Tag: immunity and the lines of immunity

Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. They are essential for fighting infections and for subsequent immunity. Phagocytes provide the second line of defence which is considered as a non-specific defence because it destroys invaders in a generalized way without targeting specific pathogens. Phagocytes are activated when the pathogen is able to cross the first line of defence.

Neutrophils are the most abundant type of granulocytes. They form an essential part of the innate immune system. Neutrophils undergo a process called chemotaxis via amoeboid movement, which allows them to migrate toward sites of infection or inflammation. Neutrophils express and release cytokines, which in turn amplify inflammatory reactions by allowing other neutrophils, monocytes, eosinophils attachment to endothelium and dilation of capillaries is seen to increase the blood flow to the affected area.

Chemical barriers protect against infection. The skin and respiratory tract secrete antimicrobial peptides such as the β-defensins. Enzymes such as lysozyme and phospholipase A2 in saliva, tears, and breast milk are antibacterial substances. Vaginal secretions serve as a chemical barrier following menarche, when they become slightly acidic, while semen contains defensins and zinc to kill pathogens. In the stomach, gastric acid serves as a powerful chemical defence against ingested pathogens.

Innate immunity refers to nonspecific defence mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells such as neutrophils that attack foreign cells in the body.  Polymorphonuclear leukocytes (PMNL - neutrophils) are the granulocytes characterized by the presence of granules in their cytoplasm. They provide innate immunity.

• Passive immunity is the transfer of active humoral immunity of ready-made antibodies (exogenous) or preformed antibodies. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially when high levels of antibodies specific to a pathogen or toxin are transferred to non-immune persons through blood products that contain antibodies. People administered with preformed antibodies get 'Passive immunity'.
• So, the correct answer is 'Passive immunity'.

Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase is an antibacterial enzyme produced by animals and humans that forms part of the innate immune system. Lysozyme is abundant in secretions including tears, saliva, human milk, sweat and mucus. It is also present in cytoplasmic granules of the macrophages and the polymorphonuclear neutrophils. Lysozyme is an enzyme and hence, acts as a chemical barrier of infection.

The second line of defence involves non-specific cellular and molecular responses of the innate immune system because these defences do not differentiate between different types of the pathogen and respond the same way upon every infection. Mucous membrane, phagocytes and lysozyme are all involved in a non-specific defence mechanism, whereas, killer T-cells or cytotoxic T-cells provide the third line of defence.

• Physiological barriers to the entry of microorganisms in the human body are tears in eyes, saliva in mouth and HCl in the stomach.
• The enzymes lysozymes are found in tears and saliva inhibits the synthesis of peptidoglycan present in the cell wall of microorganisms especially eubacteria.
• Hence Tears acts as the physiological barrier to entry of microorganisms in the human body.
• So, the correct answer is 'Tears'.