The first are phagocytes. These are cells that destroy other cells by surrounding them and engulfing them; a process called phagocytosis. Macrophages (or antigen presenting cells) are a type of phagocyte that travel through the body and engulf anything that is not recognized as part of the self. Inside the macrophage, an organelle called the lysosome, breaks down the engulfed substance into its component parts. These antigens are then presented on the surface of the macrophage bound to membrane proteins called Class II major histocompatibility complex antigens (MHC Class II). An antigen is anything that your body recognizes as foreign.
The last two important classes are both lymphocytes. T Cells (or T lymphocytes) are made from stem cells in the bone marrow, and then travel to the thymus, a small gland that sits above the heart. There they learn to recognize invading microbes by the antigens that are exposed on their surface. There are four main types of T cells. Helper T Cells are responsible for initiating the immune response. They contain a receptor, called a T-cell receptor (TCR), which can bind to a MHC Class II that is presenting a peptide in its binding site. Cytotoxic T Cells are responsible for rupturing membranes of cells that have been infected. Inducer T Cells oversee the development of T cells in the thymus and Suppressor T Cells are responsible for stopping the immune response.
The second kind of lymphocytes are the B Cells. These cells do not travel to the thymus after they have been made like the T cells do. Instead, they travel in the blood stream "looking" for foreign antigens. When a B cell "sees" an antigen, it divides rapidly into plasma cells which each produce an antibody. The antibody will bind to the foreign invader wherever it occurs in the body and mark it for destruction.