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Immunity
Overview
Barriers help animals defend against many dangerous pathogens they encounter.
The immune system recognizes foreign bodies and responds with the production of immune cells and proteins.
Two major types of defence have evolved:
Innate immunity and acquired immunity
Innate immunity
Present before exposure to pathogens and is effective from birth.
Involves nonspecific responses to pathogens.
Innate immunity consists of external barriers plus internal cellular and chemical defences.
Acquired immunity
Acquired or adaptive immunity develops after exposure to agents such as microbes, toxins, or other foreign substances
It involves a very specific response to pathogens
Innate immunity
Recognition of traits
shared by broad ranges
of pathogens, using a
small set of receptors
Rapid response
Barrier defences: skin,
mucous membranes,
secretions
Internal defences:
Phagocytic cells,
antimicrobial proteins,
inflammatory response
Natural killer cells
Acquired immunity
Recognition of traits specific to
particular pathogens, using a
vast set of receptors
Slower response
Humoral response:
Antibodies defend against
infection in bodily fluids
Cell-mediated resonse:
Cytotoxic lymphocytes defend
against infection in body cells
Barrier defences include the skin and mucus membranes of the respiratory, urinary, and reproductive tracts.
Mucus traps and allows for the removal of microbes
Many body fluids including saliva, mucus, and tears are hostile to microbes
The low pH of skin and digestive system prevents growth of microbes
White blood cells (leukocytes) engulf pathogens in the body
Groups of pathogens are recognized by receptors
After the WBC (white blood cell) engulfs a microbe, then it fuses with a lysosome to destroy the microbe
Inflammatory response occurs following an injury. Mast cells release histamine, which promotes changes in blood vessels.
These changes increase local blood supply and allow more phagocytes and antimicrobial proteins to enter tissues
Pus is a fluid filled with white blood cells, dead microbes, and cell debris, that builds up at the site of inflammation
Inflammatory response• 1.Tissue injury -> release of histamine
• 2.Causes dilation and increased leakiness in local blood vessels and movement of phagocytes to area
• 3.Phagocytes consume bacteria and cell debris, and tissue heals
Inflammation can either be local or systemic (throughout body)
Fever is a systemic inflammatory response triggered by macrophages (one type of WBC) and toxins from pathogens
• Septic shock is a life threatening condition caused by an overwhelming inflammatory response.
• Natural Killer Cells
Almost all cells in the body have a receptor on their surface
Cancerous or infected cells no longer display this receptor, and natural killer cells attack these damaged cells
Some pathogens avoid destruction by modifying their surface to prevent recognition or resisting breakdown by phagocytosis
Tuberculosis is an example
Acquired immunity
White blood cells called lymphocytes recognize and respond to antigens, or foreign molecules
Lymphocytes that mature in the thymus above the heart are called T cells, and those that mature in bone marrow are called B cells.
Lymphocytes contribute to immunological memory, an enhanced response to a foreign molecule encountered previously
B cells and T cells have receptor proteins that can bind to foreign molecules
Each individual lymphocyte is specialized to recognize a specific type of molecule
An antigen is any foreign molecule to which a lymphocyte responds
A single B or T cell has about 100,000 identical antigen receptors
Antigen binding siteAntigen binding site
Antigen receptor on a B cellAntigen receptor on a T cell
B cells give rise to plasma cells, which make proteins called antibodies or immunoglobulin
Antibody 1
Antibody 2
Antibody 3
Antibody 4Antigen
binding sites
Epitopes
Antigen
B cell receptors bind to specific, intact antigens
Secreted antibodies, or immunoglobulins, are the same shape as B cell receptors
T cells bind to antigen fragments presented on a host cell
These antigen fragments are bound to special cell-surface proteins called MHC
In infected cells, MHC molecules bind and transport antigen fragments to the cell surface, called antigen presentation
A nearby T cell can then detect the antigen fragment that is being displayed
If the MHC is presented on a normal body cell the antigen is displayed for cytotoxic T cells
If the MHC is presenting on an immune system cell such as a macrophage, or B cell the antigen is displayed for cytotoxic T cells, and helper T cells
Normal body
cell Immune cell
The acquired immune system has three important properties:
1. Receptor diversity (T cells and B cells can bind to any foreign particles)
2. A lack of reactivity against host cells (T cells and B cells never bind to normal cells, or proteins in the body)
3. Immunological memory (B cells remember coming in contact with a foreign particle)
Infected cell Antigen B cell
Cytotoxic
T cell
Cytotoxic T cell
Helper T cell Helper T cell
Helper T cell
Plasma B cell Memory B cell
Antibody
production
Antigen receptors are generated randomly
As lymphocytes mature in bone marrow or thymus, they are tested for self reactivity
Lymphocytes with receptors specific for the body's own molecules are destroyed by apoptosis
In the body there are a few lymphocytes with antigen receptors for any particular epitope
The binding of a mature lymphocyte to an antigen induces the lymphocyte to divide rapidly
This is called clonal selection
Two types of clones are produced: short-lived activated plasma cells and long-lived memory cells
Antigen
Proliferation
Memory B cellPlasma B cell
The first exposure to a specific antigen represents the primary immune response
During this time, plasma cells are made, and T cells are activated
In the secondary immune response, memory cells allow a faster, more efficient response to the same specific antigen
Acquired immunity has two branches: the humoral immune response and the cell-mediated immune response
Humoral immune response involves activation and clonal selection of B cells, resulting in production of secreted antibodies
Cell-mediated immune response involves activation and clonal selection of cytotoxic T cells
Helper T cells aid in both responses
Helper T cells respond to nearly all antigen
Helper T cells bind to antigen presenting cells
Helper T cells secrete chemical messages that stimulate other lymphocytes
Cytotoxic T cells interact the MHC on infected cells and become activated killer cells
The activated cytotoxic T cell secretes proteins that destroy the infected target cell
Neutralization occurs when a pathogen can no longer infect a host because it is bound to an antibody
Antibodies attached to antigen increase phagocytosis by macrophages
Three types of immunity
• Innate immunity
Everyone is born with a general type of protection.
Many of the germs that affect other species don’t
harm us. For example the virus that causes
leukemia in cats doesn’t affect humans.
Innate immunity includes the first line of defense –
external barriers of the body
And non-specific defenses such as fever and
inflammation.
Adaptive Immunity and Passive Immunity
• The second kind of defense is active immunity
which develops through our lives. This involves
lymphocytes and develops only as people are
exposed to diseases or immunized by vaccines.
• Passive immunity is “borrowed” from another
source and lasts for a short time. For example,
antibodies in a mother’s breast milk provide a
baby with temporary immunity to diseases the
mother has been exposed to.
Immunity
• Everyone’s immune system is unique. Some
people never seem to get infections, whereas
other seem to be sick all the time. As people
become older they usually become immune to
more pathogens. This is why adults and teens
tend to get sick less often then kids.
Problems of the Immune System
• Disorders of the immune system fall into four main
categories:
1. Immunodeficiency disorders (primary or acquired)
2. Autoimmune disorder (in which the body’s own immune
system attacks it own tissue as foreign matter)
3. Allergic disorders (in which the immune system
overreacts in response to an antigen)
4. Cancers of the immune system
Immunodeficiency Disorders
• Immunodeficiencies occur when a part of the
immune system is not present or is not working
properly
• A person can be born with an
immunodeficiency, although the symptoms
might not occur until later in life, or it can be
acquired through infection or produced by drugs
• Immunodeficiencies can affect B lymphocytes,
T lyphocytes, or phagocytes.
Innate Immunodeficiency:
•Severe combined
immunodeficiency
(SCID) also known
as “bubble boy”
disease after a boy
who lived in a
microbe free
bubble.
SCID is a serious immune system disorder
that occurs because of a lack of both B and
T lymphocytes, which makes it almost
impossible to fight infections.
Acquired Immunodeficiency• Usually develop after a disease, although can
also be the result of malnutrition, or medical
problems. Can be caused by certain
medications.
• Example:
• HIV (human immunodeficiency virus)/AIDS a
disease the steadily destroys the immune
system. It is caused by HIV, a virus that wipes
out T helper cells. Without these cells, the
body is unable to defend against normally
harmless organisms.
Immunodeficiency caused by medication
• Some medications suppress the immune system.
One of the drawbacks of chemotherapy for
treatment of cancer, is that it attacks many fast
growing, healthy cells, including immune cells.
• As well people who have had organ transplants
may need to take immunosuppresant medications.
Autoimmune Disorders
• In an autoimmune disorder, the immune system mistakenly attacks the body’s healthy organs and tissues as though they were foreign invaders. Examples:
• Lupus – chronic disease marked by muscle and joint pain and inflammation (may involve attacks on kidneys and other organs)
• Juvenile rheumatoid arthritis – a disease in which the immune system attacks certain body parts ( such as joints of the knee, hand, and foot)
Allergic Disorders
• Occur when the immune system overreacts to exposure to antigens in the environment. The substances that provoke such attacks are called allergens. The immune response can cause symptoms such as swelling, inflammation, watery eyes, sneezing. Medications that are antihistamines can relieve symptoms.
• In some cases, allergies can be life threatening if they cause anaphylaxis – which is a systemic allergic response.
• Allergic disorders include asthma, eczema, and allergies
Cancers of the immune system
• Cancer occurs when cells grow out of control. This can happen with cells of the immune system.
• Lymphoma involves the lymphoid tissue.
• Leukemia involves abnormal growth of leukocytes, is the most common childhood cancer.
• Both types of cancer in kids are curable.