## 2 Materials and Methods
### Effect of Achillea millefolium Extract on Skin Cell Viability  
To assess the cytotoxicity of the extract on skin cells, the metabolic activity and integrity of cell membranes were assessed in HaCaT and HDF cells exposed to the extract for 24 h at concentrations from 0.01% to 10.0%. The effect of the extract was shown to depend on both the concentration and the type of cells. In the test assessing resazurin reduction (AB assay), a positive effect of the yarrow extract was observed in the concentration range of 0.01% to 2.5% in the case of keratinocytes, and in the range of 0.01% to 5% in the case of fibroblasts. The greatest increase in the metabolic activity of both cell types was observed for the concentration of 1.0%, at which the viability of HaCaT cells was 112.7% and that of HDF cells was 116.9%. \begin{table}
\begin{tabular}{c c c} \hline
**MBC/MFC** & **MIC** & **Test Microorganism** \\ \hline
16 & 2 & _Candidia albicans_ ATCC10231 \\ \hline
8 & 8 & _Streptococcus agalactiae_ PCM 2683 \\ \hline \textgreater{}16 & \textgreater{}16 & _Enterococcus faecalis_ PCM 2784 \\ \hline
16 & 8 & _Proteus mirabilis_ ATTC 29906 \\ \hline
8 & 8 & _Streptococcus mutans_ ATTC 25175 \\ \hline
8 & 8 & _Staphylococcus epidermidis_ ATTC 8853 \\ \hline
16 & 8 & _Streptococcus pyogenes_ ATTC 19615 \\ \hline
16 & 16 & _Escherichia coli_ UPEC PCM 176 \\ \hline
8 & 8 & _Enterococcus hirne_ ATCC 10541 \\ \hline
16 & 2 & _Bacillus subtilis PCM 486_ \\ \hline
8 & 8 & _Staphylococcus aureus 6538P_ \\ \hline
8 & 4 & _Staphylococcus epidermidis_ PCM 2118 \\ \hline
16 & 8 & _Escherichia coli_ ATCC 8739 \\ \hline
8 & 2 & _Pseudomonas aeruginosa_ PAO1 \\ \hline
4 & 2 & _Ralstonia solanacarum_ Z1 \\ \hline \end{tabular}
\end{table} 
Table 3: MIC and MBC/MFC values of yarrow extract in mg/mL. In both cell types, the extract at a concentration of 10% inhibited cell viability, especially in the case of keratinocytes, with 18.5% inhibition of viability (Figures 3 and 4). The results obtained using neutral red dye (NR assay) also showed the most beneficial effect of the _A. millefolium_ extract in the concentration range of 0.01-2.5%. The greatest accumulation of neutral red in the lysosomes was observed at a concentration of 1.0%, which resulted in keratinocyte viability of 117.5% and HDF cell viability of 114.8%. In the case of HaCaT cells, 5.0% and 10.0% concentrations of the extract reduced viability to 94.3% and 90.7%, respectively. In the case of fibroblasts, the NR assay showed no significant cytotoxic effect at any of the concentrations used (Figures 5 and 6). Figure 4: Reduction of resazurin by yarrow extract in the concentration range of 0.01% to 10.0% in HDF cells. Data are presented as the mean \(\pm\) SD of three independent experiments, in which each extract concentration was tested in triplicate. Figure 3: Reduction of resazurin by yarrow extract in the concentration range of 0.01% to 10.0% in HaCaT cells. Data are presented as the mean \(\pm\) SD of three independent experiments, in which each extract concentration was tested in triplicate. ***\(p\) = 0.0008, *\(p\) = 0.0275 versus the control. The plant studied in this work contains several bioactive plant compounds that can have a beneficial effect on the viability of skin cells through antioxidant, anti-inflammatory, cell proliferation-stimulating, and protective effects against oxidative stress and UV radiation [27; 28]. The polyphenols contained in the extract can neutralize free radicals, thus protecting skin cells from oxidative damage. This action supports the integrity of cell membranes and mitochondrial homeostasis, increasing the viability of keratinocytes and fibroblasts [27; 29]. The literature data also indicate that polyphenolic compounds reduce the effects of UV on skin cells by reducing the expression of collagen- or elastin-degrading metalloproteinases and increasing the level of endogenous antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase.