Chapter 16 & 17 Immunology II:

Immunization and Immune Disorders

 

I Immunization

II Overview of Immunologic Disorders

III Immediate Hypersensitivity

IV Cytotoxic Reactions

V Cell-Mediated Reactions

VI Immunodeficiency Diseases

I Immunization

A. Kinds of antigens used

Dead bacteria or viruses cannot reproduce

Living but attenuated bacteria or viruses are able to reproduce in the host but cannot cause disease (pertussis, polio, MMR)

Components (usually surface proteins) of bacteria or viruses made by recombinant DNA - probably the safest

Toxoids &emdash; altered toxins that are able to stimulate immune responses but do not cause damage. (Diphtheria, tetanus)

 

B. Characteristics of an ideal vaccine

Safe

Confers immunity on everybody who gets it

Immunity is long lasting. Not all vaccines are and some require boosters at intervals, such as tetanus and diphtheria, but these are reasonable and may last 10 years or so. A good cholera vaccine is badly needed but the present one only lasts a few months.

Stable at room temperature for adequate periods of time

Inexpensive

Easy to administer

 

C. General consideration for vaccination

Vaccination is not usually effective after infection &emdash; the immune system is not going to respond any faster to the vaccine that to the real infection. A vaccine that works after exposure might be useful in a few cases where the disease takes a long time to develop. The only case in practice is rabies. The incubation time for rabies virus is long enough for the immune system to mount a response to the vaccine in the meantime.

When isn't a vaccine a good idea?

Live vaccines are not safe for people with compromised immune systems. Live virus vaccines are bad for pregnant women, because the virus may cross the placenta and the immune system of the fetus is not yet developed. Although some maternal antibodies have crossed the placenta, that may not be enough.

There is a low frequency of serious reactions to vaccines. Therefore the decision on recommending a vaccine for general use must take into account the hazards of the disease, including frequency, the hazards of the vaccine, and the effectiveness of the vaccine. The US recommendations have generally been conservative and there are vaccines given in other countries that are not widely used in the US. A notable example is the BCG vaccine for tuberculosis. Historically there were questions about its effectiveness and safety. Now it is felt the incidence of tuberculosis is not sufficient.

 

D. Passive immunity

Passive immunization is when somebody else makes the antibodies. In other words the patient gets blood serum from another individual. Can be a mixture of serum from the general population or from someone who has recently had the disease (convalescent serum) or been immunized against it (hyperimmune serum).

Serum proteins may be separated from the fluid and fractionated before use. Immunoglobulins are found in the gamma fraction of serum when it is electrophoresed. Hence gamma globulin is the fraction used for passive immunity.

Serum is used when there is an urgent need for immunity and a vaccine can't generate it fast enough. Can be against an infectious agent or against a toxin, such as a snake or spider bite.

Best from a human. Otherwise often from horses. However, if horse serum is introduced the recipient will make antibodies to the horse proteins and should not receive horse serum again.

Serum is also used for Rh- mothers who have Rh+ babies. The mother's immune system will recognize the Rh+ antigen as foreign and make antibodies against it, most frequently during delivery when blood from the fetus may enter her circulation. If a second fetus is Rh+ it may be damaged by anti-Rh+ antibodies in the mother's blood. This causes hemolytic disease of the newborn. Anti-Rh+ IgG from serum binds to the Rh+ antigen and prevents production of anti-Rh+ antibodies by the mother's immune system. It is routine for Rh- mothers now.

 

 II Overview of Immunologic Disorders

The immune system is not perfect. A variety of problems may occur. Some are due to hereditary defects in the immune system or to diseases that damage the immune system. However, in other cases, a normal immune system may react in a counter-productive way. This may cause anything from minor discomfort to life-threatening conditions.

Problems can be classified into one of four groups:

We will look briefly at Types I, II and IV

 

III Immediate Hypersensitivity

Allergen = an antigen that can provoke a hypersensitivity response.

Examples - pollen, animal dander [scales from skin, hair, feathers], dust mites, food, spores, bee venom.

Immediate hypersensitivity is what we usually regard as allergies. Time to response is usually less than 30 minutes; may be only a few minutes.

Mechanism: allergens stimulate the production of IgE antibodies. The reason is not known. These antibodies, rather than circulating like IgG or IgM, bind to mast cells (connective tissue cells found next to capillaries) and basophils, especially in the skin, respiratory tract, and vascular endothelium. Then the next time the allergen enters it binds to the IgE on the mast cells and basophils. The bound IgE triggers the release of histamine, serotonin [modified amino acids that act as signals] and a group of peptides. Histamine and serotonin cause blood vessels to dilate and smooth muscles to contract.

 

A. Localized anaphylaxis

"Hay fever" (= allergic rhinitis) - the upper respiratory system is affected: watery and itching eyes, a lot of mucus, sneezing, maybe congestion, maybe flushed skin.

Asthma results when the lower respiratory tract is affected. Potentially more serious. Wheezing, difficulty in breathing caused by constriction of the smooth muscles in the bronchial tubes. Asthma may be fatal if serious enough and not treated.

 

B. Systemic anaphylaxis

Systemic anaphylaxis is much worse and is most often caused by allergens that are injected, although not always. It can be caused by insect stings, which of course are injected. And it can be caused by food.

Results: peripheral blood vessels dilate and blood pressure drops dramatically, producing shock ("anaphylactic shock"). Also airways can constrict. It can be fatal within 15 minutes. People who know they are prone to anaphylactic shock often carry kits to administer epinephrine, which reverses the effects

 

C. Other allergic reactions

Food allergies are not always that serious and may just produce diarrhea, vomiting, abdominal pain, rash, hives, eczema, even asthma. But they can be very serious as mentioned above. Two examples of food allergies are with peanuts and seafood.

Drug allergies: penicillin is the most frequent. Results: can be anything from a relatively harmless rash to systemic anaphylaxis.

 

D. Desensitization

A series of injections of small amounts of allergen. Poorly understood but it is thought that the small amounts of allergen stimulate the production of IgG instead of IgE. If higher levels of IgG specific for the allergen are present it will compete with IgE for the allergen. Therefore little of the allergen will reach the IgE and the allergic reaction is short circuited.

This works better for some agents than other. Desensitization works well for pollen, dust mites, spores, venom. Better for some people than others. But not recommended for animal danders - there is always some risk of an adverse reaction to desensitization injections and this may be greater for animal dander. Food allergens also haven't worked well.

 

E. Genetics

The tendency to develop these allergies is at least partly genetic. However, the kind of allergen reacted to is not - it depends on what you are exposed to. It is estimated that 10% to 30% of the population have these kinds of allergies, but some scientists consider this to be low. Many people have allergies but can handle them with over the counter medications

 

 IV Cytotoxic Reactions (Type II Hypersensitivity)

These are hypersensitivity reactions where antibodies bind to antigens on the surface of cells and lyse the cells. Blood transfusion reactions and some autoimmune diseases are in this group.

Time to reaction: 5 to 12 hours.

Mechanism: IgG, IgM antibodies cause lysis of cells.

Red blood cells (and some other cell types) have a family of antigens on the cell surface &emdash; the ABO system. The A-antigen [N-acetyl galactosamine] and the B antigen [galactose] are both sugars added to the ends of a short peptide on the surface of the cell. The "O-antigen" is just absence of A or B.

Since humans are diploid, each person has two ABO genes. Therefore you can be AA, AO, BB, BO, AB or OO. We make antibodies against whatever kind of ABO antigen we don't have. So if you have Type A blood (AA or AO), you'll have antibodies to the B antigen and vice versa. If you have Type O blood, you'll have antibodies against both A and B antigens and if you have Type AB you won't have antibodies against either.

The problem: if you are Type A and you receive Type B blood, your antibodies will bind to the Type B red blood cells and lyse them. The lysed cells lead to systemic shock and kidney failure because the cell debris clogs the kidneys and this is usually fatal. This will happen on the first transfusion. Why do we already have antibodies to a foreign blood cell that has not been introduced before? Because both the type A and type B antigens are similar enough to very common antigens on the surface of some bacteria and some plant cells.

In practice blood types are matched in the lab before transfusions. There are other cell surface antigens also, although they are not as important as the ABO antigens. Ideally a Type A person would get Type A blood which had also been checked for other possible mismatches. In emergencies it is possible to use blood that is not precisely matched. The rules are:

Universal donor = Type O. As Type O cells have no ABO antigens, they can be introduced with reasonable safety into any persons of any blood type.

Universal recipients = Type AB. Since Type AB individuals do not have antibodies to either A or B they can accept any type of blood.

 

V Cell Mediated Hypersensitivity Reactions

Cell-mediated hypersensitivity is due to a special kind of T cell (TDH cells = delayed hypersensitivity T cells). Also involved: natural killer cells and macrophages.

Time to response: 24-48 hours.

Mechanism: the allergen penetrates the outer skin layers and is processed by macrophages. These present the allergen (or a digested fragment of it) on their cell surfaces and this is bound by TDH cells. These then proliferate and form memory cells. On later exposure the TDH cells respond by secreting cytokines which attract macrophages and natural killer cells to the area. Epidermal cells in the area are damaged and the result is an inflamed itchy area, possibly with blisters, or it can be more serious.

Contact dermatitisis caused by a variety of chemicals.

The skin test for tuberculosis is another example of this kind of hypersensitivity. This is because M. tuberculosis often survives inside macrophages and fragments of its surface proteins are continually presented by the macrophages.

In the tuberculin test a small amount of tuberculin (an antigenic lipoprotein from M. tuberculosis) is injected under the skin. If a person has been exposed to M. tuberculosis or has had the BCG vaccine, a reaction will occur. A raised hard region (induration) is formed &emdash; sometimes also turns red.

 

VI Immunodeficiency Diseases

A. Types of Immunodeficiency

1. Genetic ("Primary Immunodeficiency")

Some people are born with hereditary deficiencies in their immune system, either in the development of T cells or of B cells or both.

Agammaglobulinemia = B cell deficiency

DiGeorge syndrome = T cell deficiency

Severe combined immunodeficiency SCID = deficiency in both B and T cells. Obviously this is much more severe. Bubble babies &emdash; kept isolated in plastic bubbles to avoid infection.

Several different genetic defects can result in SCID. One of these, adenosine deaminase deficiency, (defect in Ada gene) was the first genetic disease to be treated successfully by gene therapy. A good copy of the Ada gene was carried on a retrovirus and inserted into human cells from the patient.

 

2. Acquired ("Secondary Immunodeficiency"). May be caused by:

a) Certain infectious diseases that damage the immune system, including tuberculosis, leprosy, measles and AIDS.

b) Several kinds of cancer (e.g. Hodgkin’s disease).

c) Immunosupressants such as radiation, some anticancer drugs.

 

B. Human Immunodeficiency Virus and AIDS

The most serious of the infections that cripples the immune system is AIDS (Acquired Immunodeficiency Syndrome). This disease is caused by a retrovirus and destroys primarily the T cells.

Retroviruses are a large family of viruses. AIDS is caused by one of these, HIV (Human Immunodeficiency Virus). Other retroviruses are known that infect humans e.g. HTLV-1 and HTLV-2 which cause leukemia. Retroviruses cause leukemia, tumors, anemia, immune dysfunction and neurological diseases in birds and mammals.

There are several variants of HIV and these are divided into the HIV-1 and HIV-2 groups. The HIV-1 strains are responsible for most AIDS cases, especially the earlier ones. HIV-2 is still mostly found in Africa and appears to be less virulent. Both types of HIV are closely related to SIV (Simian Immunodeficiency Virus), which infects African monkeys. HIV-1 probably derived by mutation form SIV within the last 100 years. HIV-2 is more closely related to SIV and emerged more recently.

AIDS was first recognized in the late 1970's in the USA. However blood samples from Africa from the late 1950’s have been found to contain antibodies against HIV. This implies that the patient had been infected with HIV and had made antibodies in response.

Retroviruses contain single stranded RNA in the virus particle. After infection the virus converts the ssRNA to a double stranded DNA molecule. This is then integrated into the chromosomes of the host cell. Once inserted, the retrovirus DNA is there permanently. The integrated DNA is used as a template to make more RNA to fill up new virus particles. Retrovirus particles have two protein shells around their RNA and then an outer envelope.

HIV invades only a few types of cells. It uses CD4 protein on the cell surface as its receptor. The CD4 protein is found on helper T cells, macrophages and a few others. Over a long period of time, the T cell population is gradually destroyed. Without helper T cells, the B cells cannot make antibody and the patient becomes susceptible to infection by opportunistic microorganisms. [Remember opportunistic pathogens are microorganisms unable to infect healthy people with intact immune systems.] In addition, microorganisms that cause only mild disease in healthy people are much more severe in AIDS patients and often get into tissues that they do not normally invade.

 

C. Common Infections in AIDS Patients:

Pneumonia due to the fungus Pneumocystis carinii is the most common by far and is seen in about 66% of AIDS patients.

Other fungal infections include Candida and Cryptococcus (both yeasts) and Histoplasma.

Cryptosporidium and Toxoplasma are protozoa.

Bacteria include Mycobacterium avium complex (MAC) as well as Mycobacterium tuberculosis and also Salmonella.

Viruses include Herpes simplex (ulcers and bronchitis), and cytomegalovirus.

Kaposi’s sarcoma is a cancer that causes purplish patches on the skin. It is rare except in AIDS patients and is apparently caused by infection with a novel virus HHV8 (human herpes virus 8).

 

D. Progression of HIV Infection

Stage 1 &emdash; Infection

Flu-like symptoms in 2-4 weeks, or maybe no symptoms at all. Antibody against HIV is made within 1-18 weeks.

Stage II - Asymptomatic

Antibody against HIV is present. No obvious disease. May last 1 &emdash; 8 years (sometimes longer).

Stage III &emdash; Immune dysfunction

White cells begin to decrease. Mild immune dysfunction. Further drop in T cell count. Then moderate immune dysfunction. Weight loss, diarrhea and fever. The helper T cell count, as defined by the CD4 antigen, begins to drop from above 500 per mm3 [normal is 800 to 1200 per mm3] to the 200-500 mm3 range. Begin to see opportunistic infections and precancerous growths. May last several years. This used to be called ARC (AIDS related complex) but the term isn't used much now.

Stage IV &emdash; Clinical AIDS

Severe immune dysfunction. Opportunistic infections. CD4 count is lower than 200 mm3 and Candida infections appear in the esophagus, bronchi and lungs. Cytomegalovirus eye infections, tuberculosis, Pneumocystis carinii pneumonia, toxoplasmosis of the brain, Kaposi's sarcoma are typical. In addition, the HIV virus itself may invade the brain via infected macrophages and cause neurological symptoms as well.

The HIV virus targets principally the T helper cells, although some other cells are infected as well. It inserts a DNA copy of itself into the genome of the host cell and can remain there inactive for months or years. Then it begins to replicate and infect other cells. When large numbers of viruses are made in a cell, it is killed. HIV also causes T cells to fuse together forming syncytia - large multinucleate cells. These clump together and disintegrate.

 

E. Location of Virus and Diagnosis of HIV

HIV infection is diagnosed nowadays by looking for antibodies [using ELISA] which are formed early in infection before the depletion of helper T cells cripples the antibody response.

False negatives are a problem, especially early in infection before many antibodies are present. False positives may also occur and individuals who tests positive are usually given a second test [Western blot or immunofluorescent antibody test]. These also look for antibodies but by different mechanisms.

HIV virus can be isolated from most HIV infected patients and grown in cultured cells. The virus is found in the blood (both free virus and in infected white cells) and in semen and vaginal secretions. Both of these are significant sources of the virus and dangerous.

Much smaller amounts are found in urine, tears, sweat and saliva and these fluids are not considered high risk (Not zero risk but unlikely to transfer the virus). As the infection progresses viruses are found in other tissues of the body, specifically, in the brain. So should consider all tissue potentially hazardous.

 

F. Transmission of HIV

Transmission is via blood or sexual contact. HIV does not survive outside the body very well and it is sensitive to heat and disinfectants. HIV is not transferred by airborne droplets, shaking hands, handling objects, or insects like mosquitoes. People living in the same house using reasonable precautions do not become infected.

Needle stick injuries may transfer HIV. Calculated risk after needle stick injury with blood containing HIV is 3 out of 1000 (0.3%). Probability of infection from mucous membrane exposure to blood is 10 times lower. [Probability of acquiring hepatitis B from a needle stick with blood containing HBV is from 6 to 30%.]

Probably the people most at risk are those handling emergencies where there may be a lot of blood and not enough time to take the usual precautions and people routinely handling blood samples.

 

Recommended precautions:

1. Gloves for touching blood, body fluids, mucous membranes, skin lesions or objects possibly contaminated by them. Discard gloves, wash hands immediately and thoroughly. Double gloving for invasive surgical procedures.

2. Masks, protective eye-ware and gowns for procedures that might release droplets of body fluids.

3. Needles and other sharp objects - don't puncture yourself; don't re-sheath, discard properly. Health care facilities now have containers marked for disposal of such objects. In some areas of the world disposable needles are not available nor are facilities to sterilize needles always available either.

4. Emergency resuscitation - use devices rather than mouth to mouth.

5. Workers with skin lesions, surface wounds etc should avoid direct patient care or handling of equipment.

6. Dental personnel should consider blood, saliva and gingival fluids potentially infective.

7. Routine housekeeping - wash floors, walls and other areas not normally associated with disease with a 1:100 dilution of household bleach. Spills &emdash; use a 1:10 dilution.

 

G. Treatment of HIV

There is no permanent cure, but drug treatment may suppress symptoms and prolong life. Most antiviral drugs have unpleasant side effects.

a) Base or nucleoside analogs. Inhibit reverse transcriptase.

AZT = azidothymidine = zidovudine (3'-azido-2',3'-dideoxythymidine); Zalcitabine = ddC (2',3'-dideoxycytidine).

b) Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTI).

Nevirapine, Delavirdine, Efavirenz.

c) Protease inhibitors. Block HIV protease, stop processing of virus proteins. Prevent assembly of virus particle.

Indinavir, Saquinavir, Nelfinavir, Ritonavir.

One major problem is the extremely high mutation rate of HIV due to errors made by reverse transcriptase. New variants of influenza appear every few years. However, HIV mutates so fast that the virus changes significantly while inside a single AIDS patient. Thus HIV develops resistance very easily. Therefore effective treatment needs a mixture of drugs. HAART can cost $25,000 per year.

There is no vaccine yet. Many problems.