V] THE HEALTHY IMMUNE SYSTEM

A. Non-specific Mechanical Defenses

1a. In order to cause a disease, microorganisms must first enter the enchanted forest in some way.

b. The skin protects us from surface pathogens.

2a. Tears, saliva and mucus help wash away pathogens

b. and contain mild bactericidal (bacteria-killing) chemicals.

c. If pathogens enter through the air way they are trapped in a layer of mucus.

d. They may be removed through the continuous, wave-like motion of cilia, moving dirty mucus up to where it can be swallowed, coughed or sneezed.

B. Non-specific Innate Immune Responses

1a. several types of leukocytes (white blood cells) that attack and eliminate anything they recognize asforeign.

b. macrophages will engulf or eat the invading cells

c. neutrophils will surround invading cells & release lysozymes that destroy the invader & themselves

2. inflammation causes more fluid in an area which allows the entry of other immune cells and chemicals into the infected area and helps contain the infection

3. fever raises body temperature over 40oC which is less favorable to viral replication

4. interferon (a chemical released by a virus-infected cell), signals neighboring cells to produce antiviral substances

5. complement (a cascade of 20 anti-infective proteins) released when the first protein in the cascade contacts the cell wall of certain bacteria and fungi

a. the complement may surround and smother an invader

b. the complement may puncture the cell membrane of the invader

c. the complement may tag the invader and attract a macrophage

C. Specific Immune Responses

1. Cell-mediated immunity by T-Cells

a. They are called ‘T cells’ because they mature in the thymus, high in the chest behind the breastbone.

b. The process starts when a macrophage engulfs the invader and processes an antigen [a specific

surface component of the pathogen, usually a protein].

antigen

pathogen

c. The macrophage displays the antigen fragments combined with a Major Histocompatibility Complex [MHC] protein on the macrophage cell surface.

antigen

MHC protein processed antigen

d. A receptor on a circulating, resting helper T cell recognizes the antigen-protein complex & binds to it.

Helper-T-cell

T-cell receptor

e. The binding process causes the helper T cell to activate the cytotoxic T cell [or Killer T cell] so that it can attack and destroy the diseased cell. The killer T cell does NOT attack the pathogen.

                                                                           

f. The helper T cell also releases lymphokines which attract other T cells and B cells

g. The body also releases suppressor T cells that will calm the body and shut off the immune response.

MHC protein__

Activated helper-T-cell

Infected cell

MHC protein

Killer-T-cell

Cell dies

Activated killer-T-cell

MHC protein__

Activated helper-T-cell

Infected cell

MHC protein

helper-T-cell

Cell dies

Activated killer-T-cell

2. Humoral response – B cells

a. B-cells are lymphocytes that grow to maturity in the bone marrow

b. B-cells produce antibodies that circulate in the blood and

lymph streams and attach to foreign antigens to mark them for destruction by other immune cells.

c. Antibodies are proteins with the same basic Y shape but with a special region that is highly specific to target a given antigen.

d. The variety of antibodies is very large. Different antibodies are destined for different purposes.

i] Some coat the foreign invaders to make them attractive to the circulating scavenger cells, phagocytes, that will

engulf an unwelcome microbe.

ii] Some antibodies combine with antigens and activate a cascade of nine proteins, known as complement, that have been circulating in inactive form in the blood. Complement helps destroy foreign invaders and remove them from the body.

iii] Still other types of antibodies block viruses from entering cells.

e. The final group of B cells are the B memory cells that retain the information about the geometry of an antigen so that on a subsequent exposure, the body will ‘recognize’ the invader

f. Activation of B cells to make antibody

The B cell uses one of its receptors to bind to its matching antigen, which the B cell engulfs.

MHC protein

antigen

B-cell

The B cell then displays a piece of the antigen, bound to a MHC protein, on its cell surface.

MHC protein

This whole complex then binds to an activated helper T cell.

activated helper-T-cell

This binding process stimulates the transformation of the B cell into an antibody-secreting plasmacell.

plasma cell

Remember some key points

1. Mechanical barriers:

skin, tears, mucous

A. Non-specific defenses

2. Innate

macrophages, neutrophils, fever, inflammation, interferon, complement

B. Specific responses

1. T -cells

macrophages, antigens, helper-T-cells, killer-T-cells, suppressor-T-cells

2. B-cells

macrophages, antigens, helper-T-cells, B-cells, B memory-cells

D. PROBLEMS IN THE IMMUNE SYSTEM

1. ALLERGIES

a. a foreign protein is recognized as an invader and the body responds to its presence

b. basophil cells [WBC] release histamine

c. histamine makes capillaries more permeable to allow WBC to leave & find invaders

d. as an indirect result tissues swell – this may be annoying or even life threatening

2. AUTOIMMUNE DISEASES

a. the self / non-self system breaks down & self antigens are treated as non-self

b. this is usually the result of an injury to specific cells and does not affect all self proteins

c. or it is possible that a few killer T cells attack your cells and suppressor T cells fail to control them

d. either way, when the suppressor T cells do not control self attacks autoimmune disease

e. eg. type I diabetes – your own killer T cells attack your beta cells in the Islets of Langerhans multiple

sclerosis – you attack your own myelin sheath cells

3. Leptin effect on immune system

a. Fat cells secrete leptin

b. Leptin suppresses the B-cell response

c. People with more body fatsecrete more leptin

d. Therefore, people with more body fat have asuppressed immune system

e. These people will heal more slowly after surgery

F. PASSIVE & ACTIVE IMMUNITY

1. Passive immunity

a. direct passage of antibodies into a person

b. placenta passes antibodies into baby from the mother

c. colostrum passes transfer factor – protein that helps T cells & B cells recognize pathogens

d. a tetanus shot is a shot of antibodies

2. Active Immunity

a. this is when something is injected into the body to trigger a T cell and B cell response

b. what is injected?

-dead pathogens

c. problem – what if the pathogen changes its antigen?

- similar pathogens eg. cowpox for smallpox

- antigens of the pathogen

- coat of a pathogen