Study Notes on MN, Big S, Little s Blood Group System

The MN big S little s blood group system is integral to antibody panels.
Notable for demonstrating dosage effects where M and N showcase this.
Includes alleles big S and little s, crucial for interpretation in antibody ID panels.
Primarily located on red blood cells, named using letters from the word "immune."

Antigens of the MN and S systems are located on glycoproteins, particularly glycophorin.
- Glycophorin A carries M and N antigens.
- Glycophorin B carries big S, little s, and U antigens.
MN and S systems were discovered following the ABO blood group system, established through immunization experiments involving rabbits and human red blood cells (1927).

M and N antigens are on the outer edge of glycophorin A, which makes them susceptible to enzyme treatment.
- Enzymes Affecting MN Antigens: (1) Bisonzyme (from figs), (2) Papain (from papaya), (3) Bromelain (from pineapple).
Dosage Importance: M and N are alleles which exhibit dosage, influencing their expression into phenotypes.
Common phenotypes in populations: Over 50% display both M and N antigens.
M and N antigens detectable as early as 9 weeks of gestation, enduring throughout life.
- Historically used for paternity testing before DNA testing.

Anti-M antibodies are naturally occurring, primarily IgM type, typically reacting best at room temperature:
- Some report IgG forms of anti-M antibodies.
- Variability in reactions based on the pH of preservatives; examples show reactions decrease at a pH of 6.5.
Anti-M does not bind complement and is more prevalent in children.
- Can be noted in bacterial infection cases.
Enhancement of antibody activity can be achieved by:
- Increasing incubation time within manufacturer specifications.
- Adjusting temperature within specified limits.
- Use of specific reagents like LISS or PEG, noting PEG helps concentrate the solution by removing water.

New antigens associated with glycophorin B are of high prevalence (99% of the population):
- Only 1% of individuals are negative for big S, little s, and U.
- U antigen is resistant to enzyme treatment and present universally, needing careful consideration during transfusions.
- Anti-U antibodies occur mainly in response to transfusion or pregnancy, are IgG, and clinically significant.
Challenges: Finding antigen-negative U blood cells is hard since they're present only in 1% of individuals (primarily African American and Sub-Saharan African populations).
Potential complications for patients with anti-U antibodies, requiring contact with the rare donor registry due to their rareness.

Anti-U antibodies have been implicated in:
- Transfusion reactions.
- Hemolytic disease of the fetus and newborn (due to their IgG nature allowing placental crossing).
Disease Associations:
- Glycophorin A and B may serve as receptors for pathogens (e.g., E. coli and Plasmodium falciparum, which cause malaria), suggesting a link to increased susceptibility to infections.

IgM vs. IgG Antibodies in MN and S Systems:
- M and N primarily produce IgM antibodies (typically not clinically significant).
- Big S, little s, and U antibodies are mainly IgG, clinically significant and capable of causing reactions.
U antibodies can develop in patients who are big S negative, little s negative.

This lecture highlights the complexities within the MN big S little s blood group system, its clinical relevance, and potential complications associated with transfusion and antibodies.