Introduction to Immunity

Core Concepts

Innate immunity: fast but general

Innate immunity covers the first two lines of defense — the surface barrier (skin, mucous membranes, secretions) and the rapid responders beneath. Responses are immediate (minutes to hours) and identical for every invader. Key players:

• Phagocytes — neutrophils (fast, abundant first responders) and macrophages (eat debris, later presenting antigen pieces to T cells).
• Natural killer (NK) cells — destroy virus-infected and tumor cells.
• Complement system — blood proteins that mark microbes for destruction, punch holes in their membranes, and attract phagocytes.
• Interferons — signaling proteins released by infected cells that warn neighbors and slow viral spread.
• Inflammation — the coordinated response (redness, heat, swelling, pain) that delivers blood, fluid, and immune cells to the injury site.

Innate immunity has no memory: the fifth exposure is handled like the first.

Adaptive immunity: specific and memorable

Adaptive responses take days on first exposure but are precise. Two branches work together:

• Humoral immunity (B cells → antibodies): B lymphocytes make and release antibodies (immunoglobulins) that bind specific antigens — neutralizing viruses, tagging bacteria for destruction (opsonization), and activating complement. Classes: IgG (most abundant, crosses the placenta, main memory antibody), IgM (first produced), IgA (mucosal surfaces, breast milk), IgE (allergic reactions), and IgD (B cell function).
• Cell-mediated immunity (T cells): helper T cells (CD4+) direct the response, activating B cells, macrophages, and other T cells; cytotoxic T cells (CD8+) directly kill infected or abnormal cells by recognizing antigen fragments displayed on their surface (via MHC molecules).

Antigen presentation is the bridge: a macrophage or dendritic cell engulfs an invader, displays a piece of it on its surface, and shows it to helper T cells, which switch on the right B and T cell responses.

Memory: primary vs secondary response

On first exposure, the adaptive response is slow (the primary response): it takes days for the specific B and T cells to proliferate, and antibody levels rise slowly. Some cells become long-lived memory cells. On second exposure, memory cells recognize the antigen immediately and proliferate fast, producing a secondary response: quicker, higher antibody levels, longer protection — often so fast that illness never develops. Vaccines exploit this: they deliver antigens (weakened, inactivated, or pieces of a pathogen) to create memory without disease.

Active vs passive immunity

• Active immunity — the person's own immune system produces the response and the memory: natural (recovering from an infection) or artificial (vaccination). Protection is long-lasting.
• Passive immunity — preformed antibodies are transferred: natural (maternal antibodies across the placenta and in breast milk) or artificial (immune globulin after exposure). Protection is immediate but temporary — no memory is created.

When immunity goes wrong: a preview

The same system that protects can cause harm when misdirected: hypersensitivity (overreaction — allergic reactions, including anaphylaxis), autoimmunity (attacking the body's own tissues), and immunodeficiency (underfunctioning — congenital or acquired, as in HIV/AIDS or chemotherapy-induced neutropenia). These states explain why some drugs suppress immunity (immunosuppressants for autoimmune disease and transplant rejection) and others harness it (vaccines, monoclonal antibodies).

Common Confusions