ImmunologyTier 1Medical Sciences concept

HLA sensitisation - relative risk by exposure type (transplant, pregnancy, transfusion)

Core concept

HLA sensitisation = development of anti-HLA antibodies from prior exposure to foreign HLA antigens -- critically important because it drives panel reactive antibody (PRA) levels and predicts difficulty finding a compatible transplant
Three exposure routes, with differing relative sensitisation risk: pregnancy, blood transfusion, prior transplant
Prior transplant (especially a failed/rejected graft) carries the highest sensitisation risk -- sustained, high-dose exposure to a full set of mismatched donor HLA antigens, often with concurrent inflammation

Exposure	Relative sensitisation risk	Mechanism
Prior transplant	Highest	Prolonged exposure to complete mismatched HLA set, especially at rejection
Pregnancy	Moderate-high, increases with parity	Exposure to paternally-inherited fetal HLA antigens; multiple pregnancies compound risk
Blood transfusion	Lower (further reduced by leukoreduction)	Exposure to donor leukocyte HLA antigens; leukoreduced products carry substantially lower risk

Sensitisation risk from pregnancy increases with each additional pregnancy -- a multiparous woman awaiting transplant is more likely to be highly sensitised than a nulliparous woman
Leukoreduction of blood products reduces (but does not eliminate) transfusion-related HLA sensitisation -- a specific rationale for leukoreduction beyond FNHTR/CMV prevention

Highly sensitised transplant candidates (high PRA) face longer waiting times and higher risk of antibody-mediated rejection if a donor expresses a targeted HLA antigen -- crossmatch and donor-specific antibody testing become critical
Trap: assuming only prior transplant recipients need sensitisation screening -- a multiparous woman or previously transfused patient can be highly sensitised without ever having had a transplant
Desensitisation protocols (plasmapheresis, IVIG, rituximab) are considered for highly sensitised candidates with a otherwise suitable but HLA-incompatible living donor

Directly determines transplant crossmatch and donor-specific antibody (DSA) results -- pre-formed DSA against a specific donor predicts hyperacute/accelerated antibody-mediated rejection
Same sensitisation principle underlies why directed blood donation from a potential future living donor relative is generally avoided -- would sensitise the recipient against that specific donor's HLA
PRA and calculated PRA (cPRA) are the standard clinical measures summarising overall sensitisation burden used in organ allocation algorithms

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