There are two different forms of specific immunity: humoral (or antibody-me diated) and cellular (or cell-mediated). Cellular immunity acts by direct cell-to-cell contact to protect the body against viruses that have infected its own cells and against tumor cells. Humoral immunity acts by the secretion of soluble proteins (antibodies or immunoglobulins) that circulate in the blood and lymph where they can combine with antigens and neutralise them.
Humoral (antibody-mediated) immunity can be either active or passive. Active immunity occurs when the antibodies are made within the body’s immune system.
It is passive when the antibodies come from outside the body. An ex-ample of this is the colostrum of mammalian milk, for a few months after birth a mammalian infant is naturally protected by passive immunity gained through antibodies secreted in its mother’s milk, this is also true in some fish eggs and larvae that contain some antibodies produced by the parent fish. Antibodies can be extracted from plasma and injected into a patient as an artificial but temporary means of providing immediate protection against certain antigens.
Antibodies are Y-shaped proteins formed from two pairs of polypeptide chains (two heavy and two light chains) held together covalently by disulfide bonds. Each antibody can bind selectively to one type of antigen. But each arm of the Y has a binding site, so each antibody molecule can bind to two molecules of the same antigen. The stem of the Y can be recognized by other cells of the body, so it helps determine what happens to an antigen-antibody complex.
The binding sites of an antibody are specific and highly selective for a single antigen. The body is capable of making an antibody that can combine with any conceivable antigen. The genetic mechanisms that allow this are complicated, but very fascinating. In addition, any particular antibody can occur as different classes of immunoglobulin (IgG, IgM, IgE, IgA, IgD), which are effective in different circumstances. They differ in the kind of protein that forms the stem of the Y (parts of the pair of heavy chains), although they have the same anti- genic specificity. The cells (activated lymphocytes) that make a specific anti-body can switch from making one class to making a different class, but always of the same antigen specificity.