1. An antibody which is capable of binding to human CD4 antigen, in which the CDRs of the light chain of the antibody have the amino acid sequences: CDR1: LASEDIYSDLA (SEQ ID NO:13) CDR2: NTDTLQN (SEQ ID NO:14) CDR3: QQYNNYPWT (SEQ ID NO:15)

10. An antibody which is capable of binding to human CD4 antigen, in which the CDRs of the light chain of the antibody have the amino acid sequences: CDR1: LASEDIYSDLA (SEQ ID NO:13) CDR2: NTDTLQN (SEQ ID NO:14) CDR3: QQYNNYPWT (SEQ ID NO:15), and

12. A humanized antibody, or an antigen-binding fragment of said humanized antibody, wherein the humanized antibody comprises a light chain variable domain comprising a framework and three complementarity determining regions (CDRs) and a heavy chain variable domain comprising a framework and three CDRs, wherein: the amino acid sequence of the CDRs consists of the amino acid sequence of the CDRs of an antibody of a first species that is capable of binding to a human cluster of differentiation (CD) antigen, wherein the first species is a non-human mammal; the amino acid sequence of the light chain framework, of the heavy chain framework, or of both the light and the heavy chain frameworks consists of the amino acid sequence of a selected human antibody variable region, wherein the light chain framework consists of four framework regions and the heavy chain framework consists of four framework regions; wherein the humanized antibody differs from the antibody of a first species at least in that, for the humanized antibody: i.) the sequence of all four framework regions of the light chain is identical to the sequence of all four framework regions of a single selected human antibody variable region; or ii.) the sequence of all four framework regions of the heavy chain is identical to the sequence of all four framework regions of a single selected human antibody variable region; or iii.) both i.) and ii); and wherein: iv.) the selected human antibody variable region of i.) is selected from the human light chain antibody variable regions with the most overall homology to the light chain variable region of the antibody of the first species; or v.) the selected human antibody variable region of ii.) is selected from the human heavy chain antibody variable regions with the most overall homology to the heavy chain variable region of the antibody of the first species; or vi.) both iv.) and v.); and vii.) wherein the most overall homology is determined on the basis of the respective variable domains, including at least CDR1 and CDR2; and wherein the humanized antibody is capable of binding the same human CD antigen as the antibody of the first species.

13. A humanized antibody according to claim 12, or an antigen-binding fragment of said humanized antibody, wherein the non-human mammal is rat.

14. A humanized antibody according to claim 12, or an antigen-binding fragment of said humanized antibody, wherein the non-human mammal is mouse.

15. A humanized antibody according to claim 12, or an antigen-binding fragment of said humanized antibody, wherein the humanized antibody or fragment thereof is a (Fab′) 2 fragment.

16. A humanized antibody according to claim 12, or an antigen-binding fragment of said humanized antibody, wherein the humanized antibody or fragment thereof is a Fab fragment.

17. A humanized antibody according to claim 12, or an antigen-binding fragment of said humanized antibody, wherein the antibody has glycosylation characteristic of CHO cells.

18. A humanized antibody according to claim 12, or an antigen-binding fragment of said humanized antibody, further comprising a human light chain constant region and a human heavy chain constant region.

19. A pharmaceutical composition comprising a humanized antibody according to claim 12, or an antigen-binding fragment of said humanized antibody, and a pharmaceutically acceptable carrier or diluent.