VIENNA, AUSTRIA—Living in overcrowded conditions appears to protect children and young adults against developing a subtype of Hodgkin lymphoma (HL), a cancer that originates from the lymphocytes (white blood cells). The protective effect suggests that infections earlier in life may stimulate the immune system to deal with future infections and cancerous cells more efficiently, according to the British researchers who made the discovery.

Presenting their results at the 2015 European Cancer Congress (ECC2015), Richard McNally, DIC, PhD, a Reader in Epidemiology at the Institute of Health and Society, Newcastle University, Newcastle, United Kingdom, said the causes of HL were not well understood at present. In order to try to achieve better knowledge of this issue, the researchers decided to analyze all 621 cases of HL in patients ages birth to 24 years recorded in the Northern Region Young Persons’ Malignant Disease Registry.

“Childhood lymphomas are more common in males, but analysis by sex has not been done very frequently. Additionally, the male-to-female ratio changes in Hodgkin lymphoma according to age, so we decided to take age into account, as well as other factors such as socio-economic deprivation,” McNally said.

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Hodgkin lymphoma occurrence peaks during young adulthood and again in persons older than 55 years. The overall 5-year survival rate is approximately 85%. For the birth to 24 years age group included in the study, 5-year survival is approximately 93%.

The researchers found five different subtypes of Hodgkin lymphoma among the patients studied: 247 cases of the nodular sclerosis (NS) type (tumor nodules are large); 105 of mixed cellularity (a mix of different types of inflammatory cells); 58 lymphocyte-rich, the subtype with the best outcome; 68 others; and 143 not otherwise specified (NOS). Overall, more males than females within the group had Hodgkin lymphoma, but the male:female ratio varied by both age group and subtype. For the NS subtype, there were 130 males and 117 females, but this was reversed at ages 20 to 24 years, with 72 females and 55 males.

Deprivation was calculated using the four components of the Townsend deprivation score: household overcrowding, non-home ownership, unemployment, and households with no car. The researchers found a lower incidence of the NS subtype among patients living in areas with more overcrowded households, and a 5% higher level of household overcrowding had the effect of halving the number of cases of the subtype.

However, for the NOS group the reverse was seen. Overcrowding was associated with a higher incidence of NOS type Hodgkin lymphoma, while deprivation seemed to have no effect on the incidence of mixed cellularity and lymphocyte-rich subtypes.

“Our findings related to the NS subtype may suggest that the recurrent infections to which children living in overcrowded conditions are likely to have been exposed stimulate their immune systems and hence protect them against developing this type of cancer later in their childhood and early adult life,” said McNally. “Those who have a genetic susceptibility to Hodgkin lymphoma and have been less exposed to infection through not living in such overcrowded conditions may have less developed immune systems as a result, and are, therefore, at greater risk of developing this subtype.”

“We knew already that recurrent infections may protect against childhood leukemia, and now it looks as we can add Hodgkin lymphoma, and, particularly its NS subtype, to the list,” concluded McNally.