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The Immune System Overview. The specific immune system is mediated by leukocytes = white blood cells ( wbcs ). P hagocytosis : wbcs engulf invading organisms and destroy them Macrophages are large, long-lived phagocytic cells c ontaining many lysosomes.

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The Immune System Overview

The specific immune system is mediated by leukocytes = white blood cells (wbcs)


Phagocytosis: wbcs engulf invading organisms and destroy them

Macrophages are large,

long-lived phagocytic cells

containing many lysosomes


Antimicrobial proteins attack microorganisms

  • Complement system—30 proteins interact in a
  • series of steps that results in lysis of invaders

Interferons—produced by virus-infected cells to help other cells resist infection; tell other cells to make proteins that inhibit viral replication


The Inflammatory Response

  • Blood supply increases to area of injury
  • Histamine made by mast cells dilates vessels
  • Phagocytes arrive to destroy pathogens

Immune system = 3rd line of defense

  • Specific
  • Diverse
  • Self-nonselfrecognition
  • Memory

Humoral response:

B cells

Cell-Mediated response: Cytotoxic T cells

Both activated by Helper T cells


B and T cells are lymphocytes (specific type of leukocyte) made in the bone marrow

B cells mature in

the bone marrow.

T cells mature in

the thymus.


Antigen—a foreign substance that elicits a specific response by lymphocytes

Antibody—antigen-binding immunoglobulin

produced by B cells

Antibodies and antigen receptors on the membranes of lymphocytes recognize and distinguish among antigens.


Antibodies = Immunoglobulins (Igs): have 2 heavy

chains and 2 light chains joined by disulfide bridges

C regions = constant

V regions = variable (antigens bind to V regions)


Each lymphocyte will recognize and respond

to ONE and ONLY ONE antigen. Specificity is determined during development before any antigens are encountered.


When an antigen binds to the receptors, those specific lymphocytes are activated and

begin to divide.

Plasma cells secrete

antibodies. They are also called effectorcells.

Clonal selection:

producing clones of

cells that will destroy

a particular antigen

Memory cells: stick

around until the next

time the antigen appears


The immune system distinguishes self from nonself —> no lymphocytes are reactive against

the body’s own molecules.

An autoimmune disorder is when this system fails and the immune system destroys the body’s own tissues.


MHC = major histocompatibilitycomplex

  • group of glycoproteins embedded in the cell membrane
  • “self-markers”
  • Class I MHCs on ALL nucleated cells
  • Class II MHCs only on macrophages,
  • B cells, and activated T cells
  • MHC = “Self”
  • Antigen = “Nonself”

Class I MHC molecules present antigens to cytotoxicT cells.

Class II MHC molecules present antigens to helper T cells.




fight against pathogens

outside of cells

Cell-mediated response—

T cells fight against

pathogens that have already infected cells


B cells and macrophages are antigen-presenting cells (APCs). Class II MHC molecules present the antigens to helper T cells and bind to CD4on the helper T cell membrane. When bound to MHC II, the helper T cells proliferate. Helper T cells activate both the humor and the cell-mediated responses.

Cytokines (interleukins) signal other lymphocytes (B or T cells) to proliferate as well.


When cytotoxic T cells are activated, they kill infected cells.

CD8 (on the membrane of cytotoxic T cells) binds to class

I MHCs. When it recognizes a foreign antigen, it kills the

cell by releasing perforin.

Cell-Mediated Response


Humoral Response

Helper T cells and free floating antigens stimulate B cells to proliferate to make Ab-secreting plasma cells & memory cells. B cells present antigens on class II MHCs to make even more B cells!


Active immunity can be acquired naturally (after exposure to an antigen) or artificially (by a vaccine).

Vaccines are inactivated toxins that still have the epitopes available to trigger an immune response. Then if the antigen is actually encountered,

the immune response will be quicker (secondary response)

Passive immunity—

mom to baby


Achoo! Allergies!

IgEsfor pollen bind to mast cell. When pollen binds, mast cell releases histamine triggers vessel dilation, increased permeability sneezing, etc.

Anaphylactic shock—acute allergic response when so many mast cells release histamine that there’s a dramatic drop in blood pressure. Epinephrine counteracts this response.


Immunodeficiency diseases—parts of the immune system fail to function properly.

AIDS is an immunodeficiency disease caused by the HIV virus. HIV infects cells that have surface CD4 receptors (helper T cells).

Fusin (CXCR-4)and CCR5

are coreceptors that help

the virus to enter cells.


Antibodies to the virus increase, and the virus levels drop initially. But its DNA is still present and replicating. And cells continue to be damaged.