What is the first step in the sequence of a type 3 hypersensitivity reaction?

In a type 3 hypersensitivity reaction, the first step involves antibodies binding to antigens. This process is crucial because it initiates the formation of immune complexes, which are aggregates formed when antibodies (typically of the IgG or IgM class) attach to soluble antigens. These immune complexes can then circulate in the bloodstream and deposit in various tissues, leading to subsequent activation of immune pathways.

Following the binding of antibodies to antigens, these immune complexes may activate the complement system, which contributes to inflammation and tissue damage. However, this activation occurs later in the sequence following the initial formation of the immune complexes. The development of tissue damage is a result of the entire cascade initiated by the initial antibody-antigen binding, so it occurs after these complexes have formed and activated complement.

In summary, the binding of antibodies to antigens serves as the foundation for the subsequent reactions and pathological effects of type 3 hypersensitivity, making it the correct first step in this immune response.