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.


Immune System I cont.

Third Line of Defense

If a pathogen manages to penetrate the first line and survive the second line, a third line of specific response is then triggered, also known as the immune response.

There are two types of immune response: humoral immune response and cell-mediated immune response.

The Lymphatic System

  • The lymphatic system is an important part of the immune system, particularly the immune response.
  • The lymphatic vessels allow accumulating lymph to carry and filter out pathogens through lymph nodes. The majority of lymphocytes are also found in the lymphatic system. For example, T cells mature in the thymus while B cells mature in bone marrow.
  • Both B and T cells recognize and respond to antigens.

Humoral Immune Response

The humoral response, which takes place in blood and lymph, mainly uses B cells to combat pathogens directly. B cells are triggered and activated by helper T cells before they can fight pathogens. When the B cells encounter a pathogen they recognize, the B cells engulf the foreign substance and then display the antigens on the outside of its cell body. Helper T cells then bind to the antigens, which triggers the B cell to turn into a plasma cell. The plasma cell then releases antibodies that find pathogens with the corresponding antigen and marks it for destruction by phagocytosis.

  • The plasma cells that survive in the body for years are called memory cells. Memory cells result in a much more rapid response if the same pathogen were to ever infect the body once again.
The Lymphatic System
Image Credit: The Emirr, CC-BY-SA 3.0

Cell-Mediated Immune Response

  • The cell-mediated immune response leads to the destruction of body cells infected by viruses. This immune response mainly involves T cells, which need to be activated by antigens. However, different T cells demonstrate different responses
  • T cells are activated when they encounter the matching antigen on a leukocyte.
  • Cytotoxic T cells make contact with virus-infected cell and releases lethal toxins to destroy the cell and the virus.
  • Regulatory T cells help end the cell-mediated response and prevent T cells from accidentally attacking its own antigens.


  • Memory B and T cells produced after an initial fight against an infection help the body quickly defend itself against another infection. This results in resistance to the pathogen, or immunity, and it can be passive or active.
  • Active immunity is a result of memory cells against a certain pathogen. Immunization is meant to build active immunity against a certain pathogen by purposely exposing the body to a mild version of a pathogen to trigger an immune response.
  • Passive immunity is the transfer of antibodies to a person who has never been exposed to the pathogen.

Cell-Mediated Immune Response‍
Image Credit: National Institutes of Health, Public Domain