Biology · MU Pleven

38.Alloantigens in man – ABO/H systems. Blood groups determination.

Explains alloantigens and blood group systems, then contrasts natural IgM antibodies with immune IgG antibodies and lists the detection methods, including the Coombs test. Moves on to the ABO groups with their antibody patterns, the genetics of the alleles and the H antigen, and the sugar differences between A and B antigens. Closes with ABO incompatibility in pregnancy, including the reasons severe disease is rare, and the rare Bombay phenotype explained through epistasis.

Inside: 5 sections · 4 labelled pictures · 10 flashcards · 4 min

Topic 38 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 5 sections, about 617 words (4 min of reading), with 10 flashcards.

In this note

  1. Alloantigens in man – ABO/H systems. Blood groups determination.
  2. Antibodies (Alloantibodies)
  3. ABO Blood Group System
  4. ABO Antigen Structure
  5. ABO System & Pregnancy

Alloantigens in man – ABO/H systems. Blood groups determination.

Alloantigens are antigens that vary between individuals of the same species.

Basics of Blood Group Antigens

  • Blood group antigens are genetically controlled, located on multiple chromosomal loci.
  • They appear by 40 days of intrauterine life and remain unchanged throughout life.
  • These antigens are present not only on RBCs but also in tissues and tissue fluids.
  • A blood group system represents a set of antigens controlled by a single locus with multiple allelic forms.

Antibodies (Alloantibodies)

There are natural antibodies (IgM) and immune/iso-antibodies (IgG).

Natural Antibodies

  • Formed against most major ABO antigens when the antigen is absent from the individual.
  • In minor blood group systems, natural antibodies are rare due to low antigenicity (exceptions: Kell & Duffy).
  • Mostly IgM.
  • React poorly at body temperature but agglutinate RBCs at 5–20°C (except anti-A and anti-B).
  • Usually do not cross the placenta.

Immune Antibodies

  • These are mainly IgG.
  • React best at body temperature and readily cross the placenta.
  • Most bind complement, except notable exceptions like Rh and MN systems.

Detection of Antibodies

  • Saline agglutination test (SAT)
  • Tests using RBCs suspended in colloid media
  • Tests using enzyme-treated cells (useful for Rh + some rare antibodies)
  • Indirect antiglobulin test (Coombs test) — most sensitive, detects IgG (incomplete antibodies)

Complete antibodies → IgM → detected by SAT

Incomplete antibodies → IgG → detected by colloid/enzyme/Coombs tests

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