I region, which presents peptides from inside the cell to be destroyed by the immune system. HLA- linked olfactory receptor genes can therefore provide a possible mechanism for detecting HLA- specific odours. Men and women are attracted to the pheromones they produce and HLA is related to the perceptions of other peoples' odours. Males produce androstenol and androstenone. Androstenol is produced by fresh male sweat and is most attractive to women, while androstenone is produced once the sweat is exposed to oxygen and is highly unpleasant to women. Studies have shown that women who are at the most fertile stage of their menstrual cycle prefer the smell of men that have higher testosterone levels. Heterozygosity of HLA can also be detected through scent: in this case, heterozygosity confers greater ability to recognize a wider variety of antigens. Females, especially when not using hormonal contraceptives, are more attracted to the scent of males heterozygous for HLA. However, the same attraction and mate preferences are not held by males for heterozygous females. Males are, however, more attracted to the scent of females with rare HLA alleles. Men's arousal can be influenced by a variety of odours. For example; pumpkin pie, liquorice, doughnuts, and lavender can increase penile blood flow consequently causing arousal. Human pheromones and facial attraction Two types of male pheromones: 5α-androst-16-en-3-one (MP1, androstenone) and 4,16-androstadien-3-one (MP2, androstadienone) and one type of female pheromone: 1,3,5(10),16-estratetrael-3-ol (FP, estratetraenol) are used as signals of mating quality. Studies showed that MP2 has the strongest response produced by female vomeronasal organ (VNO), which is the first stage of the olfactory system. Sex-difference processing in the hypothalamus has been found between female and male pheromone, including in the VNO, where opposite-sex pheromones have different surface potential. Studies have illustrated a relationship between human facial attraction and pheromones. Individuals rated those of the opposite sex as more attractive if they preferred the individual's pheromones' odour. It was also demonstrated that partners in a long-term relationship would have higher concordance between two types of signals of mating quality, vision and olfaction. In addition, no difference was found between two types of male pheromones in women's preference aside from the location of pheromones production. Body odour and menstrual cycle changes Women's fertility levels shift dramatically throughout the menstrual cycle, so the period surrounding ovulation is extremely important because it represents the peak period of reproductive fertility. As conception is