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.