Diversity and dynamics of populations and disease susceptibility

“In an endemic environment, all susceptibles will develop the disease.“ South India, like the rest of the country is known for its caste system. The origin, level of inbreeding, endogamy and sympatric isolation amongst the caste system will lead to divergence of their gene pool and Ir (Immune response) genes are no exception to this. These differences may result in differential susceptibility at the population level. The lessons from inbred strains of animals explain this phenomenon and no immunologist or geneticist of today would like to carry out an experiment by mixing up the different strains of a species. In human populations in general, and in Indian caste groups in particular, the population dynamics like migration, miscegenation, social taboos and marriage patterns skew the picture and mask the differences between these populations particularly the prevalence and susceptibility to disease. Most of the research workers are disabled having a limited knowledge and even more limited facilities to do an 'ideal' experimental study in humans. Thus, in any clinical disease or immunological study in humans, a (case), caste, sex, nativity and haplotype (HLA or Ir gene) matched controls may need to be studied to understand the immunogenetic basis of disease susceptibility. The studies hitherto carried out at Madurai have revealed: i) different caste groups possess different haplotypes, some characteristic to a caste whereas others were common to many of them, ii) genetic distance calculated based on allele frequency brought out their affinity to each other; iii) not many Brahmin populations of India, have the same of the gene pool, presumably because of their origin, though they have all adopted the Hindu philosophy and religion, iv) numerically larger and geographically adjacent patrilineal clans of a tribe are genetically closer to each other; v ) a given HLA disease association transcend ethnic barrier (eg. pulmonary tuberculosis, leprosy), due to Ir gene dependant immunogeneric predisposition, vi) a few other HLA disease associations found in some populations or caste groups and not in others (eg. psoriasis) may be due to a linked gene and hitch hike phenomenon. Another new dimension is added to this genetic epidemiology: settlements, population size and the microbial world and infections increase in size as a function of time over the decades, resulting in faster transmission of a disease. The epidemiology is also changing over a period in the same place. As a result the newborn of today are subjected to a newer set of stress and selections than they were a generation ago. The epidemiology is known to affect the thymic education of lymphocytes through MHC, resulting in a different repertoire among children brought up in different environments. This has had great implications in subsequent environmental challenges and infections. Today any problem should be investigated and tackled by a group of open minded, knowledgeable scientists cutting across the barriers of their field of specializations. This is the need of the hour in this country