How do invertebrates generate a highly specific innate immune response?

High immune specificity is usually considered an exclusive property of vertebrate adaptive immunity. Surprisingly, similar specificities were recently discovered in the invertebrates, which lack the adaptive system. Here, we propose alternative mechanisms for invertebrate specificity, including (i) high genetic diversity of receptors or effectors, (ii) synergistic interactions among immune components, and (iii) dosage effects. The latter two mechanisms act at the protein level, where they could mediate a much higher functional diversity than contained genetically. This may be essential considering the limited genetic diversity of invertebrate immunity genes. They may also contribute to immunological priming--an increased responsiveness of the invertebrate immune system after parasite challenge comparable to vertebrate immune memory. Similar processes are likely to act in the innate system of vertebrates and enhance the effectiveness of adaptive immunity.