Immune SystemImmune SystemChapter 14Chapter 14

Immune SystemImmune SystemChapter 14Chapter 14

Immunity – the ability to resist infection and disease

Humans have two major types of defense mechanisms:

Innate (Non-specific) immunity

Adaptive (Specific) immunity

Innate (Non-specific) immunity

defense responses that do not distinguish between one threat and another

are present at birth

include: physical barriers (e.g. skin), phagocytic cells (neutrophils, monocytes, macrophages, eosinophils), chemicals (complement system), inflammation, fevers, etc.

provides body with “non-specific resistance”

Adaptive (Specific) immunity

protection against specifically identified threats (i.e. may defend against one particular bacterial infection but not a different one)

most develop after birth upon exposure to an antigen (Ag); an antigen can be a pathogen (disease-causing organism), foreign protein (e.g. toxin), abnormal or infected body cell, foreign tissue transplant

usually produce a state of long-term protection known as “specific resistance” or “acquired immunity”

Adaptive (Specific) immunity

depends on coordinated activity of T & B lymphocytes

T cells- involved in “cell-mediated (aka cellular) immunity”; defense against abnormal cells & intracellular pathogens

B cells- involved in “antibody-mediated (aka humoral) immunity”; defense against pathogens (Ag’s) in body fluids (blood/lymph)

Adaptive (Specific) immunity arises throughout life by active or passive means

Active immunity – development of resistance (i.e. antibody (Ab) production) to specific disease secondary to exposure to specific Ag (pathogen)

naturally acquired active immunity – natural exposure results in immune response & development of long term immunity

induced (artificial) active immunity – deliberate “artificial” exposure to Ag (i.e. vaccine/immunization)

Passive immunity – development of immunity due to transfer of “pre-made” antibodies

naturally acquired passive immunity – antibodies transferred from mom baby across placenta or in breast-milk

induced (artificial) passive immunity – administration of antibodies to fight disease after exposure to pathogen

Properties of Specific (Adaptive) ImmunityImmunity has four general properties:

Specificity

Versatility

Memory

Tolerance

Properties of Immunity Specificity – T & B cells have specific receptors that will allow them to only recognize & target a specific Ag; this process is known as “antigen recognition”

Versatility – millions of different lymphocyte populations, each with specific Ag recognizing receptors; allows for “anticipation” of potential Ag’s

Memory – after initial exposure, long term acquired immunity occurs through the production of memory cells; secondary exposure results in stronger faster response to previously recognized Ag

Tolerance – immune cells recognize self-antigens & “tolerate” (ignore) them, only going after foreign (non-self) Ag’s

Overview of the immune response The purpose of the immune response is to inactivate or destroy pathogens, abnormal cells & foreign molecules (such as toxins)

In order for the response to occur, lymphocytes must be “activated” by the process of antigen recognition

T cells are usually activated first, & then B cells. T cells mainly rely on activation by phagocytic cells collectively known as “antigen presenting cells (APC’s)” (ie. Macrophages, dendritic cells)

Once activated, T cells both attack the invader, & stimulate the activation of B cells

Activated B cells mature into “plasma cells” which produce specific antibodies designed to destroy the particular antigen.

Cell Mediated (a.k.a. Cellular) Immunity

In order for T cells to respond, they must first be activated by exposure to an antigen

antigen is bound to membrane receptors of phagocytic antigen presenting cells (APC’s) (“antigen recognition”)

These membrane receptors on cells are called “MHC proteins” (major histocompatibility complex proteins), & are genetically determined (i.e. differ among individuals)

Antigens bound to MHC proteins “tell” the T lymphocyte what the specific foreign invader is (i.e. a specific bacteria) so that the lymphocytes can mount a cellular defense

Cell Mediated (a.k.a. Cellular) Immunity

Once a T cell is activated by the presentation of the combined MHC/Ag, it will clone (by mitosis) & differentiate into:

cytotoxic T cells helper T cells memory T cells suppressor T cells

Cell Mediated (a.k.a. Cellular) Immunity cytotoxic T cells (aka CD8 cells) - seek out the specific pathogen/infected cell that contains the targeted Ag & destroys it by secreting various chemicals

helper T cells (aka CD4 cells) – necessary for coordination of specific (adaptive) & non-specific (innate) defenses, as well as for stimulating both cell-mediated & antibody-mediated immunity.

In cell-mediated immunity they release chemicals (cytokines) that strengthen the activity of cytotoxic T cells.

In antibody-mediated immunity they release cytokines that stimulate activated B cell division & differentiation into plasma cells

Cell Mediated (a.k.a. Cellular) Immunity

memory T cells – remain “in reserve” so if same Ag appears, these cells can immediately differentiate into cytotoxic & helper T cells, causing a swift secondary response to the invasion

suppressor T cells – activated more slowly than the other T cells; inhibit the response of the immune cells to prevent potential “autoimmune” response

Antigens

bacteria

viruses

ANTIGENS

bacteria

viruses

SPECIFIC DEFENSES

(Immune response)

CELL MEDIATED IMMUNITY

APC’s phagocytize Ag & activate T cells

Direct physical &

chemical attack

Activated T cells clone & differentiate into: Cytotoxic T cells Helper T cells Memory T cells Suppressor T cells

Remain in reserve

B cell activation

stimulate

Prevent autoimmune response

Antibody Mediated (Humoral) ImmunityThe body has millions of different B cell populations, each B cell has its own particular antibody (Ab) molecule (transmembrane protein) within its cell membrane

When the corresponding Ag invades the interstitial fluid surrounding the B cell, the Ag binds to the Ab molecule, & is taken into the cell, eventually being displayed on the B cell’s MHC protein. The B cell is now “sensitized”

Antibody Mediated (Humoral) Immunity

Helper T cells (that had been previously activated to the same Ag) then attach to the sensitized B cells & activate them by secreting chemicals (cytokines) Cytokine secretion results in B cell cloning & differentiation into plasma cells & memory cells

Antibody Mediated (Humoral) Immunity

Plasma cells produce millions of copies of antibodies which are released into the blood & lymph

Antibodies seek out & bind to the Ag forming an “Ab-Ag complex”, eventually leading to the elimination of the antigen by various means

Memory cells remain in reserve to respond to any subsequent exposure by the same Ag. Upon secondary exposure, memory B cells quickly differentiate into Ab producing plasma cells

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Antibody Mediated (Humoral) Immunity

Summary of Defense and Immune Responses