The Immune System and Immunity

Immune System

• The immune system protects the body from microorganisms in the environment.
• Immunity allows the body to resist disease.
• Humans have two defense systems: innate and adaptive.
• The three lines of defense include:
• Surface barriers
• Internal defenses
• Humoral and cellular immunity

Innate Defenses

• Reacts to all foreign substances like microorganisms, viruses, toxins, chemical substances, etc.
• Prevents pathogens from penetrating the body.
• Always ready to act.
• Reacts quickly.
• The first and second lines of defense are built.

First Line of Defense: Surface Barriers

• Prevents foreign substances from penetrating the body.
• The primary components include skin and mucous membranes.
• Epidermis (keratin).
• Acidity of secretions (skin, vagina, stomach, bladder)
• Enzymes (saliva, tears, gastric juice).
• Mucus (respiratory and digestive tracts).
• Chemical substances (sebum, sweat).
• Mechanical displacement (hairs, cilia).
• Microbiota (bacterial flora).
• Non-specific defense mechanisms.
• Intact epidermis provides a mechanical barrier to prevent infiltration of pathogens.
• Skin secretions create an acidic surface that inhibits bacterial growth and contains bactericidal agents.
• Keratin provides resistance against acids, bases, and bacterial enzymes.
• Intact mucous membranes form a mechanical barrier that prevents pathogen infiltration.
• Mucus traps microorganisms in the respiratory and digestive tracts.
• Nasal cavity hairs filter and trap microorganisms.
• Cilia propel debris-laden mucus toward the nasal cavity for removal.
• Gastric juice contains concentrated hydrochloric acid and enzymes to hydrolyze proteins and destroy pathogens in the stomach.
• Vaginal acidity inhibits bacterial and fungal growth in the female genital tract.
• Lacrimal secretions (tears) and saliva lubricate and cleanse the eyes and oral cavity while containing lysozyme, which destroys microorganisms.
• Acidic urine inhibits bacterial growth and cleanses the urinary tract.

Second Line of Defense: Internal Defenses

• Limits the spread of foreign substances that have penetrated the body.
• Key internal defenses include:
• Phagocytes
• Natural killer cells
• Antimicrobial proteins
• Inflammation
• Fever
• Non-specific defense mechanisms
• Phagocytes ingest and destroy pathogens that breach surface barriers and contribute to adaptive immune responses.
• Natural killer (NK) cells promote apoptosis by directly attacking virus-infected or cancerous cells, recognizing general abnormalities rather than specific antigens.
• Inflammation prevents the spread of harmful agents, eliminates pathogens and dead cells, and promotes tissue repair by attracting phagocytes to the injury site.
• Interferons modulate the immune system and protect uninfected tissue cells from viral invasion.
• Complement proteins cause microorganism lysis, promote phagocytosis through opsonization, and intensify inflammatory and immune responses.
• Fever is a systemic response triggered by pyrogens, elevates body temperature, and inhibits microbial multiplication, and promoting tissue repair.
• Phagocytes adhere to pathogens with Toll receptors to ingest and destroy them.
• Lysosomes engulf pathogens, resulting in phagolysosome formation.
• Toxic compounds and lysosomal enzymes destroy pathogens.
• Residual material is expelled through exocytosis.
• Interferons and complement enhance innate defenses by directly killing microorganisms and preventing their reproduction.

Antimicrobial Proteins

• Augmenter les défenses innées:
• Direct killing of microorganisms.
• Prevents reproduction.
• Interferons are released proteins by virus-infected cells and some lymphocytes, acting like messengers that protect non-infected cells.

The inflammatory response (inflammation):

• Limit the damage following infection or injury
• Triggering events include physical trauma, intense heat, irritants, and infection by microorganisms
• Injured or infected tissue cells and immune cells release inflammatory chemicals:
• Histamine
• Kinins
• Prostaglandins
• Complement
• Inflammatory mediators cause blood vessels to dilate, increasing blood flow
• Local hyperemia due to vasodilation results in redness and heat
• Increased capillary permeability exudate formation (fluid containing proteins and clotting factors)
• Leakage of exudate leads to local swelling (edema)
• Pressing on nerve endings, contributing to pain
• Diapedesis occurs
• Chemotaxis of phagocytes occurs, with neutrophils quickly mobilizing at the inflammation site
• Damage to the tissue
• Substances released by tissue and immune cells activate pain receptors
• Swelling also contributes to pain
• The surge of protein-rich fluids into tissue spaces
• Dilutes harmful substances
• Brings in large quantities of oxygen and nutrients needed for repair
• Allows entry of clotting proteins, forming a clot, walls off the injured area

Clinical signs of inflammation include:

• Redness:
• Increased blood flow
• Heat:
• Increased blood flow
• Swelling/edema/inflammation:
• Plasma leakage via capillaries increases permeability
• Pain:
• Compression from edema
• Stimulation of pain receptors by inflammatory releases (histamine)
• Localized inflammatory response*
• Inflammatory chemicals cause vasodilation that increases blood flow
• Increased capillary permeability leads to fluid leakage (edema)
• Neutrophils, monocytes and lymphocytes are attracted to the injury (chemotaxis)
• Leukocyte migration to the site
• Systemic inflammatory response*
• Liver and spleen enlarge
• Release of leukocytes
• Increased metabolism due to raised temperature

Fever

• Is a systematic reaction.
• Is produced by pyrogens.

Adaptive Defenses

• Reacts against specific foreign substances:
• antigens (Ag)
• Proctects the whole organism entierement by the lyphm and the blood.
• The third line of defence and work with the second line of defence.
• The lymphatic system is involved because that's where leukocytes and macrophages patrol.
• If an antigen is recognzied, it will act.
• The organism stores the antigens in its memory.

Antigens

• They mobilize the immune system and provoque a reaction
• Large protein molecules
• Examples
• Viruses
• Bacteria
• Fungi

Lymphocyte Circulation

• Originate in red bond marrow.
• Precursor T cell move to the thymus to mature, this is where they learn immunocompetence.
• T and B cells colonize into 2nd lymphoid organs such as lymph nodes.
• Lymphocytes encounter antigens that activate them.
• Activated lymphocytes proliferate, differentiate, and circulate around the body.

Effector Humoral Immunity

• Acts effieciently against:
• Micoorganisms infections
• Certain cancerous cells
• Transplanted tissues and organs
• They need to be activated for the first time to rapidly act.
• Actication happens when an antigen enters the body and is recognised for the first time

Specificity

• Use 2 ways to act specifically :
• Humoral immunity
  • with lymphocytes B to produce antibodies (Ac)
• Cellular immunity
  • with lymphocytes T

Humoral Immunity

• Immunity is provided by antibodies
• Specifically to a antigen
• Antibodies produced by the lymphocytes B.
• The complex antigen antibodies will remove the antigen.

Antigenic determinant

• The antibody binds to a small region on the surface.
• the body reacts to the part of antigen that is immunogenic (capable of eliciting an immune response)

Self-Antigens and MHC Proteins

• Each of us has a unique suite of self-antigens (proteins that are not foreign or antigenic to us, but strongly antigenic to other individuals)
• Our cells are marked with proteins that identify them as “self”
• The major histocompatibility complex (MHC) is a group of proteins on the cell surface that are coded for by genes
• MHC proteins are not antigenic to us, but are to other people
• There are two classes of MHC proteins, both of which are important to immune function:
• Class I MHC proteinsfound virtually on all body cells
• Class II MHC proteinsfound only on certain cells in the immune response

Antibodies

• Also named Igs or Gammaglobulines
• Serum protiens produced by the lyphocytes B
• Comine sspecifically with an antigen.
• Five classes of antibodies
• Ig-M: Is the first to be secreted by the plasmons.
• It fixes and activate complement
• IgA: present in limited quantities
• IgD: Attached to the Lymphocyte B
• IgG: most numerous 75-85%
• IgE
• Antibodies do not directly destroy pathogens
• The antibodies form a antigen complex
• The antibodies inactivate marque the antigens so they destroy the macrophages.

Lymphocytes B

• Small quantity of AC
• Slow reaction
• Limited protection
• With primary exposure, an initial population of lymphocytes B
• With secondary exposure, the response is faster, amplified, and prolonged  
• Large quantity of AC
• Plus rapid
• Prolongued production

Vaccination

• Nature of the vaccine
• Pathogens
• Dead
• Pieces
• It mobilize the lyphocytes and increase the immune response

Humoral Immunity

• Acquired naturally from bacterial or viral infection can be a active or passive.
• In active, B Lymphocites will create antibodies.
• Consqeuence will be memory for long term.

Cell-Mediated Immunity

• Is carried by living cells
• When lymphocytes T have the responsiblity
• Auxiliary or Helper lymphocytes (lymphocytes TCD4)
• Cylotoxic lymphocytes (lymphocytes T CD8)

Cell-Mediated Immunity

• Lymphoctes T attack directly
• Viruses
• Tumors
• Lymphoctes T indirectly through the release of chemical substances

Adaptive Defenses: A Summary

• Antigen presenting cell (APC) engulfs an antigen (Ag) and phagocytizes it

Particularities

• Organ Grafts and Prevention of Rejection
• Organ transplantation is a life-saving procedure, but the recipient’s immune system may recognize the transplanted organ as foreign and mount an attack against it (rejection of the graft)
• To combat this, immunosuppressive drugs which depresses recipent’s immune
• There are abnormalities on immpune system
• Auto immune diseases where the body loses its self recognition
• The immune system causes tissue damage and symptoms by combating but are not a danger but cause allergies.

Types of Hypersensitivities

• Allergic reactions can be immediate or delayed:
• Immune responses to perceived (but harmless) threats results in hypersensitivity
• Type I (acute) hypersensitivitiesbegin in seconds after contact with allergen and are called allergies
• IgE antibodies (allergy can trigger reactions, like a runny nose)
• Histamines release allergic reasons for the allergic sympotoms.
• Types II, III, and IV involve antibodies (IgG and IgM) and T cells
• Take longer to occur

There are also other Hypersensitivities

• Type 2 : cytotoxic
• Type 3: is not immediate
• Typer 4 can take longer than 24 hours