The main immune cells consist of B cells, T cells and Macrophages. B and T cells, also called lymphocytes, begin life in the liver of a nine-week foetus, migrate to the bone marrow, where they begin to specialize and develop along certain lines relating to their future function. One type of B cell is the plasma cell, which produces antibodies against foreign invaders called antigens. There are a number of antibodies, designated IgG, IgA, IgM, IgE, and IgD, which specialize in fighting different types of invaders to the body. Another type of B cell is the memory cell which helps to develop rapid antibody response to antigens to which the individual has previously been exposed.
B cells are involved in humoral immunity.
There are a number of types of T cells, which, together with Macrophages, are involved in cell-mediated immunity. These include T helper or T4 cells, which stimulate B cells and other T cells to activate an immune response to antigens. T4 cells are infected and destroyed by the HIV virus. Because they are one of the first in a chain reaction of responses to infection, decimation of these cells by the AIDS virus leaves the individual vulnerable to attack from normally innocuous organisms. T suppressor or T8 cells, provide an immune feedback inhibitory mechanism, to keep the immune response in balance.