The vertebrate body is defended from infection by three lines of defense: The vertebrate body is defended from infection by three lines of defense

The vertebrate body is defended from infection by three lines of defense:

• The vertebrate body is defended from infection by three lines of defense:

• The skin and mucous membranes are the first defense against invasion.
• The body can mount a cellular counterattack if an infection manages to get past the first defense.
• Lastly, cells in the bloodstream circulate and look for foreign cells as part of the specific immune response.

The skin is the first defense against invasion by microbes.

• The skin is the first defense against invasion by microbes.

• The skin has two layers
• An outer epidermis
• A lower dermis
• A subcutaneous layer lies underneath the dermis.

The dermis of the skin is thicker than the epidermis.

• The dermis of the skin is thicker than the epidermis.

• It provides structural support for the epidermis.
• The subcutaneous layer beneath the dermis is comprised of fat-rich cells that act as shock absorbers and provide insulation.

The skin also provides chemical defense in addition to the physical defense:

• The skin also provides chemical defense in addition to the physical defense:

• Oil glands make the skin surface very acidic.
• Sweat contains the enzyme lysozyme, which attacks and digests the cell walls of many bacteria.

When an infection occurs, a host of cellular and chemical defenses swing into action, including:

• When an infection occurs, a host of cellular and chemical defenses swing into action, including:

• Cells that kill invading microbes
• Proteins that kill invading microbes
• The inflammatory response
• The temperature response

The central location for the storage and distribution for the substances involved in the second line of defense is the lymphatic system.

• The central location for the storage and distribution for the substances involved in the second line of defense is the lymphatic system.

Cells that kill invading microbes

• Cells that kill invading microbes

• There are three types of white blood cells that kill microbes.
• Macrophages kill bacteria by ingesting them.

Neutrophils ingest bacteria but, more importantly, secrete chemicals to neutralize everything living in the infected area, including themselves.

• Neutrophils ingest bacteria but, more importantly, secrete chemicals to neutralize everything living in the infected area, including themselves.
• Natural killer cells attack body cells that are infected.

Proteins that kill invading microbes

• Proteins that kill invading microbes

• The complement system is a very effective chemical defense in vertebrates.

The inflammatory response

• The inflammatory response

• Makes the aggressive cellular and chemical counterattacks more effective.
• It occurs in a sequence of stages:
• The infected or injured cell first releases chemical alarm signals, such as histamine.
• These chemicals cause the blood flow to the area to increase and for capillaries to stretch and be more permeable.
• Phagocytes migrate to the site of infection and attack the invaders; many of these cells die and form the pus associated with some infections or wounds.

The temperature response

• The temperature response

• Human pathogenic bacteria do not grow well at high temperatures.
• When macrophages attack, they send a signal to the brain to raise the body’s temperature.
• The body’s thermostat rises above the normal 37°C to produce a state of fever.
• While the fever curbs microbial growth, it can be dangerous because it might inactivate critical cellular enzymes.

Lymphocytes are white blood cells that are critical to the specific immune response.

• Lymphocytes are white blood cells that are critical to the specific immune response.

• T cell lymphocytes
• Originate in the bone marrow but migrate to the thymus gland for maturation.
• They recognize microorganisms and viruses by the chemical markers, or antigens, on their surfaces.
• B cell lymphocytes
• Complete their maturation in the bone marrow and, when an antigen is encountered, they produce antibodies
• These antibodies coat the antigen and mark the cell bearing that antigen for destruction.

No invader can escape being recognized by at least a few T cells because tens of millions of different ones are made, each specializing in a particular antigen.

• No invader can escape being recognized by at least a few T cells because tens of millions of different ones are made, each specializing in a particular antigen.

• There are four main kinds of T cells.

• Helper T cells (TH), memory T cells, cytotoxic T cells (TC), and suppressor T cells.

Both B and T cells produce memory cells.

• Both B and T cells produce memory cells.

• These provide the body with the ability to recall a previous exposure to an antigen and to mount an attack against that antigen very quickly.
• The initial immune response to an antigen encountered for the first time is delayed.
• The second infection is halted much earlier due to the presence of memory cells.

Macrophages initiate the immune response.

• Macrophages initiate the immune response.

• They inspect the surfaces of all cells they encounter.

• Every cell in the body carries special marker proteins on its surface called major histocompatibility proteins, or MHC proteins.
• The MHC protein is exactly the same on all cells in that body.
• These serve as “self” markers that enable the individual’s immune system to distinguish its cells from foreign cells.

When a foreign particle infects the body, it is taken in by cells and partially digested.

• When a foreign particle infects the body, it is taken in by cells and partially digested.

• Within the cells, the antigens are processed and moved to the surface of the plasma membrane.
• Cells that perform this function are called antigen-presenting cells and are usually macrophages.

Macrophages that encounter a pathogen, identified as anything which lacks the proper MHC protein, respond by secreting a chemical alarm signal which stimulates helper T cells.

• Macrophages that encounter a pathogen, identified as anything which lacks the proper MHC protein, respond by secreting a chemical alarm signal which stimulates helper T cells.

• The helper T cells activate two lines of immune system defense.
• Cellular response carried out by T cells.
• Humoral response carried out by B cells.

Macrophages process the foreign antigens and trigger the cellular immune response.

• Macrophages process the foreign antigens and trigger the cellular immune response.

• The activated helper T cells that are bound to an antigen-presenting cell stimulate the proliferation of cytotoxic T cells.
• These cells recognize and destroy infected body cells.

Any cytotoxic T cell whose receptor fits the particular antigen-MHC complex present in the body begins to multiply rapidly.

• Any cytotoxic T cell whose receptor fits the particular antigen-MHC complex present in the body begins to multiply rapidly.

• This quickly eliminates large numbers of infected cells.
• The cytotoxic T cells kill by puncturing a hole in the plasma membrane of the infected cell.

• Following an infection, some of the activated T cells give rise to memory cells that remain in the body, ready to mount an attack quickly if the antigen is encountered again.

B cells also respond to activated helper T cells.

• B cells also respond to activated helper T cells.

• B cells do not attack infected cells, rather, they mark the pathogen for destruction.

• Early in the humoral immune response, the markers placed by B cells alert complement proteins to attack the cells carrying them.
• Later, the markers activate macrophages and natural killer cells.

B cells produce markers, or antibodies.

• B cells produce markers, or antibodies.

• B cells can bind to free, unprocessed antigens and antigen particles enter the B cell by endocytosis.
• The antigens are then processed and placed on the surface complexed with MHC proteins.
• Helper T cells that are able to recognize the specific antigen bind to the antigen-MHC protein complex.

Helper T cells stimulate the B cell to divide

• Helper T cells stimulate the B cell to divide

• Also, free, unprocessed antigens stick to antibodies on the B cell surface, triggering even more B cell proliferation.
• The B cells divided to produce
• Plasma cells that serve as short-lived antibody factories.
• Memory cells that remain in the body after the initial infection and mount a quick attack if the antigen enters the body again.

Antibodies are proteins in a class called immunoglobulins (abbreviated Ig).

• Antibodies are proteins in a class called immunoglobulins (abbreviated Ig).

• There are 5 different immunoglobulin subclasses:
• IgM promotes agglutination (clumping) reactions.
• IgG is the major form in the blood plasma.
• IgD serves as antigen receptors on the B cell.
• IgA is the form of antibody in external secretions.
• IgE promotes the release of histamine.

The plasma cells that are derived from B cells produce lots of the same antibody that was able to bind to the antigen.

• The plasma cells that are derived from B cells produce lots of the same antibody that was able to bind to the antigen.

• These antibodies flood the bloodstream and stick to antigens on any cells and microbes that present them, flagging those foreign bodies for destruction.
• Complement proteins, macrophages, or natural killer cells then are the agents of destruction.

Memory B cells circulate through the blood and lymph for long periods of time (sometimes the entire lifetime).

• Memory B cells circulate through the blood and lymph for long periods of time (sometimes the entire lifetime).

• It is estimated that human B cells can make between 106 and 109 different antibodies.

The first time a pathogen invades a body, there are only a few B cells or T cells that may recognize the antigens.

• The first time a pathogen invades a body, there are only a few B cells or T cells that may recognize the antigens.

• Binding of the antigen to its receptor on the lymphocyte surface stimulates cell division and produces a clone.
• This process is called clonal selection.
• The result is the primary immune response, which is slow to develop and produces both plasma and memory cells.

As a result of the first infection, a large clone of lymphocytes that can recognize a pathogen remains.

• As a result of the first infection, a large clone of lymphocytes that can recognize a pathogen remains.

• The secondary immune response is a more effective response when the pathogen is encountered again.

Vaccination is the introduction of a dead or disabled pathogen into a body.

• Vaccination is the introduction of a dead or disabled pathogen into a body.

• The vaccination triggers an immune response against the pathogen, without an infection occurring.

Through genetic engineering, scientists can routinely produce “piggyback,” or subunit, vaccines.

• Through genetic engineering, scientists can routinely produce “piggyback,” or subunit, vaccines.

• These vaccines are made of a harmless virus that has a pathogen gene inserted so that the virus displays the pathogen protein on its surface
• The body responds by making an antibody against that antigen, as well as memory cells to recall that antigen.

Some viruses change their antigen makeup and prevent detection even after a vaccination.

• Some viruses change their antigen makeup and prevent detection even after a vaccination.

• Flu viral genes that code for surface proteins mutate quickly.

• Efforts are being made to develop an effective vaccine against HIV, using the “piggyback” method.

A person’s blood type indicates the class of antigens found on the red blood cell surface - ABO system.

• A person’s blood type indicates the class of antigens found on the red blood cell surface - ABO system.

• If blood is mixed from two incompatible sources, the antibodies clump together or agglutinate.

Another group of antigens found in most red blood cells is the Rh factor.

• Another group of antigens found in most red blood cells is the Rh factor.

• People can be either Rh-positive or Rh-negative.
• This has significance when a mother and her fetus have opposite Rh groups.

• Monoclonal antibodies are specific to one antigen.

Many diseases reflect an overactive immune system.

• Many diseases reflect an overactive immune system.

• An autoimmune disease is when the body attacks its own tissues.
• For example, multiple sclerosis, type I diabetes, rheumatoid arthritis, lupus, and Graves’ disease.
• An allergy occurs when the body mounts a major defense against harmless antigens.
• For example, asthma is a form of an allergic response in which histamines cause the narrowing of air passages in the lungs.

AIDS (Acquired Immunodeficiency Syndrome) is a disease caused by infection with the human immunodeficiency virus (HIV).

• AIDS (Acquired Immunodeficiency Syndrome) is a disease caused by infection with the human immunodeficiency virus (HIV).

• the virus recognizes the CD4 surface receptor on many human immune cells, especially macrophages and helper T cells

HIV attacks the immune system by inactivating cells that have CD4 receptors (CD4+ cells), especially helper T cells.

• HIV attacks the immune system by inactivating cells that have CD4 receptors (CD4+ cells), especially helper T cells.

• This leaves the immune system unable to mount a response to any foreign antigen.
• HIV’s attack on CD4+ T cells progressively cripples the immune system.