T and B Cells

T and B cells are two major types of white blood cells, known as lymphocytes, that form the backbone of the adaptive immune system. Unlike the innate immune system, which provides a broad, immediate defense, the adaptive system is highly specific and capable of forming memory. This means that once your body has been exposed to a virus, bacteria, or even a vaccine, T and B cells help ensure you can respond faster and more effectively the next time.

T cells, which develop in the thymus (hence the “T”), come in different subtypes. “Helper” T cells coordinate the immune response, signaling other immune cells into action, while “killer” T cells directly destroy infected or cancerous cells. Another group, “regulatory” T cells, keep the immune system from becoming overactive and damaging healthy tissue. B cells, which mature in the bone marrow, are responsible for producing antibodies. Antibodies are proteins that recognize and neutralize specific pathogens such as viruses or bacteria. B cells also serve as “memory cells,” allowing long-term protection after infections or vaccinations.

A total T and B cell count measures how many of these cells are circulating in your blood. Low levels can signal weakened immunity. This can occur in conditions like HIV, certain cancers, bone marrow failure, or after treatments that suppress the immune system, such as chemotherapy or organ transplantation drugs. For example, people with very low T cell counts may be unable to fight off common infections. Low B cell levels, meanwhile, often result in poor antibody responses, making vaccines less effective and infections more severe. High levels of T and B cells may be seen when the immune system is highly active, as in autoimmune conditions where the body mistakenly attacks itself, or in certain infections and blood cancers such as lymphomas and leukemias.

It is important to note that total counts only give part of the picture. Someone may have normal overall numbers but an imbalance in subtypes—for example, too few helper T cells or dysfunctional B cells. This is why more detailed immune panels often break down lymphocytes into subsets, helping clinicians identify subtle immune deficiencies or autoimmune activity that a total count alone might miss.