Cellular immunodeficiencies are rare and more difficult to diagnose than humoral immunodeficiencies, for they may result from the dysfunction or expression of cell surface activation molecules, or their cell signaling components. T cell disorders may affect directly the cytotoxic function (cytotoxic T cells), or indirectly the B cell function (lack of CD4+ T cell interaction for required costimulation and signaling). The number of T cells in the peripheral blood and/or lymphoid tissues is often decreased. Cellular immune disorders are associated with intracellular pathogens (Pneumocystis carinii, Rhodococcus equi, viruses, fungi, protozoa).

Transient CD4+ T cell lymphopenia of the foal

The healthy foal presents a remarkable physiologic age-dependent increase in the total peripheral blood lymphocyte counts and secondary lymphoid tissue weight, which reflects the intense lymphocyte priming and population expansion upon encounter with many pathogens. Lymphocyte counts in foals increase gradually up to 4,500 to 6,000 cells/uL by 6-7 months of life. Concurrently, a proportional increase in the lymphocyte subpopulation distribution (CD4+ T cells, CD8+ T cells, and B cells) is expected.

Occasionally, foals in the initial 6 months of life may present a delay in reaching normal values for CD4+ lymphocytes in peripheral blood, with consequent low CD4:CD8 ratios (less than 2.0). In addition, failure to increase the expression of major histocompatibility complex (MHC) class II in lymphocytes (but not monocytes) is often observed in this condition, and suggests poor development of these cells. Recurrent respiratory infections with common (e.g. Streptococcus zooepidemicus) or opportunistic organisms (e.g. Pneumocystis jirovecii ) are the clinical signs of an underlying immunodeficiency condition. Interestingly, foals may present normal growth and weight gain during this phase.

Absolute lymphopenia may not be obvious because of the age-dependent increase in circulating lymphocytes; therefore, lymphocyte phenotyping is recommended in foals with a history of recurrent infections from 3-6 months of life. Long-term antibiotic therapy may be necessary during this transition time. Foals tend to recover clinically and to normal lymphocyte subpopulation distributions by 8 months of life, although it may take as long as 24 months of life. The cause of this transitory condition is unknown.