C.C. Santana et al. Parasite Immunology Granuloma Poorly structured aggregates, consisting of at least four large epithelioid macrophages (Figure 1b), to slightly organized granulomas (Figure 1c) were observed in 8 (11%) out of 72 animals (Table 2 and Figure 1b). They were present in 1 (9%) out of 11 animals in the undeﬁned susceptibility state group and 7 (28%) out of 25 animals in the infected and potentially susceptible to VL group. The difference in the presence of spleen granulomas between the infected and potentially susceptible to VL and other groups was statistically signiﬁcant (χ2-test, P = 0·0102). 518 Figure 1 Morphological changes in the spleen of dogs naturally infected by L. chagasi. (a) Chronic perisplenitis; (b) Ill-structured granuloma with a loose grouping of a few epithelioid macrophages; (c) Well-structured granuloma with tightly grouped epithelioid macrophages in a round structure rimmed by lymphocytes and plasma cells; (d)–(i) Architectural deﬁnition of the lymphoid follicles (g) extensively disorganized lymphoid tissue; (h) moderately disorganized lymphoid tissue; (i) well organized lymphoid tissue (d, e, and f – details of the follicular borders); ( j ) Increased plasma cell number in the red pulp; (k) Parasite-loaded macrophages in the subcapsular area; (l) Parasite-loaded macrophages in the internal red pulp. Lengths of the scale bars on the right hand side at the bottom of the ﬁgure: a, 50 μm; b, c, j, k and l, 25 μm; d, e and f, 100 μm; g, h and i, 250 μm. Cell populations in the red pulp The frequency of plasma cells was higher in the groups with evidence of Leishmania infection than it was in the noninfected group (χ2-test, P = 0·0366, Table 2 and Figure 1j). The distribution of lymphoblasts, granulocytes, resident macrophages and megakaryocytes did not differ signiﬁcantly among the groups (Table 2). Parasite burden Amastigotes were visualized in the spleens of 10 (14%) out of the 72 animals (Table 2); all of these animals belonged to © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Ltd, Parasite Immunology, 30, 515–524