Specific vs Nonspecific Immunity
Immune response is the complex series of mechanisms that act against invasions by harmful microorganisms. Without this defense, the body is vulnerable to a whole host of infections. Although not used by non-medical people, pathology books broadly categorize immunity into specific and non-specific immunity.
Non-specific immunity, as the name suggests, is not specific to a certain group of micro-organisms. These defense mechanisms act against each and every invader of the body. It is very important to understand that this non-specific immune response is so formidable that only a minute amount of infections penetrates this first line of defense.
Skin is the first barrier and the first mechanism of non-specific defense. Skin is a multilayered structure that contains lifeless dead cells on the outer surface and live cells in deeper layers. Many organisms find it impossible to penetrate this physical barrier. Skin cells are made by cell division at the deep basal layer. As cells reach the outer surface, they lose their vitality and finally detach themselves and shed. This outward migration of cells acts against the influx of invasive organisms. Skin contains various glands. Sebaceous glands secrete sebum which has antibacterial properties. Sweat washes infections off. High salt content of sweat dries micro-organisms off. Tears and saliva are secretions that wash the cornea and mouth continuously. Many epithelial surfaces in the body contain cilia. These cilia beat rhythmically to transport matter out of the body (respiratory epithelium). Saliva contains anti-bacterial properties due to lysozymes. Some epithelia produce mucus which also acts as a barrier against infections. If and when micro-organisms penetrate these defense systems they meet the lymphocytes, macrophages which phagocytose foreign matter non-specifically. This may or may not lead to generation of a specific immune response.
When a foreign substance is phagocytosed by a macrophage, a white blood cell, or an antigen presenting cell, it gets processed inside the host cell. There are antigen binding receptors called major histocompatibility complexes (MHC type 1 and 2). MHC 1 crosslinks with CD8 type lymphocytes while MHC 2 crosslinks with CD4 type lymphocytes. There is an enormous variation among antigen receptors in both T cells and B cells. CD4 T Lymphocytes get activated by this receptor cross linkage, and they produce cytokines which promote proliferation of selected lymphocytes, formation of new lymphocytes with selected receptor types, and activation of B cells to form antibodies. These mechanisms culminate in the destruction of the foreign organisms phagocytosed previously. CD8 T lymphocytes get activated by receptor cross linkage and produce substances which are highly toxic to foreign microorganisms. Specific immune response occurs in two separate occasions. When a microorganism enters the body for the first time the response is a bit delayed till all these aforementioned processes occur to an extent that any effect is observable. This is called the primary response. The immunoglobulin formed is IgM. Primary response is of a smaller magnitude than the secondary response. After primary response, some T and B cells mature into memory cells. These cells act as a shortcut so that when the antigen enters the body a second time all the initial steps are bypassed. This secondary response is much larger and much quicker. The main immunoglobulin is IgG.
What is the difference between Specific and Nonspecific Immunity?
• Non-specific immunity is a set of defenses effective against all the invaders while specific immunity is a highly focused and targeted response.
• Non-specific immunity is the first line of defense while specific immunity is the second line of defense.
• Non-specific immunity includes effector cells like white blood cells and macrophages while specific immune response includes cells like lymphocytes, antigen presenting cells, and memory cells.
• Non-specific immunity does not form a defensive memory while specific immunity does.