Absolute Lymphocyte Count

Pathogen recognition by adaptive immunity requires antigen presentation by infected cells, which carry out this task using a protein called major histocompatibility complex. MHC complexes show significant sequence and structural overlap between humans, chimpanzees, and old world monkeys. In contrast, new world monkeys show significantly different genomic organization of their MHC loci and less polymorphism. Regardless, the MHC antigen system that our lymphocytes recognize appears to have existed during early primate evolution, before the branching of modern taxa (Kennedy 1997, Bontrop 2006). Lymphocyte cell surface markers are also largely conserved, with human T and B cell markers present in nonhuman primates. Antibodies raised against these markers cross-react with lymphocytes in chimpanzees, orangutans, baboons, macaques, capuchins, marmosets, and others. These animals represent species from the hominoids, old world monkeys, and new world monkeys. In addition, human colony stimulating factors are able to induce lymphocytosis (increased blood lymphocytes) in other primates. Taken together, these data indicate that the adaptive immune system existed in the common primate ancestor, and genetic differences are a product of species divergence (Clark 1983, Mayer 1987).

Within mammals, there is significant variation in the degree of immune defense. These differences can be quantified in terms of spleen size, number of circulating leukocytes, and ease of lymphocyte activation. The overall hypothesis contends that increased immune defense should correlate with increased disease risk due to the high cost of maintaining immune defenses. Studies have identified a number of factors that contribute to pathogen exposure and active immunity. These include increased body size, longer life span, group size and density, diet, and habitat diversity. The major confounding factor in these studies is that better studied species have more known pathogens, and thus appear to suffer from an increased pathogen load. Primates show significant differences, with prosimians exhibiting the lowest overall parasite species diversity, and old world monkeys showing the highest viral species diversity. A striking correlation involves promiscuity. Species with multiple mating partners appear to have higher leukocyte counts. These are thought to combat the increased risk of STD infection (Nunn 2002, Nunn 2003).

Surprisingly, humans demonstrate somewhat more active lymphocytes than chimpanzees, with stronger cytokine and activation marker responses to a variety of stimuli. In 2006, for example, attempts to develop a therapeutic using CD28 T-cell activation triggered an extreme inflammatory reaction in human test subjects. Tests in rats and monkeys using a dose 500 times greater showed no adverse effects. Differences in lymphocyte activity may explain inter-species differences between disease incidence and severity. Excessive activation of adaptive immunity may explain why HIV, hepatitis C, and autoimmune disorders are more severe in humans than other primates (Nguyen 2006, Soto 2010).