Aim: To describe general observations of immunological reactions to foreign materials and to realize that CP titanium gives rise to a foreign body reaction with subsequent bone embedment when placed as oral implants. To analyse the possibility of titanium allergy. Materials and methods: The present paper is of a narrative review type. Hand and Medline searches were performed to evaluate marginal bone loss of oral implants and the potential of titanium allergy. Results: Immunological reactions to foreign substances include Type I hypersensitivity reactions such as allergy, Type II hypersensitivity reactions characterised by IgM or IgG antibodies that may react with blood group antigens at transfusion, and Type III hypersensitivity caused by antigen-antibody immune complexes exemplified by acute serum sickness. There is also Type IV hypersensitivity, or delayed hypersensitivity, which is typically found in drug and foreign body reactions. It proved very difficult to find a universally acceptable definition of reasons for marginal bone loss around oral implants, which lead to most varying figures of so-called peri-implantitis being 1% to 2% in some 10-year follow-up papers to between 28% and 56% of all placed implants in other papers. It was recognised that bone resorption to oral as well as orthopaedic implants may be due to immunological reactions. Today, osseointegration is seen as an immune-modulated inflammatory process where the immune system is locally either up- or downregulated. Titanium implant allergy is a rare condition, if it exists. The authors found only two papers presenting strong evidence of allergy to CP titanium, but with the lack of universally accepted and tested patch tests, the precise diagnosis is difficult. Conclusions: CP titanium acts as a foreign body when placed in live tissues. There may be immunological reasons behind marginal bone loss. Titanium allergy may exist in rare cases, but there is a lack of properly designed and analysed patch tests at present. Schlagwörter: foreign body reactions, marginal bone loss, titanium allergy