Mycoplasma is unique among prokaryotes because of their small size, small genomes, and complete lack of cell walls; they include cell wall-less prokaryotes. The aim of this study is to evaluate the effect of vaccination at 1 day old of chicks with the killed and a live vaccine (CRDF) of Mycoplasma gallisepticum on humoral immune response and immune organs by using Enzyme-Linked Immunosorbent Assay (ELISA) to measure Ab titers and investigation on histopathological changes. 130 one-day-old broiler chicks were randomly divided into four groups, 30 birds in each group, groups treated as follows: G1: the chicks were vaccinated with a live FMG vaccine (eye drop one drop 0.03ml/dose), G2: the chicks were vaccinated with killed MG (0.3 ml s.c) vaccine, G3: the chicks vaccinated with killed MG and a live MG vaccine G4: considered as the control group, in this group the chicks were not vaccinated. Blood samples were collected at 21 and 35 days old to measure titers of specific Antibodies. At 35 days old, the chicks were dissected, and the bursa of Fabricius and spleen were removed for histological evaluations. In 21-day-old chicks, the results showed a significant difference (P≤0.05) between all vaccinated groups in Ab titers compared with the control group, with the highest mean in G3 followed by G2 and G1, respectively. While in 35 days old, showed a significant difference (P≤0.05) in G3 compared with other vaccinated groups, G2, G1, and the control group. Also, there was a significant increase in all vaccinated groups at 35 days old compared with 21 days old. In the G1 group, the histopathological examination results showed a moderate lymphocytic hyperplasia bursal follicle; in G2, varying degrees of lymphoproliferative were observed in the major bursal follicle, while in G3, marked lymphocytic hyperplasia bursal follicle observed. In G4 (control group) no obvious histopathological findings were recorded. |