Facts about K and k antigens and antibodies:
- The K antigen is highly immunogenic, with an approximately 50% probability of immunization after exposure. This is only surpassed by the ABO and RhD antigens.
- Antigens are well developed at birth and Anti-K can cause severe HDFN and fetal anemia because it suppresses fetal RBC synthesis.
- Anti-K has been implicated in severe HTRs.
- Kell is a glycoprotein encoded by the gene KEL. Its precursor, Kx, is linked to the Kell glycoprotein via Km antigen, and as the Kell antigens are formed, Kx becomes weak. In cases where the gene encoding for Kx is missing or mutated, neither Kx nor Km are formed and lead to the McLeod Phenotype. In this phenomenon, some Kell antigens are reduced but are expressed weakly. These individuals can form Anti-Kx and Anti-Km (formally known as Ant-KL).
- McLeod phenotype is X-linked and occurs mainly in males. An individual with this syndrome may exhibit neurological and muscular abnormalities, as well as hemolytic anemia. Some patients also have Chronic Granulomatous Disease (CGD), a disease that exhibits a defect in the reactive oxidative metabolism needed for granulocytes to kill bacteria. CGD patients with McLeod phenotype usually produce Anti-Kx and Anti-Km.
- Because the K antigen has a lower frequency, many facilities will allow the use of 2 heterozygous cells to rule out the anti-K antibody.
- Kpa, Kpb, Jsa, and Jsb are also antigens of the Kell blood group.
- Kpb and Jsb are high-frequency antigens.
Table 4. Kell Blood Group System Facts.| Ag/Ab | Ag Frequency (White) | Ag Frequency (Black) | Dosage | Enzyme Interaction | Antibody Class | Complement Binding | Clinically Significant |
| K | 9% | rare | Occ | Enhanced by | IgG | Some | Yes |
| k | 99% | 100% | Occ | Enhanced by | IgG | No | Yes |
| Kpa | 2% | rare | Occ | Enhanced by | IgG | No | Yes |
| Kpb | 99.9% | 99.9% | Occ | Enhanced by | IgG | No | Yes |
| Jsa | 0.01% | 20% | Occ | Enhanced by | IgG | No | Yes |
| Jsb | 99.9% | 99% | Occ | Enhanced by | IgG | No | Yes |
