Immunity in biology refers to an organism's capacity to fend off harmful microbes. Both specific and generic components contribute to immunity. Regardless of their antigenic makeup, the nonspecific components serve as barriers to or eliminators of a variety of infections.
Innate immunity serves as the generic defense whereas adaptive immunity serves as the particular defense. While adaptive immunity is gained during one's life, innate immunity is the immunity one is born with or has had since birth. Only vertebrates possess acquired immunity. Cellular immunity, humoral immunity, active immunity, and passive immunity are the four categories of adaptive immunity. Passive immunity is transient and only lasts a few weeks to months while active immunity endures for a long period in the host. The first and second lines of defense for an organism are determined genetically by innate immunity, whereas the second and third lines of defense for an organism are acquired through adaptive immunity, which is not determined genetically.
Both active and passive immunity are of two subtypes- natural and artificially acquired type. Antibodies which are made after exposure to infection are natural active immunity while antibodies transmitted from mother to baby through mother’s milk is natural passive immunity. Antibodies made after getting vaccination (Immunity post covid vaccination) are artificial active immunity and antibodies acquired from immune serum medicine are artificial passive immunity.
Thus, the human body can resist almost all types of organisms or toxins that damage the tissue and organ, this capacity is called immunity. The immune system has evolved a powerful collection of defense mechanisms to protect against potential invaders that would otherwise take advantage of the rich source of nutrients provided by a vertebrate host. At the same time, it differentiates between the individual’s cells and those of harmful invading organisms and does not attack the commensal flora that inhabits the gut, skin and many other tissues to great benefit. Innate immunity is called the first line of defense as it is always ready to prevent and eradicate infections while adaptive immunity develops later after exposure to microbes so it is even more powerful in combating infections. By convention, the term immune response refers to adaptive immunity.
Polymorphonuclear neutrophils are short-lived phagocytes
• Polymorphonuclear neutrophils (often just called neutrophils or PMNs) are another important group of phagocytes.
• Neutrophils constitute the majority of blood leukocytes like monocytes, neutrophils that migrate into tissues, particularly at sites of inflammation; however, neutrophils are short-lived cells that phagocytose material, destroy it and then die.
• Lymphocytes are responsible for the specific immune recognition of pathogens.
• So, they activate adaptive immune reactions.
• Every B cell is naturally designed to show a surface receptor specific for a particular antigen.
• This antigen receptor molecule is known as an antibody.
• If a B cell combines with its specific antigen, it will divide and differentiate into plasma cells which generate a large number of the antibody but in a secreted form.
There are several different types of T cells and they have a variety of functions.
• One division interacts with mononuclear phagocytes and helps them destroy intracellular pathogens, they are called type 1 helper T cells or TH1 cells.
• The third group of T cells is responsible for the destruction of host cells that have become infected through viruses or other intracellular pathogens, this type of activity is known as cytotoxicity
• Other groups interact with B cells and help them to divide, differentiate and make antibodies. They are the type 2 helper T cells or TH2 cells and these T cells hence known as cytotoxic T lymphocytes (CTLs or TC cells).
• Through the release of soluble proteins called which signal to other cells
• Or by direct cell-cell interactions.
Cytotoxic cells recognize and destroy other cells that have become infected
Cytotoxic cells include CTLs, natural killer (NK) cells (large granular lymphocytes) and eosinophils.
• Lymphocytes known as large granular lymphocytes (LGLs) can recognize the surface changes.
• LGL damages these target cells.
• This action is sometimes called NK cells activity.
The main purpose of inflammation is to attract leukocytes and the soluble mediators of immunity towards a site of infection.
Basophils are mast cells that have granules that contain a variety of mediators which induce inflammation in surrounding tissue and are released when the cells are activated.
Platelets are essential in blood clotting but can also activate during the immune response to release mediators of inflammation.
Components of the complement system can lyse many bacteria.
1. Complement fragments released in the reaction attract phagocytes to the site of the reaction.
2. Complement components opsonize the bacteria for engulfing and digestion.
3. Activation of the complement system increases blood flow and vascular permeability at the site of activation.
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