginal mucus produce acid which kills pathogens. For example, sebaceous glands in the skin secrete fatty acids that make the surface of the skin acidic.

If there is a cut in the skin, platelets activate the clotting cascade, creating a clot so pathogens cannot enter.

If a pathogen makes it through those physical and chemical defenses, then phagocytic white blood cells can ingest and digest pathogens. These phagocytes provide non-specific immunity to disease.

Specific immunity is provided by lymphocytes. Lymphocytes divide to produce clones of plasma cells which produce antibodies that are specific to an antigen. The binding of an antibody to an antigen stimulates destruction of the pathogen. Memory cells provide immunity against future attacks by the same antigen.
Various proteins are involved in the immune defense against pathogens. For example:

Clotting Factors
Clotting factors are proteins that catalyze the blood clotting process. For example, thrombin is an enzyme that converts fibrinogen to fibrin. Fibrin is a protein that forms a mesh around a platelet plug that forms a clot and prevents the entry of pathogen into the blood.

Immunoglobulins are antibodies. Antibodies are proteins made by plasma B cells that are specific to certain an antigens. Once an antibody binds to an antigen, it inactivates the antigen.

Digestive Enzymes
Lysozyme is an enzyme that catalyzes the destruction of the cell walls of certain bacteria. There are also enzymes in the lysosome of phagocytic white blood cells digest the pathogens that have been phagocytosed by the cell.
Image: Explain the role of various proteins in the immune defense against pathogens. 

Essential Idea: The human body has structures and processes that resist the continuous threat of invasion by pathogens.
The skin provides a primary line of defense against pathogens. The skin is a physical barrier made of several layers of cells that prevent pathogen entry into the body. In addition, the skin:

...has a slightly acidic pH which prevents some bacteria from growing.

...secretes antimicrobial fatty acids. relatively dry, which inhibits some bacterial growth. populated with beneficial bacteria that prevent other bacteria from growing.

...secretes sweat, which contains the antimicrobial lysozyme enzyme (catalyzes the destruction of the cell walls of certain bacteria).
Image: Outline the role of skin in the defense against pathogens.​

Understanding:  The skin and mucous membranes form a primary defense against pathogens that cause infectious disease.