Overview
When innate defenses cannot fully clear a pathogen, the adaptive immune system takes over. It is slower to start but exquisitely specific, targeting one particular invader, and it forms memory so future encounters are handled quickly. Its work is carried out by two families of lymphocytes—B cells and T cells—and by the antibodies that B cells produce.
Antigens and Antibodies
An antigen is any molecule the immune system recognizes as foreign, usually a protein or sugar on a microbe’s surface. Each lymphocyte carries receptors that fit only one antigen shape, like a lock and key.
An antibody is a Y-shaped protein made by B cells that binds a specific antigen. Antibodies neutralize toxins, coat microbes for phagocytes, clump pathogens together, and activate complement. Humans make five classes.
| Class | Key features |
|---|---|
| IgG | Most abundant; dominates the secondary response; crosses the placenta |
| IgM | First antibody made in a new infection; very effective at clumping microbes |
| IgA | Guards mucous membranes and is found in saliva, tears, and breast milk |
| IgE | Involved in allergies and parasite defense; triggers histamine release |
| IgD | Found on B cell surfaces; helps activate them |
B Cells, T Cells, and Antigen Presentation
B cells mature in the bone marrow and fight mainly through antibodies (humoral immunity). T cells mature in the thymus and drive cell-mediated immunity. There are two principal types:
- Helper T cells (CD4) coordinate the response, releasing signals that activate B cells and cytotoxic T cells.
- Cytotoxic T cells (CD8) directly kill body cells that are infected by viruses or have become cancerous.
T cells cannot see free antigen; it must be displayed to them. MHC (major histocompatibility complex) molecules are surface proteins that present antigen fragments. MHC class I, present on nearly all cells, shows what is happening inside a cell to CD8 T cells; if a cell displays viral proteins, cytotoxic T cells know it is infected and destroy it. MHC class II, on antigen-presenting cells such as dendritic cells and macrophages, displays engulfed material to CD4 helper T cells—the crucial handoff from innate to adaptive immunity. Because MHC molecules differ from person to person, they also explain why transplanted organs can be rejected as foreign.
Clonal Selection and Memory
The body holds millions of lymphocytes, each specific for a different antigen. When an antigen appears, it activates only the rare lymphocytes that recognize it. Those cells then divide rapidly—a process called clonal selection. The expanding clone produces two kinds of cells:
- Effector cells that fight the current infection (antibody-secreting plasma cells from B cells; active T cells).
- Memory cells that survive for years and stand ready for the future.
Primary Versus Secondary Response
The primary response, on first exposure, takes about one to two weeks to build, and IgM appears first. During this lag a person may feel sick while the response ramps up. It also leaves behind memory cells. On re-exposure, the secondary response is faster (a few days), stronger, and richer in high-quality IgG, often stopping the infection before symptoms appear. This durable memory is the foundation of vaccination, which is the subject of the next lesson.
Active and Passive Immunity
Active immunity develops when the body makes its own antibodies and memory cells—either naturally after infection or artificially after vaccination. It is long-lasting. Passive immunity comes from receiving ready-made antibodies, such as those passed from mother to fetus across the placenta, delivered in breast milk, or given as an antibody injection. It works immediately but fades within weeks to months because no memory cells form.
Clinical Relevance
The adaptive system is powerful, and its errors cause important diseases.
- Allergy and anaphylaxis: In allergy, IgE reacts to harmless substances like pollen, causing mast cells to release histamine. A severe, body-wide reaction—anaphylaxis—can drop blood pressure and close the airway within minutes and is treated urgently with epinephrine.
- Autoimmunity: The system mistakes the body’s own tissues for foreign, as in type 1 diabetes, rheumatoid arthritis, and lupus.
- Immunodeficiency: When the system is too weak, infections take hold. HIV infects and destroys CD4 helper T cells; as their numbers fall, coordination collapses and untreated infection can progress to AIDS, marked by life-threatening opportunistic infections. This is why understanding CD4 cells is central to nursing and emergency care.