Examples
If there is evidence that people who have been sexually active in a particular country are more likely to be infected with HIV, the operator of the blood service can refuse to accept donations of blood or blood components from people who have been sexually active there, even if that disproportionately affects members of a particular nationality and so might otherwise be unlawful indirect discrimination because of race.
If there is evidence that women who have recently given birth are likely to suffer detrimental effects from giving blood or blood components, then a blood service can refuse to accept donations from them. This would not be unlawful direct discrimination because of maternity.