Host Immunity

Immunoglobulin classes and functions

Immunoglobulins (antibodies) are grouped into five classes, IgG, IgM, IgA, IgE and IgD, and each class has a characteristic distribution and role. Matching a class to its function is a common examination task, so the key facts should be memorized as pairs. IgE — binds to Fc receptors on mast cells and basophils and mediates immediate hypersensitivity, or allergic, reactions. Its levels rise in allergy and parasitic infection. Example: IgE cross-linking on mast cells triggers histamine release. IgG — the most abundant immunoglobulin in serum, making up roughly 75 percent of the total, and the only class that crosses the placenta, so it protects the fetus and newborn. It also provides long-term humoral immunity. IgM — a pentamer that is the first antibody class produced during a primary immune response, so it appears early after infection. Example: a rising IgM titer early in an infection suggests a recent primary exposure. IgA — the main class in external secretions such as saliva, tears, milk and gut and respiratory mucus, where it gives localized protection of mucosal surfaces. It usually exists as a dimer in secretions. A frequent trap is to assume that antibodies lyse target cells directly. Antibodies do not lyse cells by themselves; they act through neutralization, opsonization, complement activation, or recruitment of effector cells, so direct lysis is not an antibody function in these matching questions.

Leukocyte types in blood

Leukocytes (white blood cells) are classified into granulocytes, which are neutrophils, eosinophils and basophils, and agranulocytes, which are lymphocytes and monocytes. The relative proportion of each type in healthy peripheral blood must be memorized, because questions often ask which type is most or least abundant. Neutrophils — the most abundant leukocyte in healthy human blood, normally about 50 to 70 percent of the total count. They are the first cells to arrive at sites of acute bacterial infection and are produced in large numbers in the bone marrow. Lymphocytes — the second largest group in healthy blood, and they include the B cells, T cells and NK cells of adaptive and innate immunity. Monocytes — a smaller fraction that become macrophages and dendritic cells in tissues. Eosinophils — a small fraction that rises in parasitic infection and allergy. Basophils — the least abundant leukocyte, well under 1 percent of the total count. Example: a differential count that shows neutrophils as the largest group is normal, while a very high eosinophil count suggests parasitic infection or allergy.