Functional disorders and pathological lesions of different intensities, and eventually causing death, are the result of dysfunctions of the immunological system of the honey bee (Apis mellifera L.). Viral and bacterial infections, parasitic invasions and fungal toxins are potent suppressants of various parts of the immune system. Viruses destroy cellular and cell-free immune responses. Proteinases of Paenibacillus larvae decrease the antibacterial activity of haemolymph humoral factors of the apidaecine type. Aflatoxins of Aspergillus sp. destroy the anatomical and physiological barriers of the honey bee haemocoel and haemocytic responses. Deleterious effects of Varroa destructor are realized on cellular and subcellular levels of the immune system.