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Associate Degree Nursing Physiology Review


Immune System


Immune System Functions
Physical and Chemical Barriers (Innate Immunity)
Nonspecific Resistance (Innate Immunity)
Specific Resistance (Acquired Immunity)
Types of T cells


Immune System Functions

  1. Scavenge dead, dying body cells
  2. Destroy abnormal (cancerous)
  3. Protect from pathogens & foreign molecules: parasites, bacteria, viruses

The Immune System has 3 Lines of Defense Against Foreign Pathogens:

1. Physical and Chemical Barriers (Innate Immunity)

2. Nonspecific Resistance (Innate Immunity)

3. Specific Resistance (Acquired Immunity)


Physical and Chemical Barriers (Innate Immunity)

Physical and chemical barriers form the first line of defense when the body is invaded.

Physical Barriers

  • The skin has thick layer of dead cells in the epidermis which provides a physical barrier. Periodic shedding of the epidermis removes microbes.   
  • The mucous membranes produce mucus that trap microbes.
  • Hair within the nose filters air containing microbes, dust, pollutants
  • Cilia lines the upper respiratory tract traps and propels inhaled debris to throat
  • Urine flushes microbes out of the urethra
  • Defecation and vomiting -expel microorganisms.

Chemical Barriers

  • Lysozyme, an enzyme produced in tears, perspiration, and saliva can break down cell walls and thus acts as an antibiotic (kills bacteria)
  • Gastric juice in the stomach destroys bacteria and most toxins because the gastric juice is highly acidic (pH 2-3)
  • Saliva dilutes the number of microorganisms and washes the teeth and mouth
  • Acidity on skin inhibit bacterial growth
  • Sebum (unsaturated fatty acids) provides a protective film on the skin and inhibits growth
  • Hyaluronic acid is a gelatinous substance that slows the spread of noxious agents

Nonspecific Resistance (Innate Immunity)

The second line of defense is nonspecific resistance that destroys invaders in a generalized way without targeting specific individuals:

  • Phagocytic cells ingest and destroy all microbes that pass into body tissues. For example macrophages are cells derived from monocytes (a type of white blood cell). Macrophages leave the bloodstream and enter body tissues to patrol for pathogens. When the macrophage encounters a microbe, this is what happens:
    1. The microbe attaches to the phagocyte.
    2. The phagocyte's plasma membrane extends and surrounds the microbe and takes the microbe into the cell in a vesicle.
    3. The vesicle merges with a lysosome, which contains digestive enzymes.
    4. The digestive enzymes begin to break down the microbe. The phagocyte uses any nutrients it can and leaves the rest as indigestible material and antigenic fragments within the vesicle.
    5. The phagocyte makes protein markers, and they enter the vesicle.
    6. The indigestible material is removed by exocytosis.
    7. The antigenic fragments bind to the protein marker and are displayed on the plasma membrane surface. The macrophage then secretes interleukin-1 which activates the T cells to secrete interleukin 2, as described below under specific resistance .
  • Inflammation is a localized tissue response that occurs when your tissues are damaged and in response to other stimuli. Inflammation brings more white blood cells to the site where the microbes have invaded. The inflammatory response produces swelling, redness, heat, pain
  • Fever inhibits bacterial growth and increases the rate of tissue repair during an infection.

Specific Resistance (Acquired Immunity)

The third line of defense is specific resistance. This system relies on antigens, which are specific substances found in foreign microbes.

Most antigens are proteins that serve as the stimulus to produce an immune response. The term "antigen" comes from ANTI-body GENerating substances.

Here are the steps in an immune response:

  1. When an antigen is detected by a macrophage (as describe above under phagocytosis), this causes the T-cells to become activated.

    The activation of T-cells by a specific antigen is called cell-mediated immunity. The body contains millions of different T-cells, each able to respond to one specific antigen.

  2. The T-cells secrete interleukin 2. Interleukin 2 causes the proliferation of certain cytotoxic T cells and B cells.
  3. From here, the immune response follows 2 paths: one path uses cytotoxic T cells and the other uses B cells.

Cytotoxic T Cell Pathway

  • The cytotoxic T cells are capable of recognizing antigens on the surface of infected body cells.
  • The cytotoxic T cells bind to the infected cells and secrete cytotoxins that induce apoptosis (cell suicide) in the infected cell and perforins that cause perforations in the infected cells.
  • Both of these mechanisms destroys the pathogen in the infected body cell.

Arrow IconClick here for an animation on cytotoxic T cells.

The animation is followed by practice questions. Click here for even more practice questions.


Activation of a helper T cell and its roles in immunity:

Activation of helper T cells and its roles in immunity

T Cell Pathway

  • T-cells can either directly destroy the microbes or use chemical secretions to destroy them.
  • At the same time, T cells stimulate B cells to divide, forming plasma cells that are able to produce antibodies and memory B cells.
  • If the same antigen enters the body later, the memory B cells divide to make more plasma cells and memory cells that can protect against future attacks by the same antigen.
  • When the T cells activate (stimulate) the B cells to divide into plasma cells, this is called antibody-mediated immunity.
  • Click here for an animation on the immune response.

    Lightbulb IconThe animation is followed by practice questions.


Antibodies (also called immunoglobulins or Ig's) are Y-shaped proteins that circulate through the blood stream and bind to specific antigens, thereby attacking microbes.

The antibodies are transported through the blood and the lymph to the pathogen invasion site.

The body contains millions of different B cells, each able to respond to one specific antigen.

There are 4 classes of antibodies (listed from most common to least common):

  • IgG
  • IgM
  • IgA
  • IgE
  • IgD

Each antibody is made of four polypeptide (protein) chains: 2 heavy chains and 2 light chains. Both heavy chains are identical to each other and both light chains are identical to each other. Each contains a constant region and a variable region. The constant region forms the main part of the molecule while the variable regions forms the antigen-binding site.Each antibody has 2 antigen-binding sites.

Antibody structure

Antibodies work in different ways:

1. Neutralizing an Antigen

The antibody can bind to an antigen, forming an antigen-antibody complex. This forms a shield around the antigen, preventing its normal function. This is how toxins from bacteria can be neutralized or how a cell can prevent a viral antigen from binding to a body cell thereby preventing infection.

2. Activating Complement:

Complement is a group of plasma proteins made by the liver that normally are inactive in the body. An antigen-antibody complex triggers a series of reactions that activates these proteins. Some of the activated proteins can cluster together to form a pore or channel that inserts into a microbe's plasma membrane.This lyses (ruptures) the cell. Other complement proteins can cause chemotaxis and inflammation, both of which increase the number of white blood cells at the site of invasion.

3. Precipitating Antigens

Sometimes the antibodies can bind to the same free antigen to cross-link them. This causes the antigen to precipitate out of solution, making it easier for phagocytic cells to ingest them by phagocytosis (as describe above).

Also, the antigens within the cells walls of the bacteria can cross-link, causes the bacteria to clump together in a process called agglutination, again making it easier for phagocytic cells to ingest them by phagocytosis.

4. Facilitating Phagocytosis

The antigen-antibody complex signals phagocytic cells to attack. The complex also binds to the surface of macrophages to further facilitate phagocytosis.

Binding of antibodies to antigens-effector mechanisms


There are 3 major types of T cells:

1. Cytotoxic T cells

These cells secrete cytotoxin which triggers destruction of the pathogen's DNA or perforin which is a protein that creates holes in the pathogens plasma membrane. The holes cause the pathogen to lyse (rupture).

How a cytotoxic T cell kills an infected cell


2. Helper T cells

These cells secrete interleukin 2 (I-2) which stimulates cell division of T cells and B cells. In other words, these cells recruit even more cells to help fight the pathogen.

3. Memory T cells

These cells remain dormant after the initial exposure to an antigen. If the same antigen presents itself again, even if it is years later, the memory cells are stimulated to convert themselves into cytotoxic T cells and help fight the pathogen.



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This material is based upon work supported by the Nursing, Allied Health and Other Health-related Educational Grant Program, a grant program funded with proceeds of the State’s Tobacco Lawsuit Settlement and administered by the Texas Higher Education Coordinating Board.