In this post and next few posts, we present an overview of the immune system. This post summarises the two main groups of the immune system; the innate and the acquired immunity

Any individual is always exposed to a large number of different types of microbes at any given moment. Some of these microbes are harmful while others are harmless. These microbes and even dust particles, toxins or other particulate material may enter into our body through our nasal route, oral route or a injury or may come in contact with our skin. However not all can flourish in our body due to our ‘Immune System‘.

Immune system is a very complicated network of organs, cells and other chemicals, which helps the host to eliminate the invading microbe, or other inanimate objects like dust particles or allergens. Immune system also attacks the viral infected, damaged and the cancerous cell.

(Just for info: Read the paper on roles of the immune system in cancer.

In order to eliminate non-self particle entered into the body, the immune system must be able to discriminate between the self and the nonself particles or molecules. The ability of the immune system to differentiate between the two is known as self-nonself discrimination and the ability to not attack the self tissues is known as selftolerance. If the immune system attacks the self component, it can cause autoimmune diseases.

(Just for info: Read this paper on mechanism of human autoimmunity.)

The non-self invader is recognised and eliminated by immune system, which in vertebrates are is divided into innate and adaptive immune response. Let’s see brief information on the two responses.

(Just for info: Read this paper on the Evolution of Immune system)

• Innate immune system:

The word ‘innate’ literally means ‘inborn‘, that is, the components of innate response are inherited by the individual. This response is the first line of defense against the invaders, which means, when any invader enters the body it encounters the components of innate response first. Innate immune response is non specific and offers protection against any pathogen. The reaction is of same strength and sequence everytime the invader attacks. The innate immunity is quick in its action and is launched within a few minutes. The components of innate response recognise a pathogen by its pathogen-associated molecular patterns (PAMPs), which include complex lipids, carbohydrates, etc, with help of different pattern recognition receptors (PRR).

The innate immune system can be broadly subdivided into three sub groups based on their roles and properties. They are the physical, chemical and cellular components.

– Physical components:

The physical components are the physical barriers, which prevent the entry of the microbes into the body. This includes epithelial layers (e.g. skin), cilia and mucus linings (e.g. in respiratory or GI tract). Mucus of the mucus lining can trap the invading microbes. Sometimes due to predisposition (injury or disorder) or the better mechanism of the invader, it can successfully cross the physical barrier. Then the invading microbe encounters, the chemical and the cellular components of the innate system

(Just for info: Read this paper on the role of mucus layers in gut infection and inflammation.)

– Chemical components: This includes the acids in the stomach, other antimicrobial peptides, lysozymes, humoral components (like complement proteins, defensins and opsonins). These chemicals are always present in the blood and/or other fluids of body and cause destruction of the pathogens.

(Just for info: Read this review paper on the lysozyme.)

– Cellular components: The cellular components of the innate immunity are white blood cells including basophils, eosinophils, dendritic cells, monocytes/macrophages, Langerhans cells, mast cells, neutrophils and natural killer (NK) cells (more about these in the next post).

Neutrophils, eosinophils, monocytes/macrophages, dendritic cells are phagocytes. The dendritic cells and macrophages engulf the microbes, process and present the antigens on their surface to the T cells. These cells are called as antigen presenting cells.

Basophils and mast cells are proinflammatory and are involved in allergic responses to allergens.

All these cells are always patrolling the body and on encountering an invader initiate a well orchestrate reaction.

(Just for info: Read our post on how the biofilm formation helps microbes to escape the immune attack.)

Fig 1: The components of Immune System in Vertebrates.

• The adaptive immune system:

This response against a particular antigen develops only after the individual encounters the antigen. Hence it is acquired by an individual during lifetime and not inherited. This section of immune response is specific in action against a particular invader or microbe. The specificity against an antigen is due to the antigen-specific receptors expressed on the surfaces of its components, T– and Blymphocytes. Antibodies and cytokines are also important components of the acquired immune response.

This reaction is not immediate and needs around 1-2 weeks to develop. The adaptive immune response maintains the memory of the antigens it encounters and can initiate much quicker and stronger response the next time the antigens is encountered, known as the secondary response.

Therefore, innate immunity is the initial broad-range action of the body to limit the entry and proliferation of the any microbe. When the innate immunity is unable to eliminate the invader, it activates the components of acquired immunity, through the antigen presenting cells (APC). The acquired immunity components like T and B lymphocytes, in turn leads to involvement of more innate immunity components as well as their own differentiation and clonal expansion, together amplifying the action against the invader

Therefore, innate and acquired immunity are well connected and work together with the same goal of eliminating the invader from the body.

We shall discuss the role of each of the cellular components involved in both of these responses in the next post.

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Read other posts by The Biotech Notes:

Neurons: Introduction

Difference between prokaryotic and eukaryotic replication.

High Pressure Liquid Chromatgraphy (HPLC).


Chaplin (2010) Overview of the Immune Response. J Allergy Clin Immunol. Feb; 125(2 Suppl 2): S3–23.

Nicholson (2016) The immune system. Essays Biochem. 2016 Oct 31; 60(3): 275–301.