The World Health Organization published a report entitled Blood Donor Selection: Guidelines on Assessing Donor Suitability for Blood Donation. Based on a review of scientific studies and other literature, it contains detailed recommendations for establishing blood donation programs, including which donors to accept or reject.
It mentions several autoimmune diseases. From the non-communicable section (page 49), we have
Individuals with thrombocytopenia should not be accepted as blood donors
because of the risk of bleeding at the venepuncture site and because chronic
thrombocytopenia may be associated with serious underlying haematological or
other systemic disease. A past history of autoimmune thrombocytopenia is not
a contraindication to blood donation, even if treated by splenectomy, provided
that the prospective donor has been well for five years with no evidence of
The specific reason for rejection here is twofold: The possibility of bleeding and of a secondary, more serious condition. The first risk, however, is common for all haematological disease. The section references a selection of guidelines from the UK Blood Transfusion & Tissue Transplantation Services.
There is a full section dedicated to "Immunological diseases" (which presumably encompasses autoimmune diseases) (page 57):
5.6 IMMUNOLOGICAL DISEASES
Individuals with systemic immunological diseases are generally unwell and are
therefore not suitable to donate blood. Donors should be questioned about severe
allergy to materials used in blood collection, such as latex or skin disinfectant, so that contact with these materials can be avoided. Passive transfer of IgE by blood transfusion has been reported but does not alter acceptance criteria (129,130,131).
While there is no evidence of harm resulting from blood donation by individuals with a history of anaphylaxis, the permanent deferral of such individuals is recommended as a precautionary measure (70).
Note that the recommendation stems mostly from the assumption that these diseases are severe, and could thus cause serious harm to the recipients of these donors' blood. The referenced sources are three studies on the potential transferability of IgE, and one general set of blood donor recommendations.
A later section discusses general central nervous system diseases (page 58):
5.8 CENTRAL NERVOUS SYSTEM DISEASES
Assessment of the suitability of prospective donors with central nervous system
conditions should take into account the well-being of the donor and the risk of
transfusion-transmission of variant Creutzfeldt-Jakob disease (vCJD).
It is recommended that almost all donors with central nervous system diseases (epilepsy, multiple sclerosis, etc.) be deferred, for several reasons:
- A lack of full understanding of the cause of the disease (see the section on multiple sclerosis)
- A fear that blood donation could lead to adverse effects on the donor (see the section on epilepsy)
- The possibility of the transmission of variant Creutzfeldt-Jakob disease (vCJD).
Some of these have not yet been supported by studies, although attempts have been made to prove/disprove them. The primary concern is that some variants of these diseases are not well-known enough, and there could be adverse effects that physicians are not aware of. This is a concern I've seen cited on various websites of blood donation programs.
Finally, there is a generic section on medications (page 64). This harkens back to the deferral of some donors with thrombocytopenia, insofar as currently having the disease can make one ineligible for donation. In these cases, the medication could be transferred through the blood, thus possible harming the recipient:
Deferral criteria for medications taken by donors should take into account the
underlying condition for which the medication is taken, the pharmacokinetic
properties of the medication and the effect of the medication on the quality
or safety of the donated blood (146,147,148). Donors should not omit regular
medication in order to attend a blood donor session.
There is no published evidence that medications in donated blood have caused
adverse effects in a patient receiving transfusion, although it is unlikely that
such events would be recognized. European Union legislation requires temporary
deferral based on the “nature and mode of action” of the medication (149).
Again, though this possibility has been studied, there is little evidence to back it up. Blood donation programs are advised to defer these potential donors in some cases out of caution.
There are some autoimmune diseases which will not automatically prohibit blood donation. Among these are asthma and some cases of rheumatoid arthritis and hypothyroidism. These are the less severe autoimmune diseases, and they are also well-known and treatable, meaning that some of the risks mentioned earlier don't apply to them.