Biology
Immune System I:
Lines of Defense and Lymphatic System
Big Picture
The immune system consists of three lines of defense to help protect bodies from invading pathogens, such as worms and germs. The first and second lines of defense are non-specific, so the processes are the same for all pathogens. The third line of defense is specific to the type of pathogen.
Key Terms
Pathogen: Disease-causing agent such as a bacterium, virus, fungus, or protozoan.
Nonspecific Response: A response that is the same no matter what type of pathogen is involved.
Specific Response: A response tailored to a particular pathogen.
Inflammatory Response: Nonspecific response the body first makes when tissue is damaged or infected.
Phagocytosis: Process in which leukocytes (phagocytes) engulf and break down pathogens and debris.
Leukocyte: White blood cell produced by bone marrow to fight infections (can be specific or nonspecific).
Lymphatic System: System of the body consisting of organs, lymph vessels, lymph nodes, and lymph that produces lymphocytes and filters pathogens from body fluids.
Lymph: Fluid that leaks out of capillaries into spaces between cells and circulates in the vessels of the lymphatic system.
Lymph Node: Small structures located on lymphatic vessels where pathogens are filtered from lymph and destroyed by lymphocytes.
Lymphocyte: A type of leukocyte that responds to a specific pathogen.
B Cell: Type of lymphocyte that fights infections by forming antibodies.
T Cell: Type of lymphocyte that fights infections by destroying cells infected with viruses.
Immune Response: Specific defense against a particular pathogen.
Humoral Immune Response: Immune response where B cells produce antibodies against antigens in blood and lymph.
Cell-Mediated Immune Response: Immune response where T cells destroy cells that are infected with viruses.
Antigen: Molecule that the immune system identifies as foreign and responds to by forming antibodies.
Antibody: Large, Y-shaped proteins produced by B cells that recognize and bind to antigens in a humoral immune response.
Memory Cell: Lymphocyte (B or T cell) that retains a “memory” of a specific pathogen after an infection is over and thus provides immunity to the pathogen.
Immunity: Ability to resist a pathogen due to memory lymphocytes or antibodies to the antigens the pathogen carries.
Immunization: Deliberate exposure of a person to a pathogen in order to provoke an immune response and the formation of memory cells specific to that pathogen.
First Line of Defense
The first line of defense consists of the body’s mechanical, chemical, and biological barriers.
- Examples of barriers include skin (mechanical), which physically prevent pathogens from coming in, and mucus containing enzymes (chemical), which destroys pathogens that come into contact with the enzymes.
- Non-pathogenic bacteria (biological) take up space and resources to prevent other harmful microorganisms from surviving.
The first line of defense is nonspecific - the response is always the same, no matter what the pathogen type is.
Second Line of Defense
The second line of defense protects against an agent that manages to break through the first line, such as through a cut on the skin. The second line includes the inflammatory response and phagocytosis by nonspecific leukocytes.
- The inflammatory response is the body’s first reaction against infection or tissue damage.
- Nonspecific leukocytes are used in phagocytosis, in which the leukocytes engulf and break down any type of pathogens they come across.
