CD4 and infection: If the integrity and functionality of antigen-presenting cells (APCs), such as macrophages, dendritic cells, and B lymphocytes, as well as CD4+ and CD8+ T-cells, are compromised from the start of the first immunization, owing to a higher dose of E. tenella oocysts (referred to as the “crowded dose”), the birds may develop an ineffective immune response in subsequent infections due to a lack of competent lymphoid cells [2,7,10,11,12,14].