In recent years a number of studies have reported the effects of microplastics on terrestrial invertebrates at the level of the whole organism, but considerably less is known about the sublethal effects after long-term exposure. One of the likely microplastics-induced changes in organisms are those related to innate immunity. For example, indirectly by acting on the diversity and function of intestinal microorganisms, mechanical damage to the digestive tract, effects on food quality and food intake, and release of hazardous chemical additives from microplastics. In this work, we investigated the modulation of immune processes in macro-decomposer woodlice Porcellio scaber exposed to polyester fibers and crumb rubber from end-of-life car tyres. Organisms were exposed through soil for 3 weeks and immune endpoints were followed throughout the exposure (day 1, 2, 4, 7, 14, 21). Tested immune markers were: haemocyte viability and count (total and differential), and antioxidant enzymes. Our results show that neither polyester fibers nor crumb rubber affected the survival of woodlice, but immune processes were altered during the exposure period. The most significant change was increase in total haemocyte count, and specifically granulocytes, which implies alteration of metabolic processes in haemocytes. Viability of haemocytes however was not affected which implies that effect of potential chemicals leached from microplastics (in particular crumb rubber) was not significant. Antioxidant enzyme activities varied during the exposure period but were not evidently different from controls. We did not observe a difference in effects induced by crumb rubber and polyester fibers although these particles differed significantly in their physico-chemical properties. This study shows that microplastics exposure modulates immune processes in woodlice. This implies that prolonged exposure of terrestrial invertebrates to microplastics may results in their altered immunocompetence and research in this direction is of interest.