At least one soluble receptor-binding domain, for use in a method of diagnosis in vivo or prognosis in vivo of a granulocyte-related inflammatory state in a mammal, said granulocyte-related inflammatory states in mammals are selected from acute or chronic inflammation occurring during rheumatoid arthritis or cystic fibrosis, comprising the identification and quantification of the expression of at least one membrane receptor present on the surface of target granulocytes, wherein said at least one membrane receptor is bound by the at least one RBD, and wherein said target granulocytes are lung neutrophils and blood neutrophils in the diagnosis or prognosis of granulocyte-related inflammatory state during cystic fibrosis, and blood neutrophils in the diagnosis or prognosis of granulocyte-related inflammatory state during rheumatoid arthritis, said identification and quantification of the expression of membrane receptors present on the surface of target granulocytes, taking place at a given time or during a given time interval, and allowing the diagnosis and/or prognosis of said granulocyte-related inflammatory state, wherein - for the diagnosis or prognosis of granulocyte-related inflammatory state during cystic fibrosis, expression of said at least one membrane receptor is compared between lung neutrophils and blood neutrophils of the mammal, and said at least one soluble RBD is a combination of two RBD selected from: ∘ Human T Leukaemia Virus-2 (HTLV2, SEQ ID NO: 28) with one of Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1), Feline endogenous retrovirus (RD114, SEQ ID NO: 3), Koala endogeneous retrovirus (KoRV, SEQ ID NO: 20), Bovine Leukaemia Virus (BLV, SEQ ID NO: 30), or Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21), ∘ Koala endogeneous retrovirus (KoRV, SEQ ID NO: 20) with one of Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1), Feline endogenous retrovirus (RD114, SEQ ID NO: 3), Human T Leukaemia Virus-2 (HTLV2, SEQ ID NO: 28), Bovine Leukaemia Virus (BLV, SEQ ID NO: 30), or Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21), ∘ Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1) and Feline endogeneous retrovirus (RD114, SEQ ID NO: 3), or ∘ Bovine Leukaemia Virus (BLV, SEQ ID NO: 30) and Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21); or - for the diagnosis or prognosis of granulocyte-related inflammatory state during rheumatoid arthritis, expression of said at least one membrane receptor is compared between blood neutrophils of the mammal and blood neutrophils of a control mammal, and said at least one soluble RBD is selected from Koala endogeneous retrovirus (KoRV, SEQ ID NO: 20), Amphotropic Murine Leukemia Virus (AMLV, SEQ ID NO: 1), Bovine Leukaemia Virus (BLV, SEQ ID NO: 30) and Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21). - for the diagnosis or prognosis of granulocyte-related inflammatory state during cystic fibrosis, expression of said at least one membrane receptor is compared between lung neutrophils and blood neutrophils of the mammal, and said at least one soluble RBD is a combination of two RBD selected from: ∘ Human T Leukaemia Virus-2 (HTLV2, SEQ ID NO: 28) with one of Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1), Feline endogenous retrovirus (RD114, SEQ ID NO: 3), Koala endogeneous retrovirus (KoRV, SEQ ID NO: 20), Bovine Leukaemia Virus (BLV, SEQ ID NO: 30), or Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21), ∘ Koala endogeneous retrovirus (KoRV, SEQ ID NO: 20) with one of Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1), Feline endogenous retrovirus (RD114, SEQ ID NO: 3), Human T Leukaemia Virus-2 (HTLV2, SEQ ID NO: 28), Bovine Leukaemia Virus (BLV, SEQ ID NO: 30), or Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21), ∘ Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1) and Feline endogeneous retrovirus (RD114, SEQ ID NO: 3), or ∘ Bovine Leukaemia Virus (BLV, SEQ ID NO: 30) and Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21); or ∘ Human T Leukaemia Virus-2 (HTLV2, SEQ ID NO: 28) with one of Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1), Feline endogenous retrovirus (RD114, SEQ ID NO: 3), Koala endogeneous retrovirus (KoRV, SEQ ID NO: 20), Bovine Leukaemia Virus (BLV, SEQ ID NO: 30), or Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21), ∘ Koala endogeneous retrovirus (KoRV, SEQ ID NO: 20) with one of Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1), Feline endogenous retrovirus (RD114, SEQ ID NO: 3), Human T Leukaemia Virus-2 (HTLV2, SEQ ID NO: 28), Bovine Leukaemia Virus (BLV, SEQ ID NO: 30), or Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21), ∘ Amphotropic Murine Leukemia Retrovirus (AMLV, SEQ ID NO: 1) and Feline endogeneous retrovirus (RD114, SEQ ID NO: 3), or ∘ Bovine Leukaemia Virus (BLV, SEQ ID NO: 30) and Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21); or - for the diagnosis or prognosis of granulocyte-related inflammatory state during rheumatoid arthritis, expression of said at least one membrane receptor is compared between blood neutrophils of the mammal and blood neutrophils of a control mammal, and said at least one soluble RBD is selected from Koala endogeneous retrovirus (KoRV, SEQ ID NO: 20), Amphotropic Murine Leukemia Virus (AMLV, SEQ ID NO: 1), Bovine Leukaemia Virus (BLV, SEQ ID NO: 30) and Porcine Endogeneous Retrovirus-A (Perv A, SEQ ID NO: 21).