1. A method for down-regulating cells expressing a cell-associated polypeptide antigen in an animal, including a human being, said polypeptide antigen being weakly immunogenic or non-immunogenic in the animal, by inducing a specific cytotoxic T-lymphocyte (CTL) response against cells carrying the cell-associated polypeptide antigen on their surface or harbouring the cell-associated polypeptide antigen in their intracellular compartment, the method comprising effecting, in the animal, simultaneous presentation by a suitable antigen presenting cell (APC) of 1) at least one CTL epitope derived from the cell-associated polypeptide antigen, and 2) at least one first T-helper lymphocyte (TH) epitope which is foreign to the animal. 2. The method according to claim 1, wherein the animal is a human being. 3. The method according to claim 1 or 2, wherein said at least one CTL epitope when presented is associated with an MHC Class I molecule on the surface of the APC and/or wherein said at least one first foreign TH epitope when presented is associated with an MHC Class II molecule on the surface of the APC. 4. The method according to any one of the preceding claims, wherein the APC is a dendritic cell or a macrophage. 5. The method according to any one of the preceding claims, wherein the polypeptide antigen is selected from a tumour-associated polypeptide antigen, a self-protein, a viral polypeptide antigen, and a polypeptide antigen derived from an intracellular parasite or bacterium. 6. The method according to any one of the preceding claims, wherein presentation by the APC of the CTL epitope and the first foreign TH epitope is effected by presenting the animal's immune system with at least one first analogue of the polypeptide antigen, said first analogue comprising a variation of the amino acid sequence of the polypeptide antigen, said variation containing at least the CTL epitope and the first foreign TH epitope. 7. The method according to claim 6, wherein the at least first analogue contains a substantial fraction of known and predicted CTL epitopes of the cell-associated polypeptide antigen. 8. The method according to claim 7, wherein the substantial fraction of known and predicted CTL epitopes in the amino acid sequence of the analogue are recognized by at least 90% of the MHC-I haplotypes recognizing all known and predicted CTL epitopes in the cell-associated polypeptide antigen. 9. The method according to any one of claims 6-8, wherein substantially all known CTL epitopes of the cell-associated polypeptide antigen are present in the analogue and/or wherein substantially all predicted CTL epitopes of the cell-associated polypeptide antigen are present in the at least first analogue. 10. The method according to any one of claims 6-9, wherein the at least one first analogue further comprises a part consisting of a modification of the structure of the cell-associated polypeptide antigen, said modification having as a result that immunization of the animal with the first analogue induces production of antibodies in the animal against the cell-associated polypeptide antigen. 11. The method according to any one of the preceding claims, which comprises effecting presentation to the animal's immune system of an immunogenically effective amount of at least one second analogue of the polypeptide antigen, said second analogue containing a modification of the structure of fine polypeptide antigen, said modification having as a result that immunization of the animal with the second analogue induces production of antibodies against the cell-associated polypeptide antigen. 12. The method according to claim 11, wherein the modification comprises that at least one second foreign TH epitope is included in the second analogue. 13. The method according to any one of claims 6-12, wherein the first and/or second analogue(s) comprise(s) a substantial fraction of the cell-associated polypeptide antigen's B-cell epitopes. 14. The method according to any one of claims 6-13, wherein the variation and/or modification involves amino acid substitution and/or deletion and/or insertion and/or addition. 15. The method according to any one of claims 6-14, wherein the variation and/or modification comprises that - at least one first moiety is included in the first and/or second analogue(s), said first moiety effecting targeting of the analogue to an antigen presenting cell (APC), and/or - at least one second moiety is included in the first and/or second analogue(s), said second moiety stimulating the immune system, and/or - at least one third moiety is included in the first and/or second analogue(s), said third moiety optimizing presentation of the analogue to the immune system. 16. The method according to any one of claims 6-15, wherein the variation and/or modification includes duplication of at least one B-cell epitope or of at least one CTL epitope of the cell-associated polypeptide antigen. 17. The method according to any one of claims 6-16, wherein the variation and/or modification includes introduction of a hapten. 18. The method according to any one of the preceding claims, wherein the first and/or second foreign TH epitope(s) is/are immunodominant. 19. The method according to any one of the preceding claims, wherein the first and/or second foreign TH epitope(s) is/are promiscuous. 20. The method according to any one of claims 12-19, wherein the first and/or second foreign TH epitope(s) is/are selected from a natural TH epitope and an artificial MHC-II binding peptide sequence. 21. The method according to claim 20, wherein the natural TH epitope is selected from a Tetanus toxoid epitope such as P2 or P30, a diphtheria toxoid epitope, an influenza virus hemagluttinin epitope, and a P. falciparum CS epitope. 22. The method according to any one of claims 12-21, wherein the first and/or second T H epitopes and/or first and/or second and/or third moieties are present in the form of - side groups attached covalently or non-covalently to suitable chemical groups in the amino acid sequence of the cell-associated polypeptide antigen or a subsequence thereof, and/or - fusion partners to the amino acid sequence derived from the cell-associated polypeptide antigen. 23. The method according to claim 22, wherein the first moiety is a substantially specific binding partner for an APC specific surface antigen such as a carbohydrate for which there is a receptor on the APC, e.g. mannan or mannose. 24. The method according to any one of claims 15-23, wherein the second moiety is a cytokine selected from interferon .gamma. (IFN-.gamma.), FIt3L, interleukin 1 (IL-1), interleukin 2 (IL-2), interleukin 4 (IL-4), interleukin 6 (IL-6), interleukin 12 (IL-12), interleukin 13 (IL-13), interleukin 15 (IL-15), and granulocyte-macrophage colony stimulating factor (GM-CSF), or an effective part thereof; a heat-shock protein selected from HSP70, HSP90, HSC70, GRP94, and calreticulin (CRT), or an effective part thereof; or a hormone. 25. The method according to any one of claims 15-24, wherein the third moiety is a lipid such as a palmitoyl group, a myristyl group, a farnesyl group, a geranyl-geranyl group, a GPI-anchor, and an N-acyl diglyceride group. 26. The method according to claim any one of claims 6-25, wherein the first and/or second analogue(s) has/have substantially the overall tertiary structure of the cell-associated polypeptide antigen. 27. The method according to any one of claims 6-26, wherein presentation by the APC is effected by administering, to the animal, an immunogenically effective amount of the at least one first analogue. 28. The method according to claim 27, wherein is also administered an immunologically effective amount of the at least one second analogue. 29. The method according to claim 27 or 28, wherein said at least one first and/or second analogue(s) is/are formulated together with a pharmaceutically and immunologically acceptable carrier and/or vehicle and, optionally an adjuvant. 30. The method according to claim 29, wherein said adjuvant facilitates uptake by APCs, such as dendritic cells, of the at least first and/or second analogues. 31. The method according to claim 30, wherein the adjuvant is selected from the group consisting of an immune targeting adjuvant; an immune modulating adjuvant such as a toxin, a cytokine, and a mycobacterial derivative; an oil formulation; a polymer; a micelle forming adjuvant; a saponin; an immunostimulating complex matrix (ISCOM matrix); a particle; DDA; aluminium adjuvants; DNA adjuvants; .gamma.-inulin; and an encapsulating adjuvant. 32. The method according to claim 31, wherein the cytokine is as defined as in claim 24, or an effective part thereof, wherein the toxin is selected from the group consisting of listeriolycin (LLO), Lipid A (MPL, L180.5/RaILPS), and heat-labile enterotoxin, wherein the mycobacterial derivative is selected from the group consisting of muramyl dipeptide, complete Freund's adjuvant, RIBI, and a diester of trehalose such as TDM and TDE, wherein the immune targeting adjuvant is selected from the group consisting of CD40 ligand, CD40 antibodies or specifically binding fragments thereof, mannose, a Fab fragment, and CTLA-4, wherein the oil formulation comprises squalene or incomplete Freund's adjuvant, wherein the polymer is selected from the group consisting of a carbohydrate such as dextran, PEG, starch, mannan, and mannose; a plastic polymer such as; and latex such as latex beads, wherein the saponin is Quillaja saponaria saponin, Quil A, and QS21, and wherein the particle comprises latex or dextran. 33. The method according to any one of claims 27-32, which includes administration via a route selected from the oral route and the parenteral route such as the intradermal, the subdermal, the intracutaneous, the subcutaneous; the peritoneal, the buccal, the sublingual, the epidural, the spinal, the anal, and the intracranial routes. 34. The method according to any of clai