Flashcards in BL 02-27-14 10-11am Immunohematology - Cohen Deck (54):
Transfusion - transplanting red cells
-Red cells DO NOT carry MHC antigens in humans
- The antigens they do carry are much less polymorphic in the population (many fewer
- The white cells that come along with the white cells are recognized & destroyed (which is fine when you just are wanting the red cells)
- The ability to transfuse red cells from one person to another depends on avoiding immune responses
- When problems arise, they are often immunological
Transfusion - transplating platelets
- Platelets DO bear HLA (Class I) antigens
- with repeated platelet transfusions, alloimmunization may be developed
- in that case, HLA typing becomes necessary (expensive, hard to find good matches)
Blood group ANTIGENS
- glycolipids found on the surface of ALL BODY CELLS, including red cells of course
- The lipid backbone spans the plasma membrane
- The terminal sugars confer antigenic specificity, A, B or O
Blood group SUBSTANCES
- glycoproteins with similar sugars, found in the body fluids of people who have the Secretor (Se) phenotype.
-~80% of people are secretors
- their blood type can be determined from sweat stains, cigarette butts, etc.
- NO particular advantages to being a secretor
O antigen - pros & cons
People who are O are...
- somewhat protected from pancreatic cancer
- much less likely to develop venous thromboembolic disorders
Structure of Blood group antigens
- A set of glycosyl transferases assemble the
basic “core” sugar chain which almost everybody has = called the “H” antigen
- Then a final glycosyl transferase, of which there
are three alleles, can act and produce the different antigens
Antigen differences in A, B, O, AB
- The O allele is an “amorph;” it does not
code for a working transferase & so group O people have only the basic core, the H antigen.
- People who are group A have a glycosyl
transferase allele which puts an additional sugar on the core chain
- People who are B have a different allelic form of this enzyme which adds a different sugar.
- Group AB individuals have both the A and B antigens on their red cells, because they got both the A and B transferases from their parents.
- Some people who lack the transferase gene that puts the final sugar on the “core”, and thus do not express even the H antigen
= rare Bombay phenotype (Oh,)
- All blood, even type O, is foreign to such people (can only give Bombay blood)
- Appears as type O blood on normal typing tests
Commonness of ABO antigens in environment
- ABO antigens are very simple carb structures, so it's not surprising that they are found in many places in nature.
- Thus, one will inevitably come into contact w/ these carbs in the environment during infancy
- Those which are not the same as your own are, of course, foreign to you, & you BECOME IMMUNIZED to them (type A person will make Ab to B; type O person will make Ab to both A & B, etc.)
- These Abs are called “naturally-occurring” or ISOHEMAGGLUTININS
Class of Isohemagglutinins
Measure titers of isohemagglutinins
- Can be of use in Dx of B cell immunodeficiency, since they should begin to appear in blood between 3 & 6 mo of age, as antigen exposure occurs
What kind of cells do Group A people have?
Cells with A carbohydrate antigens on them.
What kind of cells do Group B people have?
Cells with B carbohydrate antigens on them.
What kind of cells do Group O people have?
Cells with NO carbohydrate antigens on them.
What kind of cells do Group AB people have?
Cells with both A & B carbohydrate antigens on them.
What kind of antibodies do Group A people make?
What kind of antibodies do Group B people make?
What kind of antibodies do Group O people make?
anti-A & anti-B
What kind of antibodies do Group AB people make?
What are the possible genotype(s) for Group A people?
AA or AO
What are the possible genotype(s) for Group B people?
BB or BO
What are the possible genotype(s) for Group O people?
What are the possible genotype(s) for Group AB people?
What is the frequency of blood types in white & black Americans?
O most common - almost half of both whites & blacks
A almost as common as O in whites, with B much less common
A & B almost equal in blacks
AB is the rarest block type (3% in w, 4% in b)
Difficult to type A & B antigens...
- There are variant A and B types (A2, A3, Ax, Bx, etc.) in which A or B antigen is expressed weakly
- Such people may be typed incorrectly or with difficulty in the blood bank.
- Suppression of ABO antigens seen in some diseases (leukemia)
- Titers of isohemagglutinin can be low in the elderly and in hypogammaglobulinemia.
- Any of these conditions can lead to an “ABO discrepancy” (lack of correlation between ABO phenotype as determined by cell & serum typing) which must be resolved.
- DNA typing is possible.
Blood type & historically isolated populations
- Some historically isolated populations have very different distributions than we see in the US
- most South American natives are group O
- group O is rare in China
- group B is very common in Vietnam
- Used by anthropologists to estimate relatedness of peoples & times of migrations.
- Important if your donor & recipient populations are
different; it can often be a problem for travelers.
- 2nd most important blood group system (out of over 600 IDed blood group antigens!)
Rh antigens are on proteins coded for at two loci:
- One is for alleles d/D
- The other for c/C & e/E
- By far the most important allele is D, so that if you say you are “Rh positive” you really mean ►“Rh(D) positive”
What does it mean when you say "Rh+"?
This truly means that you are Rh(D) positive, as the D allele is by far the most important Rh antigen.
D allele of Rh
- D is dominant over d (the “d” allele, another amorph, is heavily mutated & does not make protein)
- People whose genotype is DD or Dd type as Rh+
- 92% of U.S. blacks are Rh+; 85% of whites.
- Rh(D) negative is rare in sub-Saharan Africa
- There are no “naturally occurring” isohemagglutinins for Rh
- It’s a protein, & not ubiquitous in nature, ►so you don’t make antibody to it unless you’re Rh(D) negative & become immunized w/ Rh(D)+ red cells.
First process in the blood bank
"Type & Screen"
All donor units of blood are...
- typed for ABO & Rh
- tested for presence of “unexpected” antibodies
- tested for syphilis, Hep B & C, HIV, and West Nile Virus antibody
- In many blood banks, red cells are also typed for a long list of “minor” blood group antigens as well
Reverse typing is also performed
- makes sure isohemagglutinins in plasma are appropriate for determined red cell type
- screens for list of other RBC antibodies
Units are then banked
Preparing the recipient for transfusion (screening, etc.)
Before transfusion, recipient is
- typed for ABO & Rh
- his plasma screened for expected & “unexpected” antibodies
- Computer can then tell which donor units are compatible with the recipient.
Donors matching Recipients
- Must ordinarily be identical at ABO & Rh
- Despite ABO/Rh match, there may still Abs in recipient’s plasma which can react with Ags on donor’s RBCs… if so, this is called crossmatch.
Consequence of crossmatch if blood transfused anyways...
- Generalized complement-mediated hemolysis
- Active complement-mediated inflammation
- Free hemoglobin deposited in kidneys leading to acute renal failure
Testing for Crossmatch
- lab test in which plasma from prospective recipient is mixed w/ red cells from prospective donor
- The big question is, will this recipient’s plasma destroy the incoming red cells?
- If so, that could be catastrophic
- Cells are 1st suspended in saline & a drop of the recipient’s plasma added.
- If there is agglutination, it means there are lots of high-avidity antibodies in the serum (probably IgM) = most dangerous b/c of their potent complement-activating ability.
- Even if this test is negative there could still be lower-avidity antibodies that would shorten red cell survival, so the bank does some further tests to push the system.
RBCs & Zeta potential
Zeta potential = net negative electrical change
- keeps RBCs apart from each other
--> don't clump in capillaries
--> makes harder to agglutinate in blood bank test
-So, tests are done in a medium that neutralizes zeta potential
Agglutination & crossmatch test results
- NO agglutination = blood & recipient are "compatible”
- Group O RBCs can be given to recipients of almost any phenotype (universal donor).
- Type AB people can be universal recipients.
Antiglobulin (Coombs) Test
= a test which uses antibody against human Ig to
detect human Ig on surface of RBCs (direct test) or in plasma (indirect)
Example/Scenario of Direct Antiglobin (Coombs) Test
What to know if pt has Ab against own RBCs... Direct test
Ab wouldn’t be enough (or of high enough avidity) to agglutinate her cells in vivo, b/c she’d be dead
- But, there could be enough to decrease RBC survival from 120 to 30 days, say, and that would make her anemic.
1. Take some of her RBCs, wash them, and then add Ab against human IgG to them.
--> If they had some human IgG sticking to their surface, this “antiglobulin” could cross-link it, and the cells would agglutinate.
Example/Scenario of Indirect Antiglobin (Coombs) Test
Want to know if preg. woman has antibody in her system to Rh(D)+ [in that case, can't use RhoGAM...already immunized]
1. Take donor red cells & add recipient plasma, and then wash off any unbound proteins.
-- If there was Ab against the cells, it might bind but not agglutinate them.
2 Now we added the antiglobulin, it could cross-link bound antibodies & agglutinate cells.
- This is an indirect antiglobulin test.
Direct Antiglobulin Test (DAT) asks...
Is there antibody already on these cells I am interested in?
You rinse off the cells & add antiglobulin to find out.
DAT detects cells that were coated w/ Ab in vivo.
Indirect Antiglobulin Test asks...
Is there unexpected antibody to red cell antigens in the plasma of this potential recipient?
You take red cells, add plasma, rinse cells (we assume they haven’t agglutinated,) and then add antiglobulin.
- If cells now agglutinate, there must have been Ab to them in the plasma, b/c antiglobulin alone won’t react with red cells.
[Memory aid: There’s always one more step in an indirect test.]
Hemolytic Disease of the Newborn - basics (aka, when & to whom occurs)
Aka erythroblastosis fetalis
- occurs in Rh(D)+ babies of Rh(D)- mothers
- In last trimester & especially at time of delivery, some red cells from baby enter mother’s circulation
- If she is Rh(D)- & baby is Rh(D)+, she may make anti-Rh(D)
= No problem for the baby, who isn’t there anymore.
- But in subsequent pregnancy w/ another Rh(D)+ fetus, mother’s antibodies, formed after the first pregnancy & boosted by the second, can cross the placenta and destroy fetus’ RBCs
- In addition, each subsequent pregnancy with an Rh(D) fetus boosts her response.
Consequences of Hemolytic Disease of the Newborn
Fetus will be born jaundiced...
- Can be dangerous
- High levels of bilirubin (a breakdown product of Hgb) can cross BBB & damage basal ganglia
- Results in cerebral palsy or, if there is very severe damage, fetal death
Prevention of Hemolytic Disease of the Newborn
- The disease is preventable if, at the time that the mother delivers her FIRST Rh(D)+ baby, she is
given IgG antibody to Rh(D) (Rh-immune globulin)
- Most familiar brand being Ortho’s RhoGAM®.
- These antibodies combine w/ fetal red cells, opsonizing them, and they are destroyed before they get a chance to immunize her.
- Note: she is not made tolerant-- just not immunized
- She must receive Rh immune globulin each time there is a chance of being immunized by Rh(D)+ cells, including all subsequent normal deliveries, abortions, fetal manipulations, amniocenteses, etc.
Indirect Antiglobulin (Coombs) Tests to see if mother has been immunized by Rh(D)+
- Rh(D)- woman is pregnant
- She had two previous miscarriages.
- Did they immunize her against Rh(D)?
- Take some of her plasma & add to ABO-compatible, Rh(D)+ cells.
- We see nothing, but did it bind?
- So we now wash the cells and add
antiglobulin; they agglutinate.
- She is, in fact, already immunized.
Rh(D)+ Newborn w/high Hgb, and with RH(D)- mother...testing for hemolytic disease of the newborn among other possible explanations
- Hemolytic disease of the newborn is possible, but there are other causes of hyperbilirubinemia.
- We take some of his red cells, wash then, and add antiglobulin.
- They are agglutinated
"Prophylactic" RhoGAM shots
- A later development in prevention of HDN is the practice of giving a shot of RhoGAM to
Rh negative women at 28 weeks gestation
= to prevent immunization by small transplacental
bleeds during 3rd trimester
- The Rh-immune globulin would also be given at or shortly after delivery, if the child turns out to be Rh(D)+
Why are there not hemolytic diseases of newborns as a result of blood cell antigens other than Rh (such as ABO)?
- Since a type A mother would be expected to ahve anti-B antibodies, why don't these anti-B's not destrol a fetus with type B blood?
- Normally, isohemagglutinins are IgM, probably b/c the ubiquitous antigens that stimulate them are T-independent (carbs often are)
- So they don’t cross the placenta.
- Anti-Rh antibodies are usually IgG (being anti-proteins), and do cross the placenta.
- Occasional people do make IgG isohemagglutinins, esp. in group O people.
- So A or B fetuses of these women are at some risk of ABO hemolytic disease.
- But, there is no “RhoGAM” for this— these mothers are already immunized so there would be no point of a shot (?)
= antibodies to one antigen which bind, fortuitously,
= a fancy name for cross-reactive antibodies
Example of Heterophile Antibodies-- Infectious Mono
Best example of a heterophile antibody:
= Ab that appears in serum of pt w/ infectious mono
= Ab is in response to a viral antigen
- BUT, it happens to react also w/ sheep RBCs
--> Gives us a quick & cheap presumptive test (the Monospot) for mono
Example of Heterophile Antibodies-- Syphilis
Ab made by people w/ syphilis
= similar phosphodiester group in bacterium Treponema pallidum & in phospholipids extracted from beef heart (cardiolipin)
- Simple test can be performed that doesn’t require Treponema
ASK YOURSELF: All normal people except Group AB have isohemagglutinins (Abs to A or B antigens), so a Group A mother has anti-B. If father is BB, fetus will be AB or B; why does her anti-B not destroy the fetus’s red cells?
b/c the antibody is IgM
- b/c its antibody to carbohydrate (--> IgM with no switching)
- so, can't cross placenta
HOWEVER: Some O mothers can make some IgG to the ABO carb antigens --> can't get newborn hemolytic disease
- normally not as serious as Rh disease
- no RhoGAM b/c the woman has been making the Ab since childhood