Viral Infection: This is often the most common cause of a relative lymphocytosis, especially in the absence of leukocytosis. Viral infections can lead to an increase in lymphocytes as the body mounts an immune response.

Chronic Lymphocytic Leukemia (CLL): Although CLL typically presents with leukocytosis, early stages or certain variants might not. It's essential to consider CLL, especially in older adults.
Lymphocytic Reaction to Stress or Inflammation: Certain stressors or inflammatory conditions can cause a reactive lymphocytosis.
Hypersensitivity Reaction: Allergic reactions or hypersensitivity to drugs can sometimes cause an increase in lymphocyte percentage.

Tuberculosis (TB): TB can cause a lymphocytosis, and it's crucial not to miss this diagnosis due to its significant implications for public health and the need for specific treatment.
Lymphoma: Although less common, lymphoma can present with an increase in lymphocytes. Missing this diagnosis could have severe consequences.
HIV Infection: Early stages of HIV infection can lead to a relative lymphocytosis. Given the importance of early diagnosis and treatment, HIV must be considered.

Castleman Disease: A rare disorder that can cause an increase in lymphocytes due to lymph node hyperplasia.
Lymphoproliferative Disorders: Other rare conditions, such as lymphomatoid granulomatosis, can also lead to an elevated lymphocyte percentage.
Certain Autoimmune Disorders: Some autoimmune diseases, like lupus or rheumatoid arthritis, can occasionally present with lymphocytosis, although this is less common.