what is it that prevents a child to suffer from a disease he/she is vaccinated against ? give reason.

A vaccine contains killed or inactive pathogens (antigens). When they are injected into the body via vaccination then our body develops primary immune response against that antigen which synthesize two types of cells: - B-cells and T-cells. B-cells secrete antibodies against those antigens which kill them and Killer T cells kill the infected cells of the body which inhabit pathogen. Some of the T and B cells are stored as memory cells which prevent our body from future attack of the same pathogens. 

Thus, B and T-cells and the antibodies prevent us from the disease against which our body has been vaccinated.