All three types have important functions in the immune system (see Understanding the Immune System). Too few B cells can lead to a decrease in the number of plasma cells, which produce antibodies. Decreased antibody production can cause an increase in bacterial infections. People who have too few T cells or too few NK cells have problems controlling certain infections, especially viral, fungal, and parasitic infections. Severe lymphocyte deficiencies can result in uncontrolled infections that can be fatal.