Bacterial Infection: This is often the most common cause of neutrophilia (high neutrophil count) and can lead to a relative decrease in lymphocytes due to the body's immediate response to infection, which primarily involves neutrophils.

Acute Stress Response: Stress, whether physical (e.g., trauma, surgery) or emotional, can lead to an increase in neutrophils and a decrease in lymphocytes due to the release of stress hormones like cortisol.
Chronic Inflammation: Conditions such as chronic obstructive pulmonary disease (COPD), rheumatoid arthritis, or other autoimmune diseases can cause a sustained increase in neutrophil production and a relative decrease in lymphocytes.
Steroid Use: Exogenous steroid use can cause neutrophilia and lymphopenia (decreased lymphocytes) due to the effects of steroids on the immune system, promoting the release of neutrophils from the bone marrow and the sequestration of lymphocytes.

Sepsis: Although it might seem similar to bacterial infection, sepsis is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. Early recognition is crucial.
Hematological Malignancies: Certain blood cancers, such as leukemia, can present with abnormal blood cell counts, including high neutrophils and low lymphocytes. Missing these diagnoses can have severe consequences.
Severe Acute Respiratory Syndrome (SARS) or COVID-19: These viral infections can initially present with lymphopenia and, in some cases, neutrophilia, especially in severe cases.

Myeloproliferative Neoplasms: Rare disorders like polycythemia vera, essential thrombocythemia, or primary myelofibrosis can lead to an overproduction of blood cells, including neutrophils.
Lymphoma: Certain types of lymphoma can cause changes in blood cell counts, although this is less common.
Congenital Immunodeficiencies: Rare genetic disorders that affect the immune system can lead to abnormal blood cell counts, including decreased lymphocytes.