In winter, the lizard, Scincus scincus, fails to form humoral antibodies to parenterally introduced rat erythrocytes. The response is slow and feeble in spring whereas, it is quick and vigorous in summer and autumn. In winter, the thymus of Scincus is involuted, white pulp of spleen highly depleted while, gut-associated lymphoid tissue is rather developed. By the beginning of spring, lymphoid organs start to regenerate. By the end of spring through midfall, the thymus presents a rich lymphoepithelial organization; splenic lymphoid aggregates are so developed that they become confluent and gut aggregates increase in number and size. The data suggest that, the kinetics and magnitude of the lizard's humoral response to foreign erythrocytes, correlate with the state of lymphoid tissue development which, in turn, is affected by seasonal variations.The different orders of reptiles possess a well developed lymphoid system and respond to challenges with diverse soluble and particulate immunogens, in a specific and temperature dependent manner (Bockman, '70; Cohen, '71; Cooper, '73, '76; Ambrosius, '76; Borysenko, '76). Although the vital activities of reptiles were reported to be influenced by seasonal factors (Cloudsley-Thompson, '71; Jcjrgensen, '76), the effect of seasonal variation on immune system in reptiles has not been investigated.Reports from our laboratory demonstrated that structure of lymphoid tissues in lizards and snakes is season-dependent (Hussein et al., '78a,b,c). The present data on the lizard, Scincus scincus, confirm this finding and illustrate the effect of seasonal variation on lizards' humoral response to heterologous erythrocytes. MATERIALS AND METHODS LizardsAdult, male and female Scincus scincusScincidae, hibernator (Badir, '58) -weighing 20-40g were collected from the desert near Alexandria. Lizards were maintained in large basins with 40 cm deep sand, in a sunny, beetles: Pimelia angulata) were given ad libitum. ErythrocytesAdult albino rats were bled, when needed, directly in Alsever's solution and erythrocytes (RRBCs) washed three times with phosphate buffered saline pH 7.1 (PBS). Sheep red blood cells (SRBCs) were obtained in Alsever's solution from the Egyptian Organization for Biological and Vaccine Production, Cairo and washed in PBS immediately before use in tests for specificity. ImmunizationAt the beginning of each season, 50 Scincus received each, a single intraperitoneal injection of 0.3-0.4 ml of 20% RRBCs suspension in PBS. Groups of four lizards were sacrificed a t 15-day intervals throughout the season. Serum was heat inactivated a t 56°C for 30 minutes and immediately tested for anti-body activity. Antibody assay (haemagglutination test)Haemagglutination tests were performed in microtiter plates (Cooke Laboratory Products, Alexandria, Virginia, USA) using two-fold dilutions of normal or experimental sera in PBS and 0.05 ml of 1% suspension RRBCs or SRBCs. Plates were maintained at + 4"C, read after 2 hours and again after 24 hours. Titers 91

In spring and autumn, the lymphoid complex of the colubrid snake, Psammophis schokari is well developed and the humoral response to intraperitoneally administered rat erythrocytes (RRBC). human serum albumin (HSA), or polyvinyl pyrrolidone (PVP) is quick and vigorous. In contrast, in summer and winter, lymphoid elements of the thymus, spleen, and part of the gut-associated lymphoid tissue are highly depleted. Humoral reactivity to RRBC and HSA is poor in summer presumably due to the deleterious effect of seasonal factors on the thymus and thymus-derived helper lymphocytes. This assumption is supported by the fact that, in summer, P. schokari responds powerfully to the T-independent antigen PVP. Humoral response to RRBC, HSA, and PVP is, however, weak in winter suggesting that a further mechanism of immunosuppression is involved.Few recent data are available on the reptilian immune system, and in snakes, lymphoid tissue structure and immunologic capacity are hardly documented (Bockman, '70; Coe et al., '76; Kawaguchi et al., '78; Hussein et al., '79).Although the effect of season on different vital activities of reptiles is dramatic (Jorgensen, '76), little is known about the influence of seasonal conditions on the lymphoid organs and immune responses of different reptilian divisions, especially Serpentes. This study was, therefore, undertaken to fill a major gap in the phylogeny of immunity, and concerns the effects of seasonal rhythms on the lymphoid complex development and humoral reactivity of the snake, Psammophis schokari to rat erythrocytes, human serum albumin, and polyvinyl pyrrolidone. MATERIALS AND METHODSAdult, male and female Psammophis schokari -Colubridae, hibernator (Marx, '68) -were collected from Egyptian deserts during different seasons. Snakes were maintained outdoors in wood and wire cages, fed twice weekly with live lizards (Acanthdactylus boskianus Audouin) and given water ad libitum. A total of 570 snakes, each weighing between 30 and 60 g m and of 40-70 cm in length from snout to cloaca, were used in the present study. IMMUNIZATIONAt the beginning of each season, three groups each comprising 30-40 snakes were injected intraperitoneally with antigen. Snakes of the first group received 0.4-0.6 ml of 20% rat erythrocyte (RRBC) suspension in phosphate-buffered saline (PBS); snakes of the second group received each 10 mg of human serum albumin (HSA) dissolved in 0.5 ml of PBS, while snakes of the last group were administered each with 10 pg polyvinyl pyrrolidone (PVP) M.W. 360,000 (GAF Corporation, New York). A further group of 20 snakes was injected with 0.5 ml of PBS and was used as controls.Three immunized snakes from each experimental group as well as control snakes were d e capitated at 15-day intervals throughout the season. Blood was drawn and allowed to clot for 2 hr at room temperature and then overnight at 4°C. Individual sera were separated by centrifugation, and were heat inactivated at 56°C for 30 min, and immediately tested for antibody activity. Anti-RRBC antibodies were...

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